ESL-HH-02-05-44
Economizer Applications in Dual-Duct Air-Handling Units
Ik-Seong Joo Mingsheng Liu, Ph.D., P.E. Graduate Student Associate Professor Energy Systems Laboratory Energy Systems Laboratory University of Nebraska-Lincoln University of Nebraska-Lincoln
ABSTRACT This paper provides analytical tools and Single-Fan, Dual-Duct (SFDD) System engineering methods to evaluate the feasibility of the economizer for dual-duct air-handling units. The The SFDD system (see Figure 1) includes an results show that the economizer decreases cooling economizer, supply and return air fans, a pre-heating energy consumption without heating energy penalties coil, and cooling and heating coils. The terminal for dual-fan, dual-duct air-handling units. The boxes modulate the cold airflow, the hot airflow, or economizer has significant heating energy penalties both to maintain the room air temperature. for single-fan, dual-duct air-handling units. The penalties are higher than the cooling energy savings Exhaust Air Louver when the cold airflow is less than the hot airflow. Exhaust Air Detailed engineering analyses are required to Damper Return Air Fan evaluate the feasibility of the economizer for single- fan, dual-duct systems. Return Air Damper Heating Coil INTRODUCTION The economizer is widely acknowledged as one Cooling Coil Outdoor Air of the popular energy conservation measures. It Supply Air Fan Damper Pre-heat Coil eliminates or reduces mechanical cooling by using Filters free cooling. For a single-duct system, the economizer reduces mechanical cooling with no Terminal Box heating penalties. For a dual-duct system, however, Cold Air Hot Air heating energy penalties may exist. Liu et al. [1997] Suppy Duct Suppy Duct pointed out that the heating energy penalty is often TTZone 1 Zone 2 higher than the cooling energy savings for single-fan, Figure 1. Schematic diagram of the SFDD system dual-duct (SFDD) air-handling units, and they developed an advanced economizer algorithm to The economizer modulates exhaust air, return air solve the economizer operational problems. and outside air dampers accordingly to maintain the However, there are no general guidelines or mixed air temperature at its setpoint. In practice, the recommendations for the economizer design for dual- mixed air temperature setpoint is slightly lower than duct systems. This paper presents economizer the cold air temperature setpoint to avoid the chilled models, performs the energy performance analyses, water valve hunting or frequently opening and and develops the design recommendations. closing. The action of the exhaust and outside air dampers opposes the return air damper. When the outside air damper is fully open, the return air SYSTEM MODELS damper is closed. When the outside air damper is in The dual-duct system supplies both hot and cold the minimum open position, the return air damper is air to each zone, where a terminal box modulates the totally open. total airflow rate and/or the mixing ratio of hot and cold air to maintain the room temperature. The dual The economizer can be activated using either duct-systems are defined as single-fan, dual-duct outside air temperature (temperature economizer) or (SFDD) systems, where a single fan is used to push outside air enthalpy (enthalpy economizer). The air through both hot and cold ducts, and dual-fan, temperature economizer is activated when the outside dual-duct (DFDD) systems, where two fans are used air temperature is within a predefined range. The to push air through cold and hot ducts, respectively.
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
enthalpy economizer is activated when the outside air air duct. When the cold airflow rate is smaller than enthalpy is smaller than the return air enthalpy. Both the outside air intake rate, however, a portion of the the temperature and enthalpy economizers use the outside air is supplied to the hot air duct. Figure 4 same control sequence after activation. Figure 2 and and Equation (3) present the economizer schedules. Equation (1) present the economizer schedules.
Exhaust Air Louver Exhaust Air Damper b Return Air Fan 1 Return Air Tr Tc,d Damper VSD Heating Coil roaTT
VSD Filters Cooling Coil Controller min
Outdoor Air Pre-heat Coil Toa Damper PP T T Te,max e,min c,d Figure 2. Economizer schedules for the SFDD system Terminal Box Cold Air Hot Air Suppy Duct Suppy Duct $ O TT TTZone 1 Zone 2 ' min eoa max, 1 ? TTT Figure 3. Schematic diagram of the DFDD system ' , eoadc max, % TT ,dcr (1) b ? TTT ' min,, ,dcoabe ' TT oar o &' min TT min,, beoa 1
o(Tr Tc,d) When the outside air temperature (T ) is higher o oa Tr Toa than the cold air temperature setpoint (T ,dc ), the economizer uses 100% outside air. When the outside min air temperature is lower than the cold air temperature Toa setpoint, the economizer maintains the mixed air Te,min Tc,d Te,max temperature at the cold air temperature setpoint. When the economizer is off, the system receives the Figure 4. Economizer schedules for the DFDD minimum outside intake. system
The minimum economizer temperature,T min,, be , $ O TT ' min eoa max, varies depending on the minimum outside air intake ? TTT ' o , eoadc max, ratio, oa min,min mm d )/( . % TT ,dcro )( (3) o ? TTT ' min,, ,dcoaoe ' TT oar 1 ' TT TT )( (2) & min TT min,, oeoa min,, rbe , rdc min When the outside air temperature is between the The heating and cooling energy consumption maximum economizer temperature (Te max, ) and the depends on economizer cycles, entering air conditions, setpoints of leaving air conditions, and cold air temperature setpoint (T ,dc ), the outside air is cold and hot airflow rates. When these parameters directly supplied into the cold air duct. The cold are given, the heating and cooling energy airflow rate equals the outside airflow rate, and all consumptions can be calculated using energy balance hot air is from the return air. principles [Joo and Liu 2002, under review]. When the outside air temperature is between the cold air temperature setpoint and the minimum Dual-Fan, Dual-Duct (DFDD) System economizer temperature (T min,, oe ), the outside The DFDD system (see Figure 3) has two supply airflow is modulated to maintain the cold deck mixed fans. Outside air is directly introduced into the cold air temperature at the cold deck setpoint. The hot
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
deck air is from the return air unless the cold airflow protected if the return and outside air is well mixed ratio ( o ) is smaller than the minimum outside air during the economizer cycle. Therefore, the intake ratio. minimum economizer temperature is selected to be 20°F (-6.7°C). When the outside air temperature is higher than the maximum economizer temperature or lower than The maximum economizer temperature depends the minimum economizer temperature, the primarily on the outside air moisture contents. For a economizer is disabled. When the cold airflow ratio dry climate, such as New Mexico, the maximum is smaller than the minimum outside air intake ratio, economizer temperature can be as high as the return outside air is allowed into the hot deck in any air temperature. For a humid climate, such as schedule. Galveston, TX, the maximum economizer temperature should be limited to 62°F (16.7°C) or is an outside air intake ratio (a ratio of the lower. To consider general conditions, the maximum o economizer temperature is selected to be 65°F outside airflow to the total airflow), which may differ (18.3°C). from the outside air intake ratio of the SFDD system ( b ) because of the dual-fan system’s characteristic The minimum outside air intake depends on the that the outside air is directly supplied to the cold building functions. It often varies from 10% (office deck. buildings) to 30% (hospital buildings). At a low outside air temperature, an office building may The minimum economizer temperature of the require 30% outside air intake due to a reduced total DFDD system, T min,, oe , varies depending on min airflow rate. Therefore, the simulation is conducted and . using minimum outside air intakes of 10%, 20% and o 30%, respectively.
