NDSU EXTENSION SERVICE EXTENDING KNOWLEDGE CHANGING LIVES

AE1393 (May 2018) Air-to-Air Exchangers For Healthier -efficient Homes

Reviewed by Kenneth Hellevang Ph.D., P.E. Extension

To reduce heating and costs, builders have used better construction techniques and materials to greatly reduce air into and out of a home. While a “tight” home will reduce the costs of heating and cooling, it also will trap and harmful particles in the home.

North Dakota State University Fargo, North Dakota 1 • AE1393 Air-to-Air Heat Exchangers Window and other Relative is the ratio of to problematic levels, leading to moisture problems are likely in a the amount of water vapor in air condensation on windows and high weatherized home without air compared with the maximum humidity that may lead to mold exchangers. This is a problem for amount of water vapor the air may growth. The recommended relative both people and the home structure. hold at a particular temperature. humidity for people is around Bringing in outside air and exhausting Dew point is the temperature at 50 percent to minimize nosebleeds, indoor air (ventilation) dilutes which the relative humidity is dry skin and other physical ailments. or removes the indoor pollutants 100 percent and condensation forms. Northern climates cannot support and moisture. The question is: How this level of humidity during the Warm air has the capacity to hold do you remove the moisture and winter. When warm, moist air comes more water vapor than cold air. On a pollutants while retaining the heated in contact with cool surfaces, moisture warm summer day, the temperature or cooled air? An air-to-air heat condenses on the surface if it is below may be 85 degrees Fahrenheit (°F), exchanger will solve that problem. the dew point. with a 50 percent relative humidity Air exchangers transfer the thermal level, making the dew point 71 °F. Just as water condenses on a glass energy of the indoor air to incoming of ice water, condensation will fresh air, allowing the moisture and As the air cools, the temperature gets form on cold surfaces in a home. pollutants to be vented but retaining closer to the dew point, or the point This can happen on windows, the heat. This publication describes where the water vapor begins to settle doors, and even inside walls. reasons to use air-to-air heat out of the air. For example, as the Sustained wet conditions may cause exchangers, of exchangers, 85 °F air cools, the relative humidity structural damage and associated the cost advantages of installing them increases, and at 70 °F, condensation problems with rot and mold. and some tips on choosing a heat forms on cool surfaces. Air at 70 °F An ideal humidity for the northern exchanger that is right for your home. and 40 percent relative humidity has a Plains in the winter is 30 percent to relative humidity of about 80 percent 40 percent, a compromise between when cooled to 50 °F. Air at 20 °F and ideal conditions for human beings 90 percent relative humidity has a Why is ventilation and the structures they inhabit. relative humidity of 23 percent when a concern? heated to 60 °F. Roughly, a 20 °F drop In days past, energy was cheaper in temperature cuts the water-holding than insulation and builders used Measuring in capacity in half and doubles less care in insulating a home. the relative humidity. As time progressed and energy prices Home Humidity Use a hygrometer (Figure 1), or increased, homeowners began to In tight homes, human activities relative humidity meter, to check reduce costs by insulating attics, such as showers, drying clothes and a structure for relative humidity. walls and basements, which cooking raise the relative humidity stopped large-scale .

Recently, due to high energy costs and better materials, homeowners and builders are stopping the small air leaks around doors, windows, and even light switch plates. In some homes, this natural air now replaces inside air every four to 10 hours, compared with every 30 minutes 40 years ago. Unfortunately, this reduction of outside air entering the structure can lead to problems with . Two of the most common quality issues are excess humidity and pollutants. Figure 1. Examples of relative humidity meters, also known as hygrometers. (Photo by Carl Pedersen)

