Solar Air Heat and Residential Ventilation Makeup Air

Home , Air changes per hour, Furnace, Solar air heat

Principal Investigator: Adam Kutrich,

Supporting Investigators: Roger Garton, Scott Randall and Jason Edens

May 2013

Table of Contents Summary ...... 4 Background ...... 5 Methods ...... 9 Products ...... 9 House Specifications ...... 9 ASHRAE 62.2 Ventilation Requirements [4] ...... 9 Ventilation Heating and Cooling Load ...... 9 HRV Modeling ...... 9 TRNSED ...... 10 RETScreen ...... 10 Model Comparison ...... 10 Results and Discussion ...... 11 Ventilation Heating Loads ...... 11 System Comparison ...... 11 Energy Comparison ...... 12 Return on Investment Comparison ...... 13 Combination Systems ...... 16 Conclusion ...... 19 Appendix A – Energy Calculation Details ...... 20 House Specifications ...... 20 ASHRAE 62.2 Ventilation Requirements [4] ...... 20 Ventilation Heating Load ...... 20 HRV Energy Savings ...... 20 Standard Recirculation Loop Solar Air Heat System ...... 21 Glazed SAH for Ventilation Makeup Air ...... 22 Effect of Increasing the Solar Collector Area ...... 25 Energy Savings Percentage for Glazed SAH ...... 27 Solar Transpired Air ...... 28 Results Summary ...... 31 Appendix B – Return on Investment Calculation Details ...... 32 References ...... 33

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Summary As homes become increasingly efficient through improved building techniques and energy efficiency measures, ventilation make-up air appliances are required to bring in fresh air to ensure indoor air quality and combustion appliance safety and efficiency. Bringing outside air up to room temperature can be a very energy intensive process. Normally, this heating load is reduced by using a heat recovery ventilator (HRV) or energy recovery ventilator (ERV), which transfers thermal energy from the exhausted air to the incoming air. Alternatively, solar air heat (SAH) collectors could be used to heat the ventilation air.

Energy calculations, return on investment calculations, and computer simulations were used to evaluate the performance of solar collectors used to provide heated ventilation air and to compare that to the performance of HRV systems. The calculations detailed in this report show that using SAH collectors to replace HRV units is not an ideal application for the solar technology. The main problem is that the home requires a constant supply of fresh air while solar collectors only work when the sun is shining on them. There is also the concern of matching the ideal flow rate through the SAH collectors required for optimal heat transfer with the required volume of air exchange in a building. In addition, HRV units can often contribute additional energy savings in the form of cooling during the summer months that SAH technology cannot produce. All of these factors result in an HRV providing more than three times the energy savings of a SAH system.

While SAH systems may not be an apples-to-apples replacement for HRV units, they still are effective at heating ventilation make-up air. Since SAH technology operates at higher efficiency when its incoming air is cooler, SAH systems used for ventilation make-up air applications produce more energy than comparable SAH systems used in conventional recirculation heating applications. For this reason, SAH technology could be effectively used in tandem with HRV technology to provide ventilation heating needs.

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Background Mechanical ventilation is needed to provide homes with both high indoor air quality and energy efficiency. Adequate ventilation keeps people comfortable and healthy, and prevents moisture buildup and mold growth [1]. For a long time, homes relied mostly on natural ventilation which is not always an efficient method for providing fresh air. There are a number of reasons why this has become an important issue in recent decades. People now spend much more of their time indoors, meaning that indoor air is almost all that they breathe. Windows are seldom open in order to save energy, so less fresh air is introduced to the home [2]. At the same time, people are now exposed to a greater variety of chemicals in the indoor environment, and more people have become sensitive to allergens in the environment, so there is an even greater need for fresh air [3]. New energy-efficient buildings with very tight envelopes can make these problems even worse if they are not ventilated.

There is some concern that tight house envelopes will trap moisture and contaminants, but they are actually more healthy in practice. A study conducted by Health Canada of 400 houses in Ontario found that the houses with the most leakage also had the worst air quality. Without dedicated ventilation equipment, houses rely on wind pressure, the convective stack effect, and heating and AC equipment to distribute fresh air. This means the amount of fresh air varies with time depending on weather conditions. The pathways that fresh air will take will be unpredictable, so there is no assurance that critical areas, especially bedrooms, will receive adequate ventilation. This situation may be made even worse since outside air could be filtered through contaminated spaces such as a moldy wall cavity before reaching the occupants. Natural infiltration can also damage building envelopes by creating moisture problems in the attic during the winter, and allowing condensation inside the walls in the summer if the house is air conditioned [2]. For all of these reasons, active ventilation is sometimes advisable even if a house is old and leaky.

Building a home with a tight envelope and mechanical ventilation provides a more comfortable and healthy environment in addition to improved energy efficiency. Fresh air is provided consistently and is distributed evenly through the entire house. Air flows wherever the resistance is lowest, so providing vents for intake and exhaust means more air is directed inside where it is needed and less air is forced through cracks in the building where it can cause problems.

For these reasons ventilation requirements have become established in building codes. ASHRAE 62.2 is the standard that sets the minimum ventilation requirements for residences [4]. Although this standard includes additional details, it is very simple to calculate this ventilation rate for most situations. There are two components: whole house continuous ventilation, and spot ventilation for bathrooms and kitchens.

 Whole house ventilation (cubic feet per minute, cfm) = Floor area (sq. ft.)/100 + 7.5*(# of bedrooms + 1)

 Each bathroom requires 50cfm intermittent ventilation, or 20cfm continuous ventilation

 Each kitchen requires 100cfm intermittent ventilation, or 25cfm continuous ventilation

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When homes are insulated well and sealed tightly, heating and cooling the ventilation air becomes a large portion of the energy load. Heat recovery ventilators (HRVs) are used to save energy while keeping the home comfortable. They work by passing the incoming and outgoing streams of air next to each other so that the exhaust air heats up the intake air (or the reverse if it is warm outside). This way the HRV recycles up to 80% of the thermal energy that would otherwise leave the building.

Figure 1: HRV Operation [5]

Energy Recovery Ventilators (HRV) are another appliance that is often used for bringing ventilation air into a building. The main difference between an HRV and an ERV is that ERVs allow moisture to move from one air stream to another. Because ERVs take advantage of the latent heat in moisture, they are often more effective at recapturing heat from the exhausted air. ERVs are recommended for warmer climates where there is more humidity and cooling loads are larger, but are not recommended for northern climates that experience colder temperatures.