TT o TT )( (4) The partial building load can be expressed using e min,, ro , rdc min the cold airflow ratio and the supply air temperature. When the cold airflow ratio is 1, the building is in The energy consumptions can be calculated full cooling. When the cold airflow ratio is 0, the using general energy balance principles. Joo and Liu building is in full heating. provided the detailed models [Joo and Liu 2002, under review]. 0 5 10 15 20 25 30 oC oF 110 oC
hot deck temperature 40 ANALYSIS 100 cold deck temperature room temprature C) Economizer performance depends on the o 35
F or 90 following parameters: minimum and maximum o 30 economizer temperatures, minimum outside air 80 intake ratios, cold airflow ratios, room conditions, 25
and deck setpoints. The parameter ranges are 70 selected carefully so that simulation results can be 20 Set-up temperature ( temperature Set-up directly used and serve as a guideline for engineers. 60 15
50 10 The temperature economizer is used in this study 30 40 50 60 70 80 90 o due to the following reasons: (1) it is more popular Outside air temperature (oF or oC) F than the enthalpy economizer, and (2) both Figure 5. Room and deck operating schedules economizers have the same performance after activation. Figure 5 presents the room and deck temperature reset schedules. The room conditions are 75oF The economizer is activated when the outside air (23.9oC) and 50% relative humidity. The cooling coil temperature is between the minimum economizer discharge temperature is 55oF (12.8oC). The hot deck temperature (20°F or -6.7°C) and the maximum temperature is 100°F (37.8°C) when the outside air economizer temperature (65°F or 18.3°C). Most temperature is 40°F (4.4°C) or lower, and the hot operating staffs turn off economizers when the deck temperature is 80°F (26.7°C) when the outside outside air temperature is 32°F (0°C) or lower to air temperature is 80°F (26.7°C) or higher. The hot avoid potential coil freezing. However, coils can be deck temperature linearly increases as the outside air
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
temperature decreases when the outside air temperature is between 40°F (4.4°C) and 80°F The lower the minimum outside intake ratio, the (26.7°C). higher the energy savings are for the same ambient and load conditions. The temperature difference San Antonio Bin data [Degelman 1984] were between the mixed air temperature without the used for the simulation. The humidity ratio has a economizer and with the economizer is larger for the very limited impact during the economizer cycle. lower minimum outside intake ratio. The results can be used for most climates. When the outside air temperature is higher than The energy performance is evaluated using the cold deck setpoint 55oF (12.8oC), the savings potential energy savings per unit total airflow rate. decrease as the outside air temperature increases. The potential energy savings is also expressed using The reason is that the temperature difference between ratios of savings over 6 Btu/lbm (14 kJ/kg) which is the mixed air temperature without the economizer required energy to cool one pound of air from 75oF and with the economizer decreases as the ambient (23.9oC) and 50% relative humidity (the room design temperature increases. conditions) to 55oF (12.8oC) and 90% relative humidity. If the savings is shown as 0.2 or 20%, for Figure 7 presents contour lines of heating energy example, the real savings will be 1.2 Btu per pound savings of the economizer. The economizer of air supplied to the system (or 2.8 kJ per 1 kilogram increases the heating energy consumption. The more of air). the hot air is supplied, the larger the penalties are for the same ambient temperature. For example, when the outside air temperature is 35oF (1.7oC), the mixed Economizer and the SFDD System air temperature is 67oF (19.4oC) without the Figure 6 presents contour lines of cooling energy economizer. When the economizer is used, it is 55oF savings of the economizer for the SFDD system. The (12.8oC). Therefore, the heating coil has to warm up chart shows three different outside air intake ratios. air from 55oF (12.8oC) to the hot deck setpoint due to The abscissa represents the outside air temperature the economizer operation. (Toa). The ordinate represents the cold airflow ratio (b =0.1) (b =0.2) (b =0.3) ( b ). The different cold airflow ratios represent min min min -5 0 5 10 15 20 -5 0 5 10 15 20 -5 0 5 10 15 20 oC 1.0 different load conditions. For example, b =0 is -0.05 0.9 5 - - .0 0. -0 - 0 0
0.00 0.00 0 0.00 . 5 . 0 -0.10 0 100% heating and =1 is 100% cooling. 5 5 b .10 0.8 -0 0 .05 0.1 -0.15 -0 - 0.7 .15 (b =0.1) (b =0.2) (b =0.3) -0.20 -0 - min min min - 0 0
. . 1 1 - - - 0 0 0 0 o 5 0 -5 0 5 10 15 20 -5 0 5 10 15 20 -5 0 5 10 15 20 5 . . 0.6 2 . . 1 C 0 1 1 - - 1 - . 0 0 - 5 0 5 5 1.0 2 0 0 . . - . - 2 0 . 2 - 0 0 1 0 5 0 0 5 . 5 - 5 0 . 8 5 5 0 6 1 . . 0 2 0 - 1 5 1 0 4 2 . . - 0 1 2 . . . 0 5 2 0 0 . . . 5 0 0 2 . 0 0 0 0.00 0.00 0 0.00 0 - 6 0 3 . - . - 0 . 4 0 . 0 0 5 0 0 0 0.5 - 0 0 -0.05 5 1 5 - 1 0 2 . . 0 . . . 3 5 0 0 7 . . . 5 0 0 0 0 4 . 0 5 3 0 5 5 0 5 0 0 0 2 3 0 5 3 0.9 5 0 0 4 . . 4
0 -0.05 . 0 4 - 0 0 . 0 0 0 . 0 0 6 0 0 . . 0 0 5 1 . 0 6 0 . . . . 0 . 3 . . 0 - 0 3 5 0 5 3 0 5 0 0 0 0 0 0 0 . 0 . . 0 0 2 7 0 5 . 0 6 0 . 5 0 2 3 0 0 . . 0 2 5 .