2 • AE1393 Air-to-Air Heat Exchangers Hygrometers can have either a dial or container, this indicates a relative a digital readout. Digital hygrometers humidity level of 100 percent. Pollutants in Homes are not always more accurate. Models The reading on the hygrometer Different pollutants exist in different are commercially available that are should be at least 95 percent and levels in different homes. Examples more expensive and generally should preferably 100 percent, Figure 2. include carbon dioxide and monoxide have a higher degree of accuracy. Take note of the reading. from gas-fueled appliances, radon The more expensive hygrometers gas from the soil surrounding Now add table salt to the cup of generally are accurate within foundations, formaldehyde from water while stirring until the water 5 percent of the actual relative building materials and cannot dissolve any more salt. humidity. All hygrometers require such as mold and tobacco smoke. Salt should be sitting on the bottom calibration to increase their level Table 1 (page 4) lists some major of the cup. Then place the cup back of accuracy. When purchasing a hy- sources of indoor and outdoor in the sealed container with the grometer, check the operating pollutants. Some of the more meter and let them sit again for two range because electronic hygrometers common pollutants deserve to three hours. The salt reduces the may have a minimum relative discussion on their creation and ability of the water to evaporate and, humidity level they can read, possible human health concerns. therefore, the humidity level. A salt for example 20 percent. solution should generate a humidity Carbon dioxide and carbon monox- To calibrate a hygrometer, obtain an reading of 75 percent, but readings ide, resulting from combustion of fuel, airtight container at least three times from 70 percent to 80 percent are can pose serious health problems. the size of the hygrometer. Examples acceptable, Figure 3. Older appliances usually generate include a bag with a zip-type the highest levels of carbon mon- Compare the two readings. If they are seal, a food storage container with a oxide due to improper combustion, both different by the same amount, tight-fitting lid or a coffee can with leaks and lack of enough fresh air for you can recalibrate your hygrometer the original lid. Place a cup with complete combustion. While carbon by that amount. Check the owners water in the sealed container along dioxide only causes problems at high manual for specific instructions with the meter for four to six hours levels, its presence usually indicates for calibrating your unit. If your or until water droplets are visible carbon monoxide is also present. unit does not have the ability to be on the inner surface of the container. High carbon dioxide levels cause calibrated, then you can adjust the When the droplets begin to drowsiness and indicate poor readings mentally. accumulate on the edge of the sealed ventilation. Carbon monoxide causes headaches and fatigue at low levels and may cause unconsciousness or

Figure 2. Calibration test, 100 percent humidity. Figure 3. Salt solution calibration test, 75 percent humidity. (Photo by Carl Pedersen) (Photo by Carl Pedersen)

3 • AE1393 Air-to-Air Heat Exchangers Table 1. Common sources and types of pollutants. Pollutant Type Outdoor sources Pollutant Types* SO2 Sulfur dioxide N0 Nitrous oxide Ambient air SO , NO, NO , O NO Nitrous dioxide 2 2 3 2 Hydrocarbons, CO, particulates and O3 Ozone lead compounds CO Carbon monoxide

CO2 Carbon dioxide Motor Exhaust pollutants including CO NH3 HCN Hydrogen cyanide Indoor Sources Pollutant Types* Building Construction Materials Soil Radon death at high levels. Ensuring an Particle board Formaldehyde outside air supply for any combustion Insulation Formaldehyde, fiberglass appliance and regular air exchanges Fire retardant Asbestos, volatile organic compounds (VOC) Adhesives Organics alleviate the problems. Paint Mercury, organics Radon enters a structure through Building Contents access holes for piping, cracks Heating and cooking CO, SO2, NO, NO2, particulates and other openings to the soil and Furnishings Organics, odors results from the decay of naturally Water service Radon occurring radioactive materials in Human Occupants the soil. Radon has the potential Metabolic activity CO2, NH3, organics, odors, viruses to cause lung cancer at high levels. Human Activities Ventilating crawl spaces and base- Tobacco smoke CO, NO2, HCN, organics, odors ments with fresh air may reduce the Aerosol devices Fluorocarbons, vinyl chloride Cleaning and cooking products Hydrocarbons, odors, NH problem, but the preferred method 3 Hobbies and crafts Organics is to vent the gravel layer below the basement floor (Figure 4). A radon test should be conducted to determine the radon level.