Another strategy to reduce the energy cost of ventilation is to use solar air heat (SAH) panels to heat the air before it goes into the building. This is done for commercial buildings using solar transpired air technology. Transpired air is a SAH technology designed specifically to heat ventilation air. It consists of an unglazed flat plate filled with small holes. Incoming air absorbs solar energy as it is drawn through the holes in the collector surface, and then it flows through a space behind the collector towards the ventilation inlet. Because of its simplicity, transpired air is a very cost-effective source of heat for commercial buildings that require a lot of makeup air. This technology provides the most benefit for buildings with large ventilation requirements, but it could be effective for residential systems as well.

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Figure 2: Solar transpired air [6]

Glazed flat plate SAH collectors can also be used to heat ventilation air. In a glazed flat plate collector, solar energy passes through a glazing and is collected by a specially coated absorber plate. When the system operates, air is driven into a cavity behind the absorber where it collects thermal energy before entering the house. These systems, shown in Figure 3, normally work on a closed loop, heating up air from inside the house and recirculating it back inside. To provide heated ventilation air, the inlet could be placed outside instead as shown in Figure 4. Glazed collectors have not been tested for this purpose, but numerical models can estimate the energy production of this type of system in order to find out how well this type of SAH technology will perform in ventilation make-up air applications.

Figure 3: Glazed solar air heat system in a recirculation configuration [7]

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Figure 4: Glazed solar air heat in a ventilation makeup air configuration

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Methods A complete list of calculations and detailed results can be found in Appendix A – Calculations.

Products The glazed solar air heat collector modeled for this study is the Solar Powered Furnace (SPF) made by the Rural Renewable Energy Alliance (RREAL). The solar transpired air product modeled for this study is MatrixAir, made by Matrix Energy. Performance ratings for the SAH technologies used in this report are provided by the Solar Rating & Certification Corporation (SRCC) and are found at www.solar-rating.org. This study also looked at two different HRV units. The VHR 1405R, manufactured by Fantech, and the ES- 150, manufactured by Nu-Air, were selected for analysis in this study because of their size, availability of data and price point.

House Specifications In order to accurately compare SAH and HRV technologies, the following building was selected and used for all modeling in this study.

 Location: St. Cloud, MN and Boulder, CO.  2,500 sq. ft.  3 bedrooms

ASHRAE 62.2 Ventilation Requirements [4] Determination of ventilation requirements for this building are based on ASHRAE 62.2 requirements.

 Whole house ventilation for 2500ft2, 3 bedrooms: 2500/100 + 7.5*(3 + 1) = 55cfm  Bathroom continuous ventilation: 20cfm  Kitchen continuous ventilation: 25cfm  Total continuous ventilation: 55cfm + 20cfm + 25cfm = 100cfm

Ventilation Heating and Cooling Load The ventilation makeup air will need to be heated or cooled most of the time in order to maintain a comfortable environment inside the house. Historical degree day data provides a good estimate of what these loads will be during a typical year.

HRV Modeling HRV technology lowers the cost of introducing ventilation air by making use of the energy available in exhaust air. Unlike SAH technology, which captures energy from the sun, HRV technology simply recovers existing energy that would be otherwise lost in exhaust air. First, the ventilation heat load without an HRV is calculated based on heating degree day data. In order to calculate the energy saved by HRV units, the total ventilation load is multiplied by the HRV effectiveness, and the electric energy used is subtracted from the total. Manufacturers of HRVs do not provide data for the cooling effectiveness, but this is known to be lower due to the heating effect of the motors, so it is estimated at 50%. HRV modeling is based on manufacturers specifications, which are often slightly higher than actual field performance. Energy lost to defrost cycling was not factored into these calculations, but will have a negative effect on system performance.

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TRNSED Energy production estimates for glazed SAH systems are obtained with TRNSED (Transient Simulation Edition). SPF TRNSED is a program specially developed by Thermal Energy System Specialists in Madison, Wisconsin and RREAL with financial support from the Clean Energy Resource Team of Minnesota (certs.org.). TRNSED is designed to model SAH systems using the TRNSYS platform. TRNSYS is a software environment used to model dynamic systems. In this environment, models are built by defining relationships between multiple components, allowing for the behavior of very complex systems to be determined. For this study, a TRNSED model of a glazed solar air heat collector was used in order to estimate the amount of heat that will be produced over the course of a typical year. The software allows the user to select the location for weather data, enter parameters for the house and furnace, and enter specifications for the solar system. The program then runs an energy simulation for the entire year using a five minute time step interval. It displays a graph of the hourly results and outputs monthly energy production as a spreadsheet. The model has been verified by collecting data from actual recirculation type installations, and the data has shown that the TRNSED software yields a conservative estimate of system performance.

RETScreen Energy production estimates for solar transpired air were obtained with RETScreen, an Excel-based application produced by Natural Resources Canada and NASA. RETScreen is used to estimate the energy production and energy savings from a variety of renewable energy and energy efficiency strategies. It uses monthly average calculations, taking into account local weather data, building design and use, renewable energy system specifications, energy efficiency strategies, etc. It also performs financial, greenhouse gas reduction and other relevant calculations.

Model Comparison Since there was not one uniform method or software program capable of accurately modeling all three of these types of systems, three different methods or programs were used. Energy modeling is best used to compare the relative benefits of different system designs. While the results are realistic compared to the real world, they are not absolute. Since different models incorporate different assumptions and input data, looking at the amount of energy saved would not give a fair comparison of these systems. In order to make accurate comparisons, the percentage of energy savings for each system is calculated. This gives a clear picture of the relative strength of each technology.

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Results and Discussion

Ventilation Heating Loads Heating Ventilation Ventilation Location Degree Days Heating Load Rate (cfm) (°F-day) (mmBtu) St. Cloud, MN 8,076 100 20.93 Boulder, CO 6,667 100 17.28

System Comparison The objective of this project was to compare the effectiveness of using solar technologies to HRVs for the purpose of heating ventilation make-up air. Heat recovery ventilators and solar air heaters are both excellent technologies when used as intended. The heat recovery ventilator is an appliance intended to minimize the loss of energy from a building, whereas the solar air heater is used to collect and add solar energy to a building.

Since HRV technology is able to operate at any time of day under any weather condition and is capable of saving energy used for cooling in the summer, it outperforms SAH technology when used to heat or cool ventilation make up air. SAH technology is limited in its energy production to daylight hours and is only capable of producing energy used for heating needs.

Even though the HRV is a financially superior choice when heating ventilation make-up air, it is only able to minimize losses and is not capable of adding energy to a home. Solar air heaters are a proven technology that collect the sun’s energy and economically add that energy to a home. The two technologies complement each other very well.

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Energy Comparison

Figure 5: Energy Savings Results

HRVs are the most effective at saving energy since they have the same effect day and night regardless of the weather. These results show that HRVs can save between 2.4 and 3.8 times the amount of energy of a similarly priced SAH system, depending on the climate and the model of HRV. Increasing the size of the solar collectors reduces this performance gap, but increases the cost of the system. The HRV performance will be reduced somewhat by the need to run the defrost cycle, but this technology clearly has the greatest impact on household energy usage.