3
. (cold airflow ratio) 0 5 0 0 6 2 . - . . 0 . 3 0 0 0 . . 4 4 0 3 - 5 5
0 0 0 0 5 0.4 0 .
. g 0 0 5 5 0 - 0 5 5 2 . 4 2 . 0 . 0 0 0 5 . 3 0.10 5 0 0.8 . . - . 4 3 2 3 . 0 5 0 . . 0 5 0
0 0. 0 55 0 5 - 0.3 4 3 . 0 . -0 5 . 0.7 0 - 3 1 - - - - 0 . 0
- 0 0 5
-0.10 0 . -
-0.10 0 0 0 . . - - . 0 1 - 0 . . 1 0 4 4 1 5 . 0 - . . - 5 - 0 3 4 5 4 5 . 5 0 0 0 5 . 0 - 2 0 0 0 - 2 2 0 0 0 . . 0 0 . 5 . 5 4 2 - 4 0 0 0.2 1 . - 5 5 . . . 0 - . 0 0 0 5 5 2 3 - 5 0 0 2 . 0 5 5 - 0 4 1 5 3 . . - 4 1 5 . -0 . 5 . 5 0.6 - . 5 0 . . 0 1 2 - 2 0 - 3 0 . . - 0 0 0 0 0 5 - 0
0 0.00 0.00 0.00 0 0 . . 0 0 0 4 0 - 5
0 0 - . - 0 2 0 3 2 0 -0.05 4 . . 1 1 0 3 . . 0 . 5 . 4 . . . . 0 5 4 0 3 3 5 0 6 - 0 0 0 5 . 0 . 0 5 0 5 5 5 0 0 0 4 0 5 0 - 0 -0.05 3 3 . 0 . - 3 5 - . -0.05
. 0 1 0 0 0 0 0 5 . 0 5 0 0 0 ...... 0.1 0 2 5 . 0 - 5 0 5 5 3 .3 2 - 0 0 0 0 0 0 0 5 - . . 0 0 5 0 0.5 3 5 6 . 0 . 5 2 0 0 0 2 . - 0 . 3 5 1 0 0 . 0 . 0 0 2 2 0 . . 0 . 1 o 2 0 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 0 F 0 . 0 (cold airflow ratio) .3 0 0.4 0 0 g . 20 T T T 0. oa oa oa 15 0.25 0.3 0 Figure 7. Heating energy penalties of the economizer 0 .15 .20 5 5 1 1 . 0 . 0 . 0 0 0.15 0 10 1 1 0.2 . . 0 0 5 0 0 0 5 0 .0 0.10 in the SFDD system 0 0 0 0 0 5 . . . . . 5
0 0 0 0 0 0 0 .10 .
0 0.1 0.05 0.05 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 oF The lower the minimum outside intake ratio, the T T T oa oa oa higher the penalties are for the same ambient and load conditions. When the minimum outside air ratio Figure 6. Cooling energy savings of the economizer is lower, the mixed air temperature is higher without in the SFDD system the economizer. The mixed air temperature under the economizer operation maintains the same regardless The economizer reduces the cooling energy of different minimum outside air intake ratios. consumption. The more the cold air is supplied, the larger the savings are for the same ambient When the outside air temperature is higher than temperature. The mixed air temperature of the the cold deck setpoint 55oF (12.8oC), the penalties economizer applied is lower than that without the decrease as the outside air temperature increases. economizer. The temperature difference between the mixed air
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
temperature without the economizer and with the economizer is less effective when the minimum economizer decreases as the outside air temperature outside air intake ratio is high. increases. (b =0.1) (b =0.2) (b =0.3) min min min -5 0 5 10 15 20 -5 0 5 10 15 20 -5 0 5 10 15 20 oC 1.0 0. 0 5 8 . 0.6 0 0 0 0 5
Figure 8 shows the total thermal energy savings 0 5 0 0 5 . 0 0 1 6 . . 5 5 1 0 5 5 . 0 4 2 3 0 0 . . . . 0 2 0 . 0 . 5 5 0 . . 0 0 . 0 0 7 0 0 5 . 3 0 0 5 3 5 0.10 3 0 5 0.6 0 5 . 3 . 4 0 . 0 5 0 6 3 5 . 0 5 . . 5 0 0.9 . 0 0 . 0 6 0 3 . 45 . 0 0 . 2 2 0 0 7 4 0 5 0 0 6 0 . . 0 . 0 0 of the economizer calculated as the sum of the 0 . . 0. 1 2 5 2 5 0 0 5 4 0 . 0
. . . 0 5 0 0 3 0 0 0 5 2 . 0 0 0 0 0 0 . .