Other household airborne hazards are a result of construction materials and cleaners. Formaldehyde, a common industrial chemical, is present in many building materials and household furnishings. The formaldehyde gas can leave materials and enter the environment throughout the lifetime of the material, but most of the gas leaves within the first year. Formaldehyde causes irritation in mucous mem- branes in the nose, throat and eyes. It needs to be vented to the outside. Formaldehyde use is restricted in construction materials today.

Particulates include larger airborne items such as the mold spores and Figure 4. Radon venting. tobacco smoke mentioned earlier. (www.epa.gov)

4 • AE1393 Air-to-Air Heat Exchangers It also includes viral and bacterial organisms, pet dander, dust and many other things. Due to a large variety of items, physical ailments vary from colds to allergies to lung disease. Some particulates may be filtered out, but others can be vented only to the outside. Air-to-Air Exchanger Operation and Construction One way to minimize air quality and moisture problems in a home, without opening a window, is by the installa- tion of a mechanical ventilation sys- Figure 5. Typical features of an air-to-air heat exchanger. tem using an air-to-air heat exchanger. An air-to-air heat exchanger brings two air streams of different tempera- The majority of air-to-air exchangers The general design of an air-to-air tures into thermal contact, transfer- installed in northern climates are heat exchanger uses a series of plates, ring heat from the exhausting inside heat recovery ventilators (HRVs). called a core, stacked vertically or air to incoming outside air during the These units recover heat from horizontally. An ideal plate has high heating season. A representative heat exhausted air and return it to , high resistance exchanger is shown in Figure 5. the building. Recent advances in to corrosion, an ability to absorb In summer, the heat exchanger can technology have increased the use noises, low cost and low weight. cool and, in some cases, dehumidify of energy recovery ventilators (ERVs) Common plate materials include the hot outside air passing through as well. In the past, ERVs mainly aluminum, different types of plastic it and into the house for ventilation. were used in climates with higher sheets and advanced composites. The air-to-air heat exchanger removes humidity that have a heavier cooling Originally, heat exchangers used the excess humidity and flushes than heating load. aluminum plates. Problems out odors and pollutants generated The main difference between the occurred with corrosion in the indoors. two is that HRVs only recover heat, damp environment, created by Heat exchangers generally are while an ERV will recover heat and condensation, and poor sound classified by the way the air moves humidity. ERVs have had problems characteristics. solved the through the unit. In a counter-flow with lower efficiencies due to corrosion and some sound problems, exchanger, hot and cold air streams oversaturation of internal desiccant but the conductivity did not equal flow parallel in opposite directions. wheels during longer periods of high that of the aluminum and the cost In a cross-flow unit, the air streams humidity, but with proper installation was higher. Current high-technology flow perpendicular to each other. and maintenance, they can create a heat exchangers use composite An axial flow unit uses a large wheel. healthier living space and greater materials meeting all the criteria. The air warms one side of the wheel, energy savings. In addition, the In addition to the core, the unit con- which transfers heat to the cold air majority of ERVs being sold today sists of an insulated container, defrost stream as it slowly turns. A heat are plate-type ERVs that do not controls to prevent moisture freezing unit uses to transfer contain a desiccant wheel. Consult on the core and fans to move the air. the heat. Other units are available with a heating/cooling contractor to All heat exchangers need insulation for specialized applications. determine if an HRV or ERV would be to increase efficiency and reduce Small structures, such as houses, most beneficial in your circumstance. condensation formation on the generally use counter-flow or outside of the unit. Different types cross-flow exchangers.

5 • AE1393 Air-to-Air Heat Exchangers of defrost mechanisms with sensors within the unit are available to control the defrost process. Fans move air to provide the necessary airflow and ventilation rate.

Counter-flow heat exchangers con- sist of a core of flat plates. As Figure 6 shows, air enters either end of the exchanger. Heat transfers through the plates to the cooler air. The longer the air runs in the unit, the greater the heat exchange. The percentage of heat recovery is the efficiency of the unit. Efficiencies usually range around 80 percent. Generally, these units are long, shallow and rectangular, with ducts at either of the long ends.