Glazed SAH collectors deliver more energy savings when used to deliver ventilation air than they do in a closed-loop system. This is because the collectors lose less heat to the outside when the inlet temperature is close to the ambient temperature. If the house requires active ventilation, glazed SAH collectors will be very effective at heating that air during the day.

In a standard SAH system, the size of the array is determined by the characteristics of the house, and the flow rate is tailored to maximize the efficiency of the system. In this case, increasing the size of the array results in diminishing returns since the flow rate is fixed.

The performance of the unglazed transpired air system is very close to the glazed system of the same size, which is a surprising result. This is partly because the transpired air system uses a lower power fan

Solar Air Heat and Residential Ventilation Makeup Air 12 Rural Renewable Energy Alliance since the static pressure across the collector is very low. Glazed SAH has a greater advantage in Boulder than in St. Cloud since the climate is windier.

Return on Investment Comparison A key indicator of a project’s value can be found by looking at the return on investment (ROI). When looking at any infrastructure designed to save energy, it is important to look at the initial cost and the value of the energy savings over the lifecycle of the equipment. The motivation for installing new energy saving equipment often is to reduce utility bill costs. Return on investment calculations are intended to demonstrate which piece of infrastructure will be a better investment over the lifecycle of the equipment.

There are many important figures to consider when performing a return on investment calculation. For instance, fuel costs today may not equal fuel costs in the future. Every return on investment calculation has inherent assumptions associated with it. The details used in the calculations performed for this report are listed in Appendix B – Return on Investment Calculation Details.

This ROI calculation does not take into account any operation and maintenance costs during the lifetime of the system. In addition, ROI calculations in general do not take into account the differences in quality and longevity of the equipment and other factors such as product warranty and serviceability. It is important to also consider these other factors outside the ROI calculation when determining the ideal solution for a particular application.

Return on investment calculations are done for both HRV units, a 33 ft2 MatrixAir transpired plate collector from Matrix Energy and a 32 ft2 Solar Powered Furnace from RREAL. All ROI numbers shown here are based on the energy production calculations from the above models. ROI calculations are shown for Saint Cloud, MN only.

Years to Simple Payback: Natural Gas Propane Electricity Fuel Oil Fantech VHR 1405R 8 3 2 3 Nu-Air ES-150 8 3 3 3 RREAL SPF32 16 8 6 7 MatrixAir Transpired Plate (33 ft2) 15 7 6 6

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Figure 6: Return on Investment Against Natural Gas

Figure 7: Return on Investment Against Propane

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Return on Investment Against Electric $32,000.00 $30,000.00 $28,000.00 $26,000.00

$24,000.00 $22,000.00 $20,000.00 $18,000.00 $16,000.00 Fantech $14,000.00 NuAir $12,000.00 $10,000.00 SPF $8,000.00

CummulativeCash Flow Matrix $6,000.00 $4,000.00 $2,000.00 $0.00 -$2,000.00 0 5 10 15 20 25 Year After System Installation

Figure 8: Return on Investment Against Electricity

Return on Investment Against Fuel Oil $65,000.00 $60,000.00 $55,000.00

$50,000.00

$45,000.00 $40,000.00 $35,000.00 Fantech $30,000.00 NuAir $25,000.00 $20,000.00 SPF $15,000.00

CummulativeCash Flow Matrix $10,000.00 $5,000.00 $0.00 -$5,000.00 0 5 10 15 20 25 Year After System Installation

Figure 9: Return on Investment Against Fuel Oil

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Combination Systems SAH collectors produce less savings than HRVs, but they deliver additional heat to the house during the day. Combining the two technologies could take advantage of the strengths of both. A combination system would allow the HRV unit to conserve energy from the exhaust air while also allowing the SAH collector(s) to capture the suns energy and use it to help meet ventilation makeup air needs.

There are two ways of designing a tandem system, the first of which is shown in Figure 10. This system uses the SAH technology to preheat the incoming air first, then feed its heated air to the HRV. This configuration optimizes the SAH technology by providing it with a cold inlet air temperature. The trade- off here is that the HRV unit’s effectiveness may be reduced because the SAH may have already significantly increased the air temperature to the point where the HRV contributes limited or no energy to the ventilation makeup air.

Glazed collectors do not lend themselves well to this application since the temperature rise they can generate is so large. The collector temperature quickly rises above the indoor air temperature unless it is a very cloudy day (shown in Appendix A, Figure 17). This large temperature rise produced by glazed SAH collectors would not only decrease the effectiveness of the HRV unit, but could also risk damaging HRV components. In addition, if the temperature rise is such that outlet air from the SAH is warmer than the building temperature, the HRV could end up transferring heat in the opposite direction, causing it to effectively cool the incoming air. This could be prevented by installing a bypass whereby the air heated by the SAH collector(s) can be routed around the HRV when appropriate. However, this would probably not provide a large benefit, since there would only be short windows of time when the two devices would truly work in tandem. Any additional energy savings would probably not justify the extra cost and complexity of this system.

A transpired air collector is better suited as a preheater for HRV units. The temperature rise from the transpired air technology analyzed here is about 20°F under typical wind conditions on a fairly sunny day according to test results for the transpired air collector (see Appendix A, Figure 21). This means the transpired plate will provide substantial extra heat with only minor loss of effectiveness to the HRV and little or no risk of damaging the HRV components. Transpired air is meant to work with HVAC systems, so no extra controls or ducts are needed.

It is not clear how using SAH technology as a preheater would affect the operation of the HRV and the final energy savings. It cannot simply be stated the using the two technologies together would result in the cumulative energy savings of both systems operating independently. At times, the effectiveness of the HRV would be reduced since there would be a lower temperature differential between the incoming and outgoing air. During colder times, the HRV’s effectiveness may be increased since it will need to spend less time running a defrost cycle. Since the manufacturers give limited performance data at varying temperatures for their HRV units, it is difficult to precisely determine what the overall energy production would be, though either type of SAH technology would clearly deliver a substantial amount of heat to the house.

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Figure 10: Glazed SAH installed in line with HRV

A second method for incorporating both SAH and HRV technologies into a combination system is shown in Figure 11. This configuration uses the HRV as the first air tempering device, then uses the SAH to further increase the air temperature. It should be stated that this configuration is only compatible with glazed SAH technologies, not transpired plate technologies. This method eliminates some of the concerns associated with the previous configuration, where the SAH may overheat the HRV. Since the inlet air will be below room temperature, the SAH collectors will produce more energy savings than a standard closed-loop system.

Similar to the previously discussed method, bypasses would also be required for this configuration. The system would need to bypass the SAH during the nighttime when there is no energy available in the SAH and during the summer months when air heating is not necessary.