0 0 0 3 0 2 0 . .4 0 3 . . 0 0 0 . .55 0 5 0 . 5 0 5 0
0 . . 5 0 4 1 1 5 5 0 . 1 . 0 1
. 5 0.8 4 . 0 0 .
0
heating energy penalties and the cooling energy 0 0 0 0 0 .5 .2 . 0 5 0 5 0.4 0 0 savings. When the hot airflow rate is larger than the 0.7 . 0 45 0 . 0 .0 0. 2 .3 5 35 0 5 0.25 0 .1 0 0
0 0.00 0 0 . 5 0 0 cold airflow rate, the savings are negative. When the 1 0 0.6 0 1 0 4 0 .30 . 0 0 0 . . . 5 0 1 0 . . 3 . . 2 0 . 0 1 0 . 0 0 . 5 0 0 5 1 0 0 5 3 0 5 0 0 . 0 0 0 .25 . . 0 3 0 0 0 0 . . 0 0 5 .0 0 .1 0 0 . 2 5 5 0 cold airflow rate is larger than the hot airflow rate, 2 5 0
. 5 .
5 0 0 0.5 0 2
0 0 0 0 . 0 2 0 .
. . . . 5 0 0 5 0
0 0 5 1 0 0 1 . 0 . . .2 0 0 0 0 0 . 5 0 0 0 the savings are positive. The smaller minimum ratio) airflow (cold 5 0 0
0.4 . . g
0 0 0 . 0 0. 0 1 0 0 .1 0 .1 0 5 5 . 0 outside intake ratio shows higher savings when the 0
0
0. 0 0.3 05 . 0.00 0 0.0
0 5
0. 0 05 0 .
cold airflow rate is larger than the hot airflow rate, 0.10 0 0
. 0.2 0.00 0 0.00 0.05 and shows higher penalties when the hot airflow rate 0.00 is larger than the cold airflow rate for the same 0.1 0.00 o 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 F ambient and load conditions. T T T oa oa oa Figure 9. Cooling energy savings of the economizer (b min=0.1) (b min=0.2) (b min=0.3) -5 0 5 10 15 20 -5 0 5 10 15 20 -5 0 5 10 15 20 oC in the DFDD system 1.0 0. 0.8 0.65 65 0 0 0 5 0 5 0 5 0 .5 .7 .4 . 0 5 6 1 1 3 5 . 0 5 5 . . 0 . 2 3 0.6 5 0 0 . .6 0 6 4 5 0 0 . 0 70 . 0 . 0 0 0 5 0 .4 0 . 0 45 0.20 . 0 0.10 0 0.10 . 5 0.9 5 0.55 3 0 0 . 0 .5 0 5 0.00 4 0 0 0 3 . 0.05 4 0.05 0 .
0 0.10 . 0 . 4
5 4 0.15 0.05 0
0.00 . 0.00 . 0 0 3 0.50 0 0 5 . . 5 0 5 3 1 0 0 . For a typical office building, the outside air 0 . 3 0 5 2 . 5 3 5 0 5 3 . 5 0 . 2
0 2 . 0 2 0 0 . . 0 0 4 . . 0.8 5 3 0 2 2 0 0 0
. .
0 0 0 .40 intake ratio is from 0.1 to 0.2. The cooling energy 0.2 0. 5 0 35 .1 0.7 0 0 0 0.3 0. .2 .20 0 15 0 savings is up to 30% for min =0.1 and up to 20% 0.25 0.1 0.1 5 0.6 0.2 5 0 5 0 1 1 0 0 . . 0.15 . 0 05 0 1 0 1 . . 0 5 0 5 . 0.10 0 .05 0 1 0 0 0 5 for =0.2 when the outside air temperature is . . 0 0.00 0 min 0 .
0 0.00 0.00 0.5 .05 0.00 0.00 0 0.00 o o -0.05 -0.05 -0.05 below 60 F (15.6 C).