Cross-flow heat exchangers also use flat plates, but the air flows at right angles (Figure 7). The units have a Figure 6. Counter-heat exchanger: The airstreams flow in opposite directions. smaller footprint and may even fit in a window, but lose some of the counter-flow efficiency. Efficiencies typically do not exceed 75 percent. These units are often cube-shaped with all connections on one face of the cube. The vast majority of heat exchangers used in residential applications use the cross-flow design.

Choose the model that would best fit your particular needs. Characteristics such as space available for installation, exchange rate needed and the desired efficiency should be considered. Unfortunately, nearly every manufacturer has differ- ent ways of reporting these numbers. For example, ventilation rates depend on the resistance to airflow. A with an airflow rate of 150 cubic feet per minute (cfm) actually may produce this flow only at very low . Likewise, a unit may have a stated efficiency of 85 percent, Figure 7. Cross-flow heat exchanger: The airstreams flow at right angles to but may not be better than a unit with each other. an 80 percent efficiency, depending (RenewAire Ventilation) on the test temperature.

To standardize manufactures’ efficiency claims, the Home Ventilating Institute (HVI) tests

6 • AE1393 Air-to-Air Heat Exchangers air-to-air heat exchangers and other at specific airflow rates (cfm) and efficiency, but may be beneficial ventilation equipment. The tests are temperatures. These numbers can be for ease of operation. used to generate an air-to-air heat compared from one unit to the other Installation costs can be $500 and up, exchanger specification sheet. to enable proper comparisons at depending on the home size and the This sheet, shown in Figure 8, similar airflow rates. system’s requirements. Installation normalizes exchangers to a given can range from splicing into an set of pressures and temperatures, original system to fully ducting the enabling efficiencies and airflow Cost structure. A structure already using rates to be compared across models. An inexpensive heat exchanger ducts for heating and/or cooling The ventilation performance may cost as little as $500 to purchase. most likely already has the ducting numbers relate the airflow rates to A top-of-the-line model may cost to ensure all the air runs through a given , while the energy more than $2,000. While some of the the exchanger. Simply attaching the performance relates a set of given more expensive heat exchangers have system to a supply end may be all outdoor temperatures to different better efficiency, this is not always that is required. types of efficiencies. the case. Much of the increased cost occurs from consumer features such Many homes have electric baseboard The most important efficiency is the as easily cleaned cores, advanced or hot . Adding an air- sensible recovery efficiency since most defrost controls and sensors to turn to-air heat exchanger with these types heat exchange occurs during this type the unit on and off. These features of heating systems requires some of process. The sensible recovery generally do not affect the overall thought. The most common mistake efficiency provides unit efficiencies

Figure 8. Heat recovery design specification sheet. (Home Ventilating Institute)

7 • AE1393 Air-to-Air Heat Exchangers with do-it-yourself installations is without exchanging air in another system drawing stale air from failing to vent the entire home portion of the home. Figure 10 shows the kitchen, bathroom and utility properly (Figure 9). The problem can a more complete ventilation system room and distributing warmed out- be seen in the upper left of Figure 9. that serves the entire living space. side air to the bedrooms and The airflow from the supply to the living rooms. Figure 12 shows a Air-to-air heat exchangers also may return duct never enters the majority unit installed in the basement, be installed in a number of different of three rooms. Fresh air constantly again connected to a duct system. locations. Figure 11 shows an attic circulates through part of the home, installation connecting to an extensive recycling that portion of the home

Figure 9. Figure 10. A simple air-to-air heat exchange duct system will not Multiple supply and exhaust vents provide complete vent the entire structure properly. ventilation for the entire structure.