Modeling of this system is also difficult since the inlet temperatures to the SAH technology would be dynamic based on the outlets of the HRV, ambient temperature and solar irradiance. A worst case scenario could be assumed in which the inlet temperature for the SAH technology is equal to that of the house temperature. This would never be the case because this scenario assumes that the HRV has 100% effectiveness at all times. However, assuming this worst case scenario allows us to establish a lower bound for the energy produced by this combination system by adding the energy produced by the HRV to the energy produced by the SAH in a standard recirculation loop. By this reasoning, it can be stated that this combination system will produce equal to or greater than the energy savings shown in the table below.

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Minimum Energy Savings Produced by Combination System Location Fantech HRV with 26ft2 of SAH Nu-Air HRV with 26ft2 of SAH Saint Cloud, MN 65% 72% Boulder, CO 70% 77%

Figure 11: HRV installed in line with glazed SAH

Since modeling of the first method of combining these technologies is difficult and only a lower bound for the second scenario has been established it is difficult to numerically compare these two configurations. However, it is highly likely that the second method will produce more energy than the first simply because it does not reduce the HRV effectiveness while still allowing the SAH to provide significant heat to the incoming air, at times rising its temperature well above the building air temperature.

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Conclusion This report has been a study of HRV and SAH technologies and various combinations of both as a means of tempering incoming ventilation makeup air. While it is difficult to directly compare such drastically different technologies, this report as attempted to put numbers to this comparison through the use of computer modeling software.

Since HRV technology has the advantage of being able to run continuously and also save energy used for summertime cooling loads, it is overall more effective at delivering energy to the incoming ventilation makeup air. While there are various HRV units on the market, of the two looked at in this study the Nu- Air ES-150 was the most effective.

As with most projects, cost and return on investment are often a driving factor when deciding whether or not to move forward. This report shows HRV technology having significantly better return on investment figures. Since SAH technology cannot deliver as much energy to the incoming ventilation makeup air as HRV technology, its return on investment is less lucrative.

This report further explored the option of using SAH and HRV technologies together. While modeling of these systems together is difficult and not feasible to do accurately with the currently available tools, it is apparent that these two technologies can work well in tandem. Using an HRV to first temper incoming ventilation makeup air, then feeding this pre-tempered air to an SAH collector is an effective way of using both of these technologies together.

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Appendix A – Energy Calculation Details The following parameters and calculations estimate the performance of SAH and HRV systems in various configurations.

House Specifications In order to accurately compare SAH and HRV technologies, the following building was selected and used for all modeling in this study.

 Location: St. Cloud, MN and Boulder, CO.  2,500 sq. ft.  3 bedrooms

ASHRAE 62.2 Ventilation Requirements [4] Determination of ventilation requirements for this building are based on ASHRAE 62.2 requirements.

Ventilation Heating Load Historical degree day data provides a good estimate of what the ventilation heating load will be during a typical year. The heating load is calculated by converting the flow rate to ft3/day, multiplying by the heating degree days and multiplying by the specific heat of air, which is 0.018Btu/(ft3 * °F):

( ) ( )

Heating Ventilation Ventilation Location Degree Days Heating Load Rate (cfm) (°F-day) (mmBtu) St. Cloud, MN 8,076 100 20.93 Boulder, CO 6,667 100 17.28

HRV Energy Savings The following energy savings were calculated based on degree days and manufacturer’s specifications for each HRV model.

Heating energy savings = (Heating load) * (Apparent sensible effectiveness)

Time in heating mode is an estimate of the percentage of the year that the HRV works to heat the house, rather than cooling it. This is a weighted average based on the heating and cooling degree days for each month of the year.

( ) ( ) ( ) ( )

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( ) ( ) ( ) ( )

Ventilation Heating Electric Apparent Time In Power Heating Energy Energy Net Energy HRV Model Location Sensible Heating Rating (W) Load Savings Consumed Saved Effectiveness Mode (mmBtu) (mmBtu) (mmBtu) Fantech VHR 1405R St. Cloud, MN 70% 102 20.93 14.65 65.93% 2.01 60% Fantech VHR 1405R Boulder, CO 70% 102 17.28 12.10 66.84% 2.04 58% Nu Air ES-150 St. Cloud, MN 73% 80 20.93 15.28 65.93% 1.58 65% Nu Air ES-150 Boulder, CO 73% 80 17.28 12.62 66.84% 1.60 64%

Standard Recirculation Loop Solar Air Heat System Most glazed SAH systems operate on a closed loop, heating up air from inside the house. Standard systems are sized using the rule of 2-4 cfm per square foot of collector area to achieve good efficiency. Collectors are typically available in 26 sq. ft., 32 sq. ft., and 40 sq. feet sizes. To meet the required 100 cfm ventilation air requirement, one 26 sq. ft. collector would be the recommended size. The heat produced by this system is estimated using TRNSED, with the following input parameters.

 Building or House for Simulation: 2500 square foot – 5%/10% windows – Super Insulated  Minimum House (Furnace) Thermostat Setting: 65 °F  Shut-off Temperature (Max House Temp): 80 °F  Auxiliary Heating Inlet Temperature: House Temperature  Backup Furnace/Auxiliary System o Maximum Heating Rate of Furnace/Aux.: 120,000 Btu/hour o Flow Rate of Furnace/Aux. Heater: 2,000 cfm o Maximum Power of Aux. Blower Fan: 200W o Efficiency of Backup Furnace/Aux. System: 92%  Internal Gains: Ignore Internal Gains  Slab Heat Loss: Ignore Slab Heat Loss  Solar Collector System o Collector Inlet Temperature: House Temperature o Collector Orientation . Slope of Collector Surface: 90° . Azimuth of Collector Surface: 0° o SPF Collector Selection: Manufacturer’s Recommended Values o Collector Type: SPF26 . Number of Collector Modules in Series: 1 . Number of Collector Modules in Parallel: 1 o Controller Settings . Upper Dead Band dT for Blower: 30 °F . Lower Dead Band dT for Blower: 8 °F o Blower Fan Settings . Total Blower Fan Flow Rate: 100 cfm

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. Maximum Power of Blower Fan: 44.4 W o Summer Operation: Do Not Shut System Down for Part of Year (No DHW Heating)

BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 983,415 198,475 20.2% 198,475 1.38 33.61 13,605,862 12,517,394 1.6% February 1,152,297 286,143 24.8% 286,143 1.66 25.02 9,931,531 9,137,008 3.0% March 1,312,661 313,480 23.9% 313,480 2.15 18.00 7,032,413 6,469,820 4.6% April 843,981 204,751 24.3% 204,751 2.00 8.06 3,154,227 2,901,889 6.6% May 791,581 178,815 22.6% 178,815 2.32 1.61 629,747 579,367 23.6% June 724,912 136,823 18.9% 136,823 1.97 - - - 100.0% July 777,595 63,068 8.1% 63,068 0.86 - - - 100.0% August 836,927 142,340 17.0% 142,340 1.48 - - - 100.0% September 887,699 275,869 31.1% 275,869 2.41 0.76 299,180 275,246 50.1% October 932,706 307,821 33.0% 307,821 2.22 6.53 2,553,754 2,349,454 11.6% November 779,517 222,048 28.5% 222,048 1.51 18.26 7,120,153 6,550,541 3.3% December 701,208 138,901 19.8% 138,901 1.01 30.96 12,286,449 11,303,533 1.2% Total 10,724,499 2,468,533 23.0% 2,468,533 20.97 142.82 56,613,316 52,084,251 4.5%

Figure 12: TRNSED results for standard glazed SAH system with 1 26 sq. ft. collector (St. Cloud, MN)

Energy Energy Fan Energy Incident Delivered Fan Delivered to Solar Collected Collection Power - Consumed Solar to Building Power - Building by Fraction Energy Efficiency Auxiliary by Auxiliary Radiation by Solar Solar Auxiliary Air System System System System BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 1,159,291 368,778 31.8% 368,778 2.38 17.52 6,850,717 6,302,659 5.5% February 1,059,781 343,014 32.4% 343,014 2.01 13.88 5,426,876 4,992,726 6.4% March 1,221,537 364,699 29.9% 364,699 2.72 10.63 4,142,690 3,811,274 8.7% April 950,453 255,503 26.9% 255,503 2.57 4.37 1,710,365 1,573,536 14.0% May 734,130 172,417 23.5% 172,417 2.38 2.07 810,735 745,876 18.8% June 636,725 113,988 17.9% 113,988 1.76 0.38 147,075 135,309 45.7% July 699,251 26,998 3.9% 26,998 0.37 - - - 100.0% August 850,940 99,656 11.7% 99,656 1.10 - - - 100.0% September 1,040,959 286,669 27.5% 286,669 2.14 0.44 172,365 158,576 64.4% October 1,246,815 467,379 37.5% 467,379 2.89 3.50 1,368,262 1,258,801 27.1% November 1,107,874 383,083 34.6% 383,083 2.37 11.72 4,574,726 4,208,748 8.3% December 1,174,591 377,881 32.2% 377,881 2.31 18.50 7,234,505 6,655,745 5.4% Total 11,882,346 3,260,065 27.4% 3,260,065 24.99 83.00 32,438,315 29,843,250 9.8%

Figure 13: TRNSED results for standard glazed SAH system with 1 26 sq. ft. collector (Boulder, CO)

Glazed SAH for Ventilation Makeup Air Several parameters from the first example have to be adjusted in order for the model to represent a ventilation makeup air system. The first difference is the collector inlet temperature needs to be changed to the ambient temperature. Also, since the house needs a constant supply of fresh air, the

Solar Air Heat and Residential Ventilation Makeup Air 22 Rural Renewable Energy Alliance controller needs to be on all the time instead of turning on and off as temperatures change. To accomplish this, both dead band settings are set to -100 °F. The following parameters were used in this model.

 Building or House for Simulation: 2500 square foot – 5%/10% windows – Super Insulated  Minimum House (Furnace) Thermostat Setting: 65 °F  Shut-off Temperature (Max House Temp): 80 °F  Auxiliary Heating Inlet Temperature: House Temperature  Backup Furnace/Auxiliary System o Maximum Heating Rate of Furnace/Aux.: 120,000 Btu/hour o Flow Rate of Furnace/Aux. Heater: 2,000 cfm o Maximum Power of Aux. Blower Fan: 200W o Efficiency of Backup Furnace/Aux. System: 92%  Internal Gains: Ignore Internal Gains  Slab Heat Loss: Ignore Slab Heat Loss  Solar Collector System o Collector Inlet Temperature: Ambient (Outdoors) Temperature o Collector Orientation: . Slope of Collector Surface: 90° . Azimuth of Collector Surface: 0° o SPF Collector Selection: Manufacturer’s Recommended Values o Collector Type: SPF26 . Number of Collector Modules in Series: 1 . Number of Collector Modules in Parallel: 1 o Controller Settings . Upper Dead Band dT for Blower: -100 °F . Lower Dead Band dT for Blower: -100 °F o Blower Fan Settings . Total Blower Fan Flow Rate: 100 cfm . Maximum Power of Blower Fan: 44.4 W o Summer Operation: Do Not Shut System Down for Part of Year (No DHW Heating)

Solar Air Heat and Residential Ventilation Makeup Air 23 Rural Renewable Energy Alliance

BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 983,415 450,930 45.9% 450,930 9.18 34.79 14,307,948 13,163,312 3.3% February 1,152,297 512,716 44.5% 512,716 8.29 26.22 10,540,128 9,696,918 5.0% March 1,312,661 552,512 42.1% 552,512 9.18 19.41 7,583,385 6,976,715 7.3% April 843,981 324,417 38.4% 324,417 8.88 8.72 3,406,730 3,134,192 9.4% May 791,581 243,874 30.8% 243,874 7.88 1.83 716,509 659,188 27.0% June 724,912 191,098 26.4% 191,098 6.50 - - - 100.0% July 777,595 91,442 11.8% 91,442 3.21 - - - 100.0% August 836,927 179,142 21.4% 179,142 5.10 - - - 100.0% September 887,699 330,317 37.2% 330,317 8.24 1.00 389,987 358,788 47.9% October 932,706 406,293 43.6% 406,293 9.18 7.21 2,819,441 2,593,885 13.5% November 779,517 355,754 45.6% 355,754 8.88 19.63 7,648,220 7,036,362 4.8% December 701,208 315,522 45.0% 315,522 9.18 32.46 12,990,198 11,950,982 2.6% Total 10,724,499 3,954,017 36.9% 3,954,017 93.68 151.26 60,402,546 55,570,343 6.6%

Figure 14: TRNSED results for solar ventilation air heat system with 1 26 sq. ft. collector (St. Cloud, MN)