(cold airflow ratio) 0.00 0.00 0.4 10 g -0.10 -0. -0.15 5 10 -0.1 0. -0.20 - 0.3 - - 0 - 0 0 -0.25 .2 0 -0 - . . 0 . 1 1 0 - - . 5 5 0 0 0 0 0 5 The economizer has no heating energy savings or .3 1 -0 - - . - . 5 . 5 0 0 1 0 1 .2 0
-0.05 - - - . 0 . 0 . 0 0 0.00 - 0 0.00 0.00 0 1 2 5 1 3 .3 - . 0.2 0 - 0 - . . 0 5 0 0 0 5 2 - 2 - - . .2 - . 0 0 0 0 2 0 5 0 2 30 - 3 0 .4 - . . 5 . 0 5 5 . 0 - . penalties for the DFDD system. The return air is - 3 3 -0.4 0 0 .4 0 5 -0 2 - - 0 0 0 . 0 - . 2 5 3 5 3 . -0 0 . 5 4 5 0 . -0.5 4 5 0. - 40 0.1 0 .3 - .55 -0 50 -0 -0. -0 directly introduced to the hot deck. The heating coil -0.60 .45 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 oF inlet temperature is the same as the return air T T T oa oa oa Figure 8. Total energy savings of the economizer in temperature regardless of the economizer use when the SFDD system the cold airflow ratio is higher than or equal to the outside air intake ratio. When the cold airflow ratio When the outside air temperature is low, the cold is lower than the outside air intake ratio, a portion of airflow can be higher or lower than the hot airflow outside air is supplied to the hot deck. The rate depending on the building characteristics. If the economizer is disabled, and only the minimum cold airflow is higher than the hot airflow, the outside air is allowed to the system. economizer can be installed to decrease the overall thermal energy consumption. However, if the cold Economizer and Retrofit airflow is lower than the hot airflow, the economizer An economizer retrofit is not recommended for should not be installed. Detailed engineering the SFDD system in typical commercial buildings. analyses are required to justify the feasibility of the However, it may be cost effective for the DFDD economizer for each case. system. When the SFDD system is retrofitted to the DFDD system, the economizer should be retained. Economizer and the DFDD System Figure 9 presents contour lines of the cooling Figure 6 presents cooling energy savings of energy savings of the economizer for the DFDD adding an economizer when the SFDD system is system. The economizer saves cooling energy when converted to the DFDD system. The savings are the cold airflow is larger than the outside air intake. generally 5% to 40% depending on ambient and load Below the zigzag line, there are no cooling energy conditions. consumptions because the cold deck mixed air temperature is lower than the cold deck setpoint. The higher the cold airflow ratio, the larger the energy savings are for the same ambient temperature. The
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
(b min=0.1) (b min=0.2) (b min=0.3) (b min=0.1) (b min=0.2) (b min=0.3) o o -5 0 5 10 15 20 -5 0 5 10 15 20 -5 0 5 10 15 20 C -5 0 5 10 15 20 -5 0 5 10 15 20 -5 0 5 10 15 20 C 1.0 1.0
0.05 0.05 0.05 0.9 0.05 0.9 0.10 0 0 0 . .1 .1 0 0 0
5
0 0 0 .1 . 0 0 0.15 1 0.15 . 0.15 . 0 0.8 05 0.8 0
0 0 0.20 0.20 0.20 .0 5 0 5 . 1 II II II 0 . I I I 0.7 5 0 0.7 . 0 0.25 0.25 0.25 10
0
0 .
0 . 0.30 0 0.30 0 0.30 5 0 . 0 5 . 1 0.6 1 . 0.6 0 0 5 0 0 5 . 2 .
0 0.35 0 0.35 0
5 . 2 0 0 0 0 5 . .
1 . . 0 0.5 0 5 1 0.5 0.40 3
5 . 5 0 0 0 0 0 . . 5 . 3 1 0 1 5 0 . 0 0. 1 0 . 0.45 0 1 05
(cold airflow ratio) airflow (cold 0 ratio) airflow (cold . 0
0 0 0 0.4 1 0.4 0
g g 0 . 0 0 0 . . 1 0 .
. 1 1 . 3 4 . 5 4 0.50 2 2
0 0 0 0 . 5 0
0 0 0.35
0 . .
1 1
0
5 5
0 0.55 0.55 . . 2 1 0 0.3 0 0.3 0 0 5 0 5 . . 0 . 0 4 . 5
0 0 4 0 0.40 0.40 0 2 0 . . . 5 0
4 0 0 5 . . 6 5 . 0.25 0.25 . 0 1 0 0 1 0 0 . 0 5 0 3 0 5 . 0 . 1 4 . 0 0 0 0 0 0 2 5 5 5 . . 0 6 0 0 . 6 3 0 0 0.2 0.2 . 5 0
. 0 0 . . 5 1 0.45 0.45 0 . 2
0 5 5 0 5 0 .6 0 0 0 0 . . 0
. 5 0.30 0.30 5
. 0 0 5 .
0 0 3 .1 0 5 III
0 0 . 5 0 III 0 III
0 .
.
3 0.0 0 1 5 .7 5 0.50 0.50 IV 0.1 .05 0.1 0 0 IV 0.05 0 IV
30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 oF 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 oF
Toa Toa Toa Toa Toa Toa Figure 10. Heating energy savings of retrofitting the Figure 11. Heating energy savings of retrofitting the SFDD system to the DFDD system and adding an SFDD system with the economizer to the DFDD economizer system and keeping the economizer
Figure 10 presents contour lines of heating Region I: the heating energy savings only energy savings of adding an economizer when the depends on cold airflow ratio. The hot deck mixed SFDD system is converted to the DFDD system. The air temperature is constant at 55oF (12.8oC) for the savings are generally 5% up to 15%. At low ambient SFDD system and at 75oF (23.9oC) for the DFDD temperatures, savings are low for the high outside air system. The lower the cold airflow ratio, the higher intake ratio due to the pre-heating penalties. The the savings are. more outside air the system takes, the more pre- heating penalties it generates. Region II: the heating energy savings decrease when the outside air temperature is close to 65°F For example, when the outside air temperature is (18.3°C). The temperature difference between the 50oF (10.0oC), the cold airflow ratio is assumed to be hot deck air temperature and the mixed air 0.4. The minimum outside air intake ratio is 0.2. temperature decreases for the SFDD system, even Then, the cooling energy savings is 25% (Figure 6). though the temperature difference between the hot The heating energy savings is 12% (Figure 10). The deck air temperature and the hot deck inlet mixed air total thermal energy savings is 37% or 2.22 Btu/lbm. temperature remains constant for the DFDD system.