Figure 11. Attic installation of air exchanger. Figure 12. Basement installation of air exchanger. (NDSU Extension) (NDSU Extension)

8 • AE1393 Air-to-Air Heat Exchangers Inspect the condensation pan and air needed to keep the building Heat Exchanger drain tubing. To be certain nothing occupants healthy. Dangerous levels Maintenance is blocking the tubing, pour some of pollutants, such as chemicals, To ensure the HRV is working water into the pan near the drain. particulates, radon and even excess properly, regular maintenance needs If the water does not drain, the water vapor that could cause to be performed. The maintenance tubing will need to be cleaned. structural damage and health schedule will depend on the particu- problems, are removed. Different At least once a year clean the lar unit installed; refer to the owners types of heat exchangers exist to heat exchanger core. Make sure manual for specific instructions. meet the many conditions needed to follow the instructions in the by homeowners, whether imposed owners manual on the proper Make sure the power to the unit is off by installation, environmental or cleaning and maintenance of the before performing any maintenance. energy considerations. Begin with the filters. Clean or change core. Again, ensure the power is off the filters every one to three months, before performing any maintenance. With the tighter homes built today, depending on the manufacturer’s In addition to the core, the fans excess humidity leading to window recommendations. Washable filters should be cleaned at least once a condensation and other moisture should be cleaned by following the year. Wipe the blades clean and problems are likely without a manufacturer’s recommendations. oil the motor only if recommended heat exchanger. The heat exchangers by the manufacturer. provide a direct, quick return on the When changing the filters, vacuum investment and the peace of mind An air-to-air heat exchanger recycles the area surrounding the filters. that fresh air is available to breathe the heat from vented indoor air to After cleaning the filters, check the at all times. outside air intakes to ensure nothing heat the incoming fresh outside is blocking the screens and hoods.

Figure 13-A. Typical installation of a heat exchanger. Figure 13-B. Filters in a heat exchanger. (Photo courtesy of Shirley Neimayer, University of Nebraska - Lincoln). (Photos courtesy of Shirley Neimayer, University of Nebraska - Lincoln).

9 • AE1393 Air-to-Air Heat Exchangers ■ For a private home, the number of bedrooms determines the typical Cost Effectiveness number of occupants. of Heat Exchangers In the example, a three-bedroom home has an occupant level of four, or the A simple payback method, where the number of bedrooms plus one. To determine the ventilation airflow rate, energy savings pay for the purchase the following formula is used: and installation in a calculated time Recommended ventilation rate = (0.01 x floor area, square feet) + frame, shows the cost-effectiveness 7.5 (number of bedrooms + 1) of adding a system. Ventilation rate of example = (0.01 x 1,500 sq. ft.) + 7.5 (3 bedrooms + 1) = As a guide, the following set of 45 cubic feet per minute equations shows the cost-effectiveness The ventilation airflow rate often is expressed as cubic feet per minute or cfm. of an air-to-air heat exchanger installed in a home with low ■ The recommended ventilation rate is 45 cfm for this example home. infiltration levels in Fargo, N.D. For the sample calculation, the Using a heat exchanger to warm this air to the indoor temperature recovers heating costs associated with warming the cold air to . following conditions exist: The exact amount of energy depends, of course, on the difference in • Floor area: 1,500 square feet (ft2) temperature between outside and inside air. • Number of bedrooms: 3 • Infiltration rate: 0.1 air exchanges ■ A measure of this is a heating degree day (HDD). per hour (ACH) or 10 hours for a Commonly, an HDD is calculated by taking the mean difference between complete air exchange 65 °F and the average daily temperature. The various weather agencies • Fuel oil cost per gallon $3.80 around the state have tables of normal HDDs for a given area. For this • Electricity cost per kilowatt-hour example, Fargo, N.D., with an HDD of 9,000 is used. (kwh): $0.10 The equations for determining the amount of energy saved (Btu) in a year use the cfm, HDD, the efficiency rating of the heat exchanger (EF) Standard recommended ventilation and a constant for the specific heat and specific weight of air (25.92). rates have been set by the American The formula is as follows: Society of Heating, Refrigerating Heat saved each year (Btu) = cfm x HDD x EF x 25.92 and (ASHRAE Standard 62.2-2007). Btu – British thermal units These standards do not take into Cfm – ventilation airflow rate in cubic feet per minute account special circumstances such HDD – heating degree day as specific sensitivities or hobbies that create air quality issues. Standards EF – heat exchanger efficiency vary according to the building, 25.92 – constant for specific heat and weight of air its use and the number of occupants Using 45 cfm and 9,000 HDD, the heat energy saved by a 70 percent efficient heat (ASHRAE Standard 62.2-2007). exchanger would be: The benefits include moisture Heat energy saved = 45 x 9,000 x 0.70 x 25.92 removal, decreasing the potential Heat energy saved = 7,348,320 Btu per year of structural damage, elimination of harmful pollutants and reduced As mentioned earlier, the exchanger needs a defrost control to keep ice energy costs. Any system installed from forming. Defrosting is generally done using an electric resistant heater. also will increase the resale value This electric cost needs to be subtracted from the energy savings cost. of a building. The cost can be determined using the following formula: Cost of defrosting = power consumed by defrost device x hours of operation x cost of electricity Assuming a 70-watt (W) heater, 500 hours of operation per year at temperatures below freezing and $.10 per kwh, the electric cost to operate the defroster, after converting watts to kilowatts (kW), is: Cost = 70W x 500 hours per year x 1kW/1,000W x $0.10/kwh = $3.50 per year