Energy Energy Fan Energy Incident Delivered Fan Delivered Solar Collected Collection Power - Consumed Solar to Building Power - to Building Fraction Energy Efficiency Auxiliary by Auxiliary Radiation by Solar Solar by Auxiliary Air System System System System BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 1,159,291 534,314 46.1% 534,314 9.18 18.82 7,371,515 6,781,794 7.3% February 1,059,781 472,414 44.6% 472,414 8.29 14.99 5,857,648 5,389,036 8.1% March 1,221,537 502,255 41.1% 502,255 9.18 11.64 4,536,246 4,173,347 10.7% April 950,453 347,753 36.6% 347,753 8.88 5.01 1,959,400 1,802,648 16.2% May 734,130 236,157 32.2% 236,157 8.67 2.39 935,579 860,733 21.5% June 636,725 148,814 23.4% 148,814 6.09 0.40 157,873 145,243 50.6% July 699,251 34,393 4.9% 34,393 1.26 - - - 100.0% August 850,940 113,217 13.3% 113,217 3.31 - - - 100.0% September 1,040,959 326,344 31.4% 326,344 6.45 0.53 208,500 191,820 63.0% October 1,246,815 542,931 43.5% 542,931 9.05 4.01 1,570,427 1,444,793 27.3% November 1,107,874 506,340 45.7% 506,340 8.88 12.65 4,931,597 4,537,069 10.0% December 1,174,591 542,818 46.2% 542,818 9.18 19.82 7,770,363 7,148,734 7.1% Total 11,882,346 4,307,750 36.3% 4,307,750 88.40 90.27 35,299,147 32,475,216 11.7%

Figure 15: TRNSED results for solar ventilation air heat system with 1 26 sq. ft. collector (Boulder, CO)

Figure 17 shows the hourly performance of this system for one week at the end of January/beginning of February.

Solar Air Heat and Residential Ventilation Makeup Air 24 Rural Renewable Energy Alliance

Figure 16: Sample of collector temperatures for a ventilation system with 1 26 sq. ft. SAH collector (St. Cloud, MN), taken from TRNSED simulation

Effect of Increasing the Solar Collector Area Increasing the area of solar panels used can increase the heat output of the system. The next four charts show the results of using one 40 sq. ft. collector and two 40 sq. ft. collectors in parallel. All the other input parameters are the same as the 26 sq. ft. example.

Energy Energy Fan Energy Incident Delivered Fan Delivered to Solar Collected Collection Power - Consumed Solar to Building Power - Building by Fraction Energy Efficiency Auxiliary by Auxiliary Radiation by Solar Solar Auxiliary Air System System System System BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 1,536,789 569,670 37.1% 569,670 9.18 34.60 14,201,527 13,065,405 4.2% February 1,800,702 649,165 36.1% 649,165 8.29 25.81 10,398,011 9,566,170 6.4% March 2,051,303 699,887 34.1% 699,887 9.18 18.95 7,417,968 6,824,531 9.3% April 1,318,895 410,780 31.1% 410,780 8.88 8.59 3,356,899 3,088,347 11.7% May 1,237,009 307,738 24.9% 307,738 7.74 1.75 685,768 630,907 32.8% June 1,132,825 241,638 21.3% 241,638 6.47 - - - 100.0% July 1,215,152 110,372 9.1% 110,372 2.97 - - - 100.0% August 1,307,870 225,678 17.3% 225,678 5.05 - - - 100.0% September 1,387,213 416,821 30.0% 416,821 8.23 0.93 362,722 333,704 55.5% October 1,457,546 514,262 35.3% 514,262 9.18 7.00 2,738,123 2,519,073 17.0% November 1,218,155 449,922 36.9% 449,922 8.88 19.42 7,562,168 6,957,195 6.1% December 1,095,782 398,861 36.4% 398,861 9.18 32.25 12,902,981 11,870,742 3.3% Total 16,759,242 4,994,793 29.8% 4,994,793 93.22 149.29 59,626,166 54,856,074 8.3%

Figure 17: TRNSED results for solar ventilation air heat system with 1 40 sq. ft. SAH collector (St. Cloud, MN)

Solar Air Heat and Residential Ventilation Makeup Air 25 Rural Renewable Energy Alliance

Energy Energy Fan Energy Incident Delivered Fan Delivered to Solar Collected Collection Power - Consumed Solar to Building Power - Building by Fraction Energy Efficiency Auxiliary by Auxiliary Radiation by Solar Solar Auxiliary Air System System System System BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 1,811,632 675,726 37.3% 675,726 9.18 18.49 7,237,630 6,658,620 9.2% February 1,656,126 598,032 36.1% 598,032 8.29 14.75 5,767,400 5,306,008 10.1% March 1,908,903 635,850 33.3% 635,850 9.18 11.46 4,459,659 4,102,887 13.4% April 1,485,279 440,345 29.6% 440,345 8.88 4.78 1,870,640 1,720,989 20.4% May 1,147,230 299,031 26.1% 299,031 8.64 2.38 930,243 855,823 25.9% June 995,014 184,249 18.5% 184,249 5.82 0.40 156,117 143,627 56.2% July 1,092,724 43,441 4.0% 43,441 1.25 - - - 100.0% August 1,329,769 143,050 10.8% 143,050 3.26 - - - 100.0% September 1,626,713 406,322 25.0% 406,322 6.37 0.49 190,324 175,098 69.9% October 1,948,405 686,011 35.2% 686,011 9.03 3.82 1,494,388 1,374,837 33.3% November 1,731,281 640,532 37.0% 640,532 8.88 12.43 4,844,677 4,457,103 12.6% December 1,835,540 687,247 37.4% 687,247 9.18 19.49 7,645,287 7,033,664 8.9% Total 18,568,617 5,439,837 29.3% 5,439,837 87.95 88.47 34,596,364 31,828,655 14.6%

Figure 18: TRNSED results for solar ventilation air heat system with 1 40 sq. ft. SAH collector (Boulder, CO)

BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 3,073,579 692,874 22.5% 692,874 9.18 34.26 14,068,369 12,942,900 5.1% February 3,601,405 791,046 22.0% 791,046 8.29 25.45 10,259,430 9,438,676 7.7% March 4,102,606 853,205 20.8% 853,205 9.18 18.57 7,278,021 6,695,779 11.3% April 2,637,789 500,602 19.0% 500,602 8.88 8.38 3,273,752 3,011,852 14.3% May 2,474,018 374,873 15.2% 374,873 7.72 1.69 663,904 610,791 38.0% June 2,265,650 286,162 12.6% 286,162 6.23 - - - 100.0% July 2,430,304 128,715 5.3% 128,715 2.62 - - - 100.0% August 2,615,741 274,313 10.5% 274,313 5.01 - - - 100.0% September 2,774,425 505,490 18.2% 505,490 8.06 0.80 313,545 288,461 63.7% October 2,915,092 626,510 21.5% 626,510 9.18 6.78 2,645,942 2,434,267 20.5% November 2,436,311 547,747 22.5% 547,747 8.88 19.13 7,459,509 6,862,748 7.4% December 2,191,563 485,397 22.1% 485,397 9.18 31.91 12,791,378 11,768,068 4.0% Total 33,518,484 6,066,935 18.1% 6,066,935 92.40 146.99 58,753,850 54,053,543 10.1%