If the SFDD system has the economizer, no Region III: the savings depends on the outside cooling energy savings can be achieved when it is air temperature and the minimum outside intake ratio. converted to the DFDD system. Neither system As the outside air temperature decreases, the savings requires cooling energy. decrease. The lower the minimum outside air intake ratio, the higher the savings are for the same outside Figure 11 presents heating energy savings of air temperature. retrofitting the SFDD system to the DFDD system when the economizer remains. The savings range Region IV: the cold airflow rate is lower than the from the maximum of 50% to 70% depending on outside intake rate. A proportion of outside air is outside air intake ratios. The heating energy savings introduced into the hot deck. The heating energy are divided into four regions. The cold deck savings depend on the outside air temperature and the temperature setpoint line divides Region I and outside air intake ratio. The savings drastically Region II, and it also divides Region III and Region decrease when the outside air temperature is close to IV. The preheating energy penalty line divides 65°F (18.3°C). Region I and Region III (below this line, pre-heating penalties occur in the DFDD system because the Figures 6 through 11 present the potential energy mixed air temperature is lower than the cold deck savings under the pre-defined room temperature and temperature). The minimum outside intake line deck schedules. If actual room conditions or actual divides Region II and Region IV. cold and hot deck temperatures are different from the schedules, the potential energy savings can still be determined using Figures 6 through 11, provided that
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
the cold airflow ratio is corrected using the following savings for adding an economizer is 22% (See Figure equations. 9). The total cooling energy savings is 594,000 Btu/hr (174.1 kW) using Equation (8). i) Room temperature correction If the cold air temperature is reset to 60oF TT (15.6oC), the cold airflow ratio is 0.57 by Equation rr 21 (5) 21 TT (6). The cooling energy savings is 27%. The total hc cooling energy savings is 729,000 Btu/hr (213.6 kW). The savings for the room temperature changes or the ii) Cold deck temperature correction hot air temperature resets can be calculated as the same way using Equation (5) or (7). TT P 2 hc (6) 21 1 TT hc The annual potential energy savings can be calculated using Equation (9) based on hourly iii) Hot deck temperature correction savings and the number of operating hours. The cold airflow ratio has to be estimated depending on TTTT )( building characteristics. Based on the bin hh 212 hc 2 (7) 1 temperature and the estimated cold airflow ratio, the TT hc 1 percentage of energy savings can be determined using Figures 6 through 11. APPLICATION P E yr , NE iihr (9) The potential hourly energy savings can be i determined using Figures 6 through Figure 11. To determine the hourly energy savings, the total building airflow rate, the outside air intake rate, the CONCLUSION cold airflow rate, the outside air temperature, the Models for thermal energy consumptions are room air temperature, and the cold and hot air developed to investigate the economizer effects for temperature must be given. With the given outside the SFDD system and the DFDD system. A method air temperature, the outside air intake ratio and cold is developed to determine the potential energy airflow ratio are used to determine the percentage of savings. energy savings in the charts. The economizer reduces both the heating and Typically, room and deck temperature cooling energy consumption for the DFDD systems. corrections are required during the process. These The economizer may increase or decrease the overall corrections, using Equation (5) through Equation (7), thermal energy consumption depending on the lead a new cold airflow ratio that yields a new building characteristics for the SFDD systems. The savings percentage with which the actual hourly detailed engineering analyses are required to justify energy savings can be calculated. the installation of the economizer for each case. When the SFDD system is converted to the DFDD Hourly energy savings, Ehr [Btu/hr], can be system, the economizer should be retained. calculated with the total building airflow rate and the percentage of savings in the charts as the following NOMENCLATURE equation. c = Specific heat for dry air (Btu/(lbm oF ) or p J/(kg oF )) mE PP lbmBtu )(6 (8) hr E = Energy consumption (Btu/lbm or kJ/kg) h = Air enthalpy (Btu/lbm or J/kg) For example, a hypothetical commercial building 2 m = Airflow rate (lbm/hr or kg/s) has a DFDD system and the floor area of 100,000 ft E = Energy savings (Btu/hr or kW, Btu/yr or (9290 m2). The system operates with the total 3 3 kWh/yr) building airflow rate of 100,000 ft /min (47.2 m /s). T = Air temperature (oF or oC) The minimum outside intake ratio is 20%, and the = Outside air intake ratio ( m / m ) cold airflow ratio is 0.5. The outside air temperature oa d is 55oF (12.8oC). The cold deck temperature is set at = Cold airflow ratio ( mc / md ) 55oF (12.8oC), and the hot deck temperature is set at = Energy savings (non-dimensional) 92.5 oF (33.6oC). As a result, the total cooling energy
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
Subscripts p = Preheating r = Room air b = Base case system, single-fan, dual-duct
constant volume system c = Cooling, cold deck REFERENCE d = Designed Degelman, L., 1984. Bin Weather Data. American dew = Dew point Society of Heating, Refrigerating and Air- e = Economizer Conditioning Engineers, Inc. Atlanta GA f = Fan Liu, M., Claridge, D. E. and Park, B. Y., 1997. “An h = Heating, hot deck Advanced Economizer Controller for Dual-Duct Air- m = Mixed Handling Systems – with a Case Application”, max = Maximum ASHRAE Transaction, Vol. 103, Part 2. min = Minimum Joo, I. and Liu, M., 2001. “Performance Analysis of o = Optimized system, dual-fan, dual-duct Dual-Fan, Dual-Duct Constant Volume Air-Handling constant volume system Units”, ASHRAE under review. oa = Outside air
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
Homes produced with airtight duct systems (around 15% savings in Htg and Cooling Energy) Palm Harbor Homes 22,000 Southern Energy Homes 8,000 Cavalier Homes 1,000 = = = Subtotal 31,000
Technical measures incorporated in BAIHP homes include some or many of the following features - better insulated envelopes (including Structural Insulated Panels and Insulated Concrete Forms), unvented attics, “cool” roofs, advanced air distribution systems, interior duct systems, fan integrated positive pressure dehumidified air ventilation in hot humid climates, quiet exhaust fan ventilation in cool climates, solar water heaters, heat pump water heaters, high efficiency right sized Figure 1 OA Intake Duct in Back Porch heating/cooling equipment, and gas fired combo space/water heating systems.
