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The temperatures decreased when the HPWH operated at the end of the day, but it is difficult to distinguish the cause between HPWH operation and the sun setting. During the first half of the day attic temperatures increased when the sun rose. In one instance, the HPWH operated during the first half of the day and the east side decreased by less than 2°F, which is where the duct pointed. The north and south sensors decreased by less than 0.4°F. The sensor at the high center of the attic continued to increase while the HPWH operated. Figure 25 shows a zoomed image of the temperatures during this time. Because the HPWH could reduce the attic temperature in only one area of the attic, the HPWH most likely had no impact on the temperature in the living zone or on the energy consumed by the HVAC equipment.
3.7 Effect of Incoming Air Temperature Figure 26 plots the daily COP values as a function of the daily DHW consumption and the intake air across the evaporator coil. Previous field monitoring and laboratory studies have shown that intake air wet bulb temperature across the evaporator coil improved COP as energy transfer is more effective at greater Ts between the ambient air and evaporator coil (Shapiro and Puttagunta 2013; Sparn et al. 2011). This trend is not evident in the data. However, the highest measured COP occurred in the warmer and more humid attic in Savannah. These results can be
Units in LaFayette likely experienced some recirculation because the relief vent at the bottom of the exhaust duct kit effectively cooled the mechanical closet. Mechanical closets would warm if the HPWH did not run, which means fewer water draws occurred.
The chart is predominantly colored in the blue range, which means the intake air temperature varied only slightly, and no trend in color pattern emerges. A larger sample size with more variation would be needed to verify the trend of increased COP values with increased intake temperature.
Figure 26. COP as a function of daily DHW consumption and intake air wet bulb temperature Table 10 reports the average air intake and exhaust temperatures and absolute humidities across the evaporator coil during heat pump operation at each monitored site.
Unit E1 (set point of 120°F) had the highest average intake air temperature and humidity, which resulted in the highest average daily COP of 3.1. The change in the intake airstream’s temperature and humidity were significantly reduced when the set point was adjusted to 150°F (E2), which lowered the COP to 1.9. This was likely due to increased HPWH operating duration. This shows that increased HPWH runtimes will have a greater impact on attic air temperatures; however, the attic temperatures promptly return to preoperating conditions when the HPWH stops. Unit C had the greatest decrease in temperature and humidity across the coil. Coincidentally, Unit C also had the third-greatest average daily DHW demand.
Table 10. Average Intake and Exhaust Air Temperatures and Absolute Humidities, Measured In Grains per Pound, across the Heat Pump during Operation Water Heater
3.8 Error Induced by Measurement Instrumentation The inlet and outlet temperatures measured by the Badger In-line Btu meters were housed in heavy brass couplings (Figure 27), which affected the temperature readings because their thermal mass was in contact with the thermocouples. The heavy mass caused temperature readings to skew toward ambient air temperatures. Once this profound effect was noticed, the metal couplings were insulated (Figure 28). When a DHW draw occurred, the outlet water temperature reading took about 2 min to reach the maximum measured temperature (Figure 29), which still was cooler than the tank set point.
Figure 27. Brass fittings housing the thermocouples measuring outlet (left); inlet water temperatures and flow rate before being insulated (right) Figure 28.
Images of an insulated water meter and thermocouple brass fittings at the LaFayette site; front grille/filter showing obvious signs of debris (right) Figure 29. Effect of Badger Meter’s thermal mass on water temperature measurements Daily average hot water outlet temperatures during draws for each site are characterized by histograms in Figure 30. When the tank set point was known to be 120°F, the daily average measured hot water outlet temperature was approximately 110°F for all units. E2 had a tank set point temperature of 150°F, and the daily average measured hot water outlet temperature was 128°F. The daily hot water outlet temperature approched the set point temperature when hot water draws were sustained for long enough periods of time to overcome the thermal mass of the brass thermocouple housings. Average daily outlet temperature occurences lower than 100°F are attibuted to days when the maximum draw duration was less than 2 min.
COP calculations used inlet/outlet water temperatures during DHW draws when the measured temperatures were adversely affected by the sensor housing’s thermal mass. Most draws were less than or equal to 2 min (Table 11). However, most draw volumes were during events that were longer than 2 min. This systemic measurement error would tend to propagate through the analysis and result in reducing the calculated values of COP below actual performance.
Figure 30. Histograms of daily average hot water temperatures measured leaving the tank during draw periods.
Number of occurrences equals number of days.