10 • AE1393 Air-to-Air Heat Exchangers To analyze the fuel Determining Fuel Savings and Payback Period To determine the money saved, the total heat energy divided by the Btu savings, the energy content and the efficiency of the gives the gallons of fuel saved.

Heat Energy Saved per year content of the fuel Gallons per year = —————————————————— BTU content/gallon x efficiency of fuel and the efficiency of Using fuel oil as an example, it has 140,000 Btu per gallon and the typical the appliances using fuel oil furnace has an efficiency of 65 percent (.65). Using these numbers, we can determine the fuel saved each year.

the fuel need to 7,348,320 BTU per year Gallons per year = —————————————— = 81 gallons per year be known. 140,000 BTU per gallon x .65

Multiplying this by the cost of fuel oil and subtracting the defrost heat gives the total savings.

Cost savings = (gallons x cost per gallon) – cost to defrost

Using the values calculated previously and a cost of $3.80 per gallon of fuel oil, the savings each year would be:

Cost savings = (81 gallons per year x $3.80 per gallon) – $3.50 cost of defrosting = $304.30 per year

Given purchase, installation and miscellaneous costs of $1,000, a simple payback method shows the number of years an air-to-air heat exchanger pays for itself in saved heating costs.

Total cost to purchase and install unit Years to payback exchanger = ————————————————— Cost savings per year

$1,000 purchase and installation cost Years to payback exchanger = ————————————————— = 3.3 years $304.30 savings per year

Table 2 (at right) lists Table 2. Energy content for common household heating fuels and accepted efficiencies.

the energy content, Standard Fuel Type Btu Units Sold Efficiencies

in British thermal (%) Fuel oil 140,000 gallon 50% to 80% units (Btu) of several Electric resistance 3,413 Kilowatt-hour 100% Natural gas 100,000 CCF 75% to 96% common heating fuels. LP gas 95,000 gallon 75% to 97%

11 • AE1393 Air-to-Air Heat Exchangers For more information about energy from the NDSU Extension Service: www.ndsu.edu/energy

Reviewers

Laney’s Inc., Fargo, N.D. Home Heating, Fargo, N.D. RenewAire LLC, Madison, Wis. One Hour Heating & Air Conditioning, Fargo, N.D.

Cover photos courtesy of the U.S. Environmental Protection Agency ENERGY STAR Program and RenewAire Ventilation of Madison, Wisc.

Disclaimer The report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assume any legal liability or responsibility for the accuracy, com- pleteness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This publication was authored by Kenneth Hellevang, Extension Engineer and Carl Pedersen, former Energy Educator

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