HOMES BY THE FLORIDA HOME ENERGY AND RESOURCES ORGANIZATION (FL.H.E.R.O.) Over 400 single and multifamily homes have been constructed in the Gainesville, FL area with technical assistance from FL H.E.R.O. These homes were constructed by over a dozen different builders. In this paper data from 310 of these homes is presented. These homes have featured better envelopes and windows, interior and/or duct systems with adequate returns, fan integrated positive pressure dehumidified air ventilation, high efficiency right sized
heating/cooling equipment, and gas fired combo Figure 2 OA Intake Duct in Soffit space/water heating systems. The innovative outside air (OA) system is described below.
The OA duct is located in the back porch (Figure 1) or in the soffit (Figure 2). The OA is filtered through a 12"x12" filter (which is readily available) located in a grill (Figure 3) which is attached to the OA duct box. The flex OA duct size varies depending on the system size - 4" for up to 2.5 tons, 5" for 3 to 4 ton and 6" for a 5 ton system. The OA duct terminates in the return air plenum after a manually adjustable butterfly damper (Figure 4).
Figure 3 Filter Backed Grill Covering the OA Intake
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
Measured Home Energy Ratings (HERS) and airtightness on these FL. H.E.R.O. homes is presented next in figures 5 through 8. Data is presented for both single family detached (SF) and multifamily homes (MF). See Table 2 below.
Table 2. Summary statistics on FL.H.E.R.O. Homes n = sample size
SF MF Median cond area 1,909 970 % constructed with 2x4 frame 94% 100% or frame and block
Figure 4 Butterfly Damper for OA control Avg. Conditioned Area, ft2 1,993 1,184 (n=164) (n=146) The damper can be set during commissioning and Avg. HERS score 87.0 88.0 closed by the homeowner in case the OA quality is (n=164) (n=146) poor (e.g. forest fire). This system introduces filtered Avg. ACH50 4.5 5.2 and conditioned ventilation air only when the cooling (n=164) (n=146) or heating system is operational. The ventilation air Avg. Qtot (CFM25 as %of 6.9% 5.0% also positively pressurizes the house. Data on the floor area) (n=25) (n=72) amount of ventilation air or positive pressurization is Avg. Qout (CFM25 as %of 3.0% 1.4% not available from a large sample of homes. A few floor area) (n=15) (n=4) measurements indicate that about 25 to 45 cfm of ventilation air is provided which pressurizes the house in the range of +0.2 to +0.4 pascals.
SF MF Sample Size, n 164 146 Average HERS 87.0 88.0 Median HERS 86.7 88.7 Minimum HERS 86.0 88.1 Maximum HERS 90.3 89.9
Figure 5 HERS Scores for FL H.E.R.O. Homes
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
SF MF Sample Size, n 164 146 Average ACH50 4.5 5.2 Median ACH50 4.4 5.3 Minimum ACH50 2.1 2.2 Maximum ACH50 8.6 8.4
Figure 6 ACH50 Values for FL H.E.R.O. Homes
SF MF Sample Size, n 25 72 Average Qtot 6.9% 5.0% Median Qtot 6.3% 4.8% Minimum Qtot 3.0% 1.26% Maximum Qtot 17.8% 16.3%
Figure 7 Qtot Values for FL H.E.R.O. Homes
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
SF MF Sample Size, n 15 4 Average Qout 3.0% 1.4% Median Qout 2.5% 1.6% Minimum Qout 0.9% 0.01% Maximum Qout 7.0% 2.2%
Figure 8 Qout Values for FL H.E.R.O. Homes
research project for the industry. Data is available for other typical non BAIHP, new Florida homes (FPL , 1995 and Cummings et al, The BAIHP team has conducted diagnostic tests 2001). The FPL study had a sample size of over 300 (blower door, duct blaster, pressure mapping, single family homes and the median Qout was 7.5% , moisture meter readings) on about 40 such problem three times that of the FL. H.E.R.O. homes. In the homes from five manufacturers in the past two years Cummings study of 11 homes the measured average and shared the results with MHRA. These homes values were : ACH50= 5.7, Qtot=9.4% and were newly built (generally less than 3 years old) and Qout=4.7%. Although the sample sizes are small the in some cases just a few months old when the FL. H.E.R.O. homes appear to have significantly problems appeared. The most frequent causes were: more airtight duct systems than typical homes. $ Leaky supply ducts and/or inadequate return air pathways resulting in long term negative The remainder of the paper presents status of other pressures. tasks of the BAIHP project. $ Inadequate moisture removal from oversized a/c systems and/or clogged condensate OTHER BAIHP TASKS drain, and/or continuous running of the air Moisture Problems in HUD code homes handler fan. The BAIHP team expends considerable effort $ Presence of vinyl covered wallboard or working to solve moisture problems in existing flooring on which moist air condenses manufactured homes in the hot, humid Southeast. creating mold, buckling, soft walls etc. $ Low cooling thermostat set point (68-75F), Some manufactured homes in Florida and the below the ambient dew point. Gulfcoast have experienced soft walls, buckled $ Tears in the belly board and/or poor site floors, mold, water in light fixtures and related drainage and/or poor crawlspace ventilation problems. According to the Manufactured Housing creating high rates of moisture diffusion to Research Alliance (MHRA), who we collaborate the floor. with, moisture problems are the highest priority Note that these homes typically experience very high
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
cooling bills as the homeowners try to compensate The volunteers build affordable housing for and with for the moisture problems by lowering the thermostat buyers who can't qualify for conventional loans but setpoints. These findings have been reported in a peer do meet certain income guidelines. For some reviewed paper presented at the ASHRAE IAQ 2001. affiliates, reducing utility costs has become part of conference (Moyer et al) the affordability definition. To help affiliates make decisions about what will The Good News: be cost effective for their climate, BAIHP researchers As a result of our recommendations and hands-on have developed examples of Energy Star homes for training, BAIHP partner Palm Harbor Homes (PHH) more than a dozen different locations. These are has transformed duct design and construction available on the web at practices in all of its 15 factories nationwide http://www.fsec.ucf.edu/bldg/baihp/casestud/hfh_esta producing about 11,000 homes/yr. All Palm Harbor r/index.htm . The characteristics of the homes were Home duct systems are now constructed with mastic developed in conjunction with Habitat for Humanity to nearly eliminate air leakage and produced with International (HFHI), as well as Executive Directors return air pathways for a total cost of <$10/home!! and Construction Managers from many affiliates. The PHH factory in AL which had a high number of Work is continuing with HFHI to respond to affiliates homes with moisture problems has not had a single requesting a home energy rating through an Energy problem home the past year! and Environmental Practices Survey. 