Figure 19: TRNSED results for solar ventilation air heat system with 2 40 sq. ft. SAH collectors (St. Cloud, MN)

Solar Air Heat and Residential Ventilation Makeup Air 26 Rural Renewable Energy Alliance

Energy Energy Fan Energy Incident Fan Delivered to Solar Collected Collection Delivered to Power - Consumed Solar Power - Building by Fraction Energy Efficiency Building by Auxiliary by Auxiliary Radiation Solar Auxiliary Air Solar System System System System BTUs BTUs % BTUs kWh kWh BTUs BTUs - January 3,623,264 822,604 22.7% 822,604 9.18 18.14 7,112,024 6,543,062 11.2% February 3,312,253 728,635 22.0% 728,635 8.29 14.50 5,668,185 5,214,730 12.3% March 3,817,807 774,756 20.3% 774,756 9.18 11.16 4,345,916 3,998,242 16.2% April 2,970,558 536,648 18.1% 536,648 8.88 4.67 1,824,907 1,678,914 24.2% May 2,294,459 357,602 15.6% 357,602 8.56 2.34 917,468 844,070 29.8% June 1,990,028 218,797 11.0% 218,797 5.67 0.39 154,276 141,934 60.7% July 2,185,449 53,351 2.4% 53,351 1.23 - - - 100.0% August 2,659,539 166,779 6.3% 166,779 2.71 - - - 100.0% September 3,253,426 494,027 15.2% 494,027 6.11 0.47 183,209 168,552 74.6% October 3,896,810 832,844 21.4% 832,844 8.90 3.66 1,433,083 1,318,437 38.7% November 3,462,562 779,954 22.5% 779,954 8.88 12.14 4,731,997 4,353,438 15.2% December 3,671,080 837,425 22.8% 837,425 9.18 19.10 7,501,743 6,901,604 10.8% Total 37,137,233 6,603,421 17.8% 6,603,421 86.77 86.57 33,872,808 31,162,984 17.5%

Figure 20: TRNSED results for solar ventilation air heat system with 2 40 sq. ft. SAH collectors (Boulder, CO)

Energy Savings Percentage for Glazed SAH The TRNSED simulation calculates the heating load for the whole house, but does not show what part of this is due to the ventilation load. An additional simulation step is required to determine what this load is so that the percentage of energy savings can be calculated. This is done by comparing two TRNSED runs where the only variable changed is the rate of infiltration. The difference in energy delivered to the building between these two simulations is equal to the heating energy required to raise the ventilation air to room the house’s setpoint.

Inside the TRNSED model, infiltration is specified in air changes per hour. The volume of the building can be used to convert the ventilation flow rate to air changes per hour:

 Volume of conditioned space: 30,217 ft3  (100 ft3/min)*(60 min/hour)*(air change/ 30,217 ft3) = 0.20 air changes per hour

Increasing the infiltration rate by this amount in TRNSED gives 19.6mmBtu for St. Cloud and 15.4mmBtu for Boulder.

The next step is to calculate the electrical energy used by the fan. Since the fan in a glazed SAH system has to overcome a significant amount of static pressure, the ventilation system can be made more efficient by bypassing the collectors and using a lower powered fan when there is no solar energy available. A good fan for this purpose is the Panasonic WhisperGreen FV-13VKM3, which consumes

15.6W while delivering 111cfm at a static pressure of 0.25in H2O. The total ventilation fan energy consumed can be determined by first calculating the percentage of the year that the solar fan will operate, and then having the direct ventilation fan run the rest of the time:

Energy used by solar fan if it ran the entire year:

Solar Air Heat and Residential Ventilation Makeup Air 27 Rural Renewable Energy Alliance

 (44.4W)*(365*24h)*(3.412Btu/Wh) = 1.33mmBtu

Energy used by direct ventilation fan if it ran the entire year:

 (15.6W)*(365*24h)*(3.412Btu/Wh) = 0.466mmBtu

TRNSED gives the energy consumed by the solar fan, ESF. The total fan energy consumed is calculated by adding in the contribution of the direct ventilation fan for the remainder of the year:

 Total fan energy consumed = ESF + 0.466*(1.33 – ESF)/1.33

Combining these two new calculations with the TRNSED results gives the following results for percentage of energy saved by the solar ventilation makeup air systems:

VMUA Fan Ventilation Fan Energy Net Energy Percentage Heating Energy Energy Heating Technology Consumed Saved Energy Saved (mmBtu) Consumed Load (mmBtu) (mmBtu) Saved (mmBtu) (mmBtu) Closed-loop SAH, 26 sq. ft. 2.47 0.072 0.441 19.6 1.96 10% Ventilation SAH, 26 sq. ft. 3.95 0.32 0.354 19.6 3.28 17% Ventilation SAH, 32 sq. ft. 4.50 0.319 0.354 19.6 3.83 20% Ventilation SAH, 40 sq. ft. 5.00 0.318 0.355 19.6 4.33 22% Ventilation SAH, 40 sq. ft. (x2) 6.07 0.315 0.356 19.6 5.40 28%

Solar Transpired Air The recommended airflow rates for the MatrixAir transpired plate SAH technology are 3-8 cfm per square foot of collector area. A lower flow rate results in a higher temperature rise, while 7-8 cfm per square foot gives optimal efficiency per collector area. For this case, a high temperature rise is desired to deliver as much heat as possible to the incoming air. Since the flow rate is 100 cfm, 33 square feet of collector is used to get 3 cfm per square foot. Figure 21 shows test results for one type of transpired air collector. For this system the temperature rise will be up to 27.5°F on a sunny day with medium wind.

Solar Air Heat and Residential Ventilation Makeup Air 28 Rural Renewable Energy Alliance

Figure 21: Test results for MatrixAir transpired collectors

RETScreen was used to estimate the heat produced by this system over a typical year for both St. Cloud, MN and Boulder, CO. The RETScreen setup is shown below for Boulder. For the weather data, wind speeds are measured at 10m above ground level. Since the collectors will be close to the ground, the wind speeds were reduced by 50%.