36 affiliates have been contacted and home energy ratings are Field Monitoring being arranged using combinations of local raters, Several houses and portable classrooms are being Building America staff, and HFHI staff. monitored and the data displayed on the web. (Visit http://www.infomonitors.com/). Of special interest is HFHI has posted the examples of Energy Star the side-by-side monitoring of two manufactured Habitat homes on the internal web site PartnerNet homes on the campus of the North which is available to affiliates nationwide. Carolina A & T U. where the advanced home is saving about 70% in heating energy and nearly 40% “Green” Housing in cooling energy, proving that the Building America A point based standard for constructing green goal can be met in manufactured housing. Other homes in Florida has been developed and may be monitored sites include the Washington State U. viewed at http://www.floridagreenbuildings.org/. Energy House in Olympia, WA; the Hoak residence The first community of 270 homes incorporating in Orlando, FL; two portable classrooms in these principles is now under construction in Marysville, WA; a classroom each in Boise, ID and Gainesville, FL. The first home constructed and Portland, OR. See other papers being presented at certified according to these standards has won an this symposium for details on two recently completed NAHB energy award. projects giving results from duct repairs in manufactured homes (Withers et al) and side by side BAIHP researchers are participating as building monitoring of insulated concrete form and base case science - sustainable products advisor to the HUD homes (Chasar et al). Hope VI project in Miami, redeveloping an inner city area with over 500 units of new affordable and “Cool” Roofs and Unvented Attics energy efficient housing. Seven side-by-side Habitat homes in Ft. Myers, FL. were tested under unoccupied conditions to Healthy Housing examine the effects of alternative roofing strategies. BAIHP researchers are participating in the After normalizing the data to account for occupancy development of national technical and program and minor differences in thermostat set points and standards for healthy housing being developed by the equipment efficiencies, the sealed attic saved 9% and American Lung Association. the white roofs saved about 20% cooling energy compared to the base case house with a dark shingle A 50-year-old house in Orlando is being roof for the summer season in South Florida. Visit remodeled to include energy efficient and healthy http://www.fsec.ucf.edu/%7Ebdac/pubs/coolroof/exs features as a demonstration project. um.htm for more information. EnergyGauge USA® Habitat for Humanity This FSEC developed software uses the hourly Habitat for Humanity affiliates work in the local DOE 2.1E engine with FSEC enhancements and a community to raise capital and recruit volunteers. user-friendly front end to accurately calculate home
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44
energy ratings and energy performance. This software is now available. Please visit Special thanks to Bert Kessler of Palm Harbor http://energygauge.com/ for more information. Homes, Mike Dalton of Stylecrest Sales, Mike Wade of Southern Energy Homes and David Hoak of Alten Industrial Engineering Applications Design for the hundreds of hours they have each The UCF Industrial Engineering (UCFIE) team contributed to the success of BAIHP. supported the development and ongoing research of the Quality Modular Building Task Force organized We are grateful to our sponsors, industry partners, by the Hickory consortium, which includes thirteen collaborators and colleagues for this opportunity to of the nation's largest modular homebuilders. UCFIE make a difference. led in research efforts involving factory design, quality systems and set & finish processes. UCFIE used research findings to assist in the analysis and design of two new modular housing factories – Excel REFERENCES homes, Liverpool, PA and Cardinal Homes - Cummings, J.B., Withers, C., McIlvaine, J., Sonne, Wyliesburg, VA. J., Fairey, P., and Lombardi, M., “Field Testing to Characterize the Airtightness and Operating CONCLUSIONS Pressures of Residential Air Handlers,” FSEC-CR- The entire BAIHP team of over 20 researchers and 1285-01, Florida Solar Energy Center, Cocoa, FL., students are involved in a wide variety of activities to November 30, 2001. enhance the energy efficiency, indoor air quality and durability of new housing and portable classrooms. FPL, 1995. “New Home Construction Research Project Findings, Results & Recommendations,” In addition to energy efficiency, durability, health, Final Report to the Florida Public Service comfort and safety BAIHP builders typically Commission, June 1995. consider resource and water efficiency. For example, in Gainesville, FL BAIHP builders have incorporated Moyer, N., Beal, D., Chasar, D., McIlvaine, J., the following features in developments: Withers, C. and Chandra, S. “Moisture problems in S Better planned communities manufactured housing: Probable causes and cures”, S More attention given to preserving the Proc. ASHRAE Indoor Air Quality 2001, Nov, 2001 natural environment S Use of reclaimed sewage water for landscaping S Use of native plants that require less water S Storm water percolating basins to recharge the ground water S Designated recreational areas S Better designed and built infrastructure S Energy efficient direct vented gas fireplaces (not smoke producing wood)
ACKNOWLEDGEMENTS This research was sponsored, in large part, by the U.S. Department of Energy, Office of Building Technology, State and Community Programs under cooperative agreement no. DE-FC36-99GO10478 administered by the U.S. DOE Golden field office. This support does not constitute an endorsement by DOE of the views expressed in this report.
The authors appreciate the encouragement and support from George James, program manager in Washington DC and Keith Bennett, project officer in Golden CO.
Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002