Solar Air Heat and Residential Ventilation Makeup Air 29 Rural Renewable Energy Alliance

Technology Solar air heater Load characteristics Application Ventilation Process

Unit Base case Proposed case Facility type Residential Indoor temperature °F 65.0 65.0 Air temperature – maximum °F 80.0 80.0 R-value – wall ft² - ºF/(Btu/h) 23.7 23.7

Design airflow rate cfm 100 100 Operating days per week – weekdays d/w 5.0 5.0 Operating hours per day – weekdays h/d 24.0 24.0 Operating days per week – weekends d/w 2.0 2.0 Operating hours per day – weekends h/d 24.0 24.0

Percent of month used Month

Unit Base case Proposed case Heating million Btu 18 18

Resource assessment Solar tracking mode Fixed Slope ˚ 90.0 Azimuth ˚ 0.0

Daily solar Daily solar Show data radiation - radiation - horizontal tilted

Month kWh/m²/d kWh/m²/d January 2.36 4.50 February 3.38 4.61 March 4.59 4.12 April 5.39 3.27 May 6.12 2.80 June 6.97 2.75 July 6.79 2.83 August 5.84 3.12 September 5.01 3.84 October 3.64 4.05 November 2.68 4.41 December 2.14 4.52 Annual 4.58 3.73

Annual solar radiation – horizontal MWh/m² 1.67

Solar Air Heat and Residential Ventilation Makeup Air 30 Rural Renewable Energy Alliance

Annual solar radiation – tilted MWh/m² 1.36

Solar air heater Type Transpired-plate Design objective High temperature rise Manufacturer Matrix Energy Model MatrixAir TR - Black Solar collector absorptivity 0.94 Performance factor 0.86 Solar collector area ft² 33 51 Solar collector shading - season of use % 0% Wind speed Incremental fan power W/ft² 1.3 Electricity rate $/kWh 0.000

Summary Incremental electricity – fan MWh 0.4 Heating delivered million Btu 4.7 Building heat loss recaptured million Btu 0.2

Heating Ventilation Fan Energy Energy Net Energy Location Heating Load Consumed Savings Savings (mmBtu) (mmBtu) (mmBtu) St. Cloud, MN 21 4.5 0.466 19% Boulder, CO 18 4.7 0.466 24%

Results Summary The following chart summarizes the resulting energy savings from each technology studied:

Solar Air Heat and Residential Ventilation Makeup Air 31 Rural Renewable Energy Alliance

Appendix B – Return on Investment Calculation Details The following numbers and assumptions were used when performing return on investment calculations for this report. All return on investment calculations are projections and are not absolute.

Equipment Costs*:

Fantech VHR 1405R [9] [10] [11] $1035.93 Nu-Air ES-150 [12] $1129.00 RREAL SPF26 [13] $1100.00 Matrix Air Transpired Plate (33 ft2) [14] [15] $1107.81

*Equipment costs do not include additional components such as fasteners, hangers, etc., and labor costs that may be required for installation. These costs have been omitted from these calculations because they are often site specific and can vary from case to case. It should be noted that installation costs can be a significant portion of the actual cost of the system. Installation costs will impact the payback periods listed above.

Fuel Prices:

Natural Gas [16] $8.66/therm Propane [17] $2.189/gallon Electricity [18] $0.1096/kWh Fuel Oils [19] $3.419/gallon

Energy Content for Fuels: [20]

Natural Gas 1,029,000 BTU/therm Propane 91,333 BTU/gallon Electricity 3412 BTU/kWh Fuel Oils 138,690 BTU/gallon

Annual Fuel Utilization Efficiency: [21]

Natural Gas 80% Propane 80% Electricity 97%** Fuel Oils 87%

**Based on indoor electric furnace or baseboard heat

Annual Inflation Rate: 2.44% [22]

Fuel Escalation Rates: [23]

Natural Gas 4.81% Propane 5.32% Electricity 3.37% Fuel Oils 9.36%

Solar Air Heat and Residential Ventilation Makeup Air 32 Rural Renewable Energy Alliance

References

[1] Home Ventilating Institute, "Home Ventilating Institute (HVI Indoor Air Quality (IAQ) Position Paper," 1 October 2009. [Online]. Available: http://www.hvi.org/publications/pdfs/HVI_IAQPositionPaper_01Oct09.pdf. [Accessed 16 January 2013].

[2] G. Cooke, "Natural Versus Mechanical Ventilation," January 2005. [Online]. Available: http://www.hvi.org/publications/pdfs/HPAC_CookeJanFeb05.pdf. [Accessed 16 January 2013].

[3] D. W. Wolbrink, "Mold, Moisture, and Houses - Ventilation Is an Effective Weapon," 2009. [Online]. Available: http://www.hvi.org/publications/pdfs/MoldPaper_final1June09.pdf. [Accessed 16 January 2013].

[4] ASHRAE Standard 62.2-2010: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2010.

[5] Prostar Mechanical Technologies, LTD, "HRV (Heat Recovery Ventilators)," 2012. [Online]. Available: http://www.prostar-mechanical.com/HRV/Greentek%20HRV.htm. [Accessed 29 January 2013].

[6] Matrix Energy, "Solar Air Heating Systems," 2011. [Online]. Available: http://www.matrixairheating.com/products.html. [Accessed 29 January 2013].

[7] Rural Renewable Energy Alliance, "Solar Air Heat Basics," 2010. [Online]. Available: http://www.rreal.org/solar-powered-furnace/solar-air-heat-basics/. [Accessed 29 January 2013].

[8] RETScreen International, "RETScreen Version 4," Natural Resources Canada, Varennes, Quebec, 2012.

[9] AC Whole Salers, [Online]. Available: www.acwholesalers.com.

[10] Grainger, [Online]. Available: www.grainger.com.

[11] Electrical Supplies Online, [Online]. Available: www.electricalsuppliesonline.com.

[12] TMS Johnson, Price Quote, 2012.

[13] Rural Renewable Energy Alliance, MSRP Pricing, 2012.

[14] Matrix Air, West Preparatory School, Toronto ON.

[15] Matrix Air, Arts Education Building at Bemidji State University, 2012.

Solar Air Heat and Residential Ventilation Makeup Air 33 Rural Renewable Energy Alliance

[16] Energy Information Administration, "Minnesota Annual Average," 2011. [Online]. Available: http://www.eia.gov/dnav/ng/ng_pri_sum_dcu_SMN_a.htm.

[17] Energy Information Administration, "Minnesota Annual Average," 2011. [Online]. Available: http://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=M_EPLLPA_PRS_R20_DPG&f=M.

[18] Energy Information Administration, "Minnesota Annual Average," 2011. [Online]. Available: http://www.eia.gov/electricity/data.cfm#sales.

[19] Energy Information Administration, "Minnesota Annual Average," 2011. [Online]. Available: http://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=M_EPD2F_PRS_R20_DPG&f=M.

[20] Energy Information Administration, "Annual Energy Review 2011," 27 September 2011. [Online]. Available: http://www.eia.gov/totalenergy/data/annual/pdf/aer.pdf.

[21] Energy Information Administration, "Minimum Requirements beginning on May 2013," [Online]. Available: http://energy.gov/energysaver/articles/furnaces-and-boilers.

[22] Bureau of Labor Statistics, [Online]. Available: http://www.bls.gov/cpi/tables.htm.

[23] Energy Information Administration, "Based on 15 year historical average for Minnesota," [Online].

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