Table 11. Statistics of Number of Occurrences of Short DHW Draw Durations
Despite the adverse impact of the water temperature measurements, the COP value measured in Savannah, Georgia, with a 120°F set point is the highest known value for A.O. Smith Voltex HPWH realized in a field monitoring study.
3.9 LaFayette Survey Results Southface wrote a survey that LHA issued to the tenants of all 30 duplexes with ducted HPWHs to assess their satisfaction about DHW demand. Figure 32 through Figure 34 represent the survey results vis-à-vis the occupants’ satisfaction with their hot water supply.
More than half the respondents indicated that they plan their shower timing to avoid running out of hot water (Figure 31). However, more than 70% have either never or seldom experienced a shortage of hot water while showering or bathing (Figure 32). These data do not show whether the respondents’ behavior in timing showers is due to experiences in the LaFayette homes or is learned behavior from past experiences, especially because they have lived in these homes for less than 1 year. Hot water shortages were predominately during showers, because 80% reported never running out of hot water while using the kitchen sink.
In regards to overall satisfaction, 93% reported they agreed or strongly agreed they experience satisfactory hot water supply. LHA reported that it has not received any direct complaints about shortages of hot water. LHA does not intend to adjust the tank set point temperature beyond 120°F or switch HPWH operation into hybrid mode, because the current settings appear to satisfy the tenants.
Previous research has identified HPWH operation noise as a barrier to acceptance of installation inside the living space, such as in the utility closets in the LaFayette community (Chasar and Martin 2013). This was one factor that led the research team to recommend a solid door on the utility closet and ducting of the HPWH to and from the encapsulated attic. Both the HPWH and the HVAC air handler were located inside different closets inside the living space. The resident survey asked several questions to help identify whether this particular installation, which runs 100% in heat pump mode, has any negative noise impact.
Figure 37. Does the noise disturb your daily activities? If yes, please explain.
Examination of more detailed questions allowed the researchers to identify that, of the 18 residents who indicated they have heard noise from the utility closet, 11 were related to the HPWH, four to the air handling unit, and three could be either. Even though 18 residents reported hearing noise, only two indicated it disturbed their daily activities. LHA has reported that it has not received complaints from residents and does not plan to make changes to HPWH operation based on noise.
4 Conclusions The effect of ducting an HPWH’s supply or exhaust airstream does not diminish its efficiency if supply-side ducting does not reduce intake air temperature, which expands HPWH application to confined areas. Exhaust ducts should be insulated to avoid condensation on the exterior;
however, this increases the risk of condensation in the duct’s interior near the HPWH, because the temperatures vary greatly between the compressor and the duct and standing bulk water around the condenser. All ducts should be at least 8 in. in diameter to prevent airflow restriction for A.O. Smith Voltex (PHPT-60/80) HPWHs. The ducted HPWH’s air-conditioning impact on HVAC equipment loads appears to be minimal when it is connected to an attic and not the living zone, because attic temperatures are impacted only during HPWH operation and immediately return to normal levels after the HPWH stops. An HPWH is not suitable as a replacement dehumidifier under normal operation in sealed attics, because peak moisture loads decreased only if the heat pump operated during the morning.
Do HPWHs installed in or connected to encapsulated attics perform differently in different climate zones?
HPWHs installed in or connected to encapsulated attics appear to perform better in Climate Zone 2 than in Climate Zone 4 because of increased attic air temperature and humidity. The heat pump intake air temperature and humidity appear to be the dominant variables that affect HPWH performance.
How do the real-world COPs vary when they are subjected to different use patterns, and how do they compare to other field studies?
HPWH COP values are dependent on several variables, including intake air temperature and humidity, inlet water temperature, number of heat pump operation events, total DHW demand, DHW demand during a heat pump operation event, and tank set point temperature. Because of this complexity, strong correlations between COP and any single variable were not necessarily established. The values calculated in this study are similar to other field-monitoring studies and laboratory studies of unducted HPWHs.
Did the HPWH satisfy DHW demand in the efficiency operating mode?
The HPWH satisfied DHW demand in the efficiency operating mode, because no complaints were reported to LHA or from the homeowner in Savannah. A survey was administered to the residents in 30 duplex units in LaFayette, and 93% of the tenants agreed that they had satisfactory DHW supply. However, the homeowner in Savannah increased the tank set point temperature to 150° from 120°F, but not necessarily due to unsatisfactory supply from the HPWH in efficiency mode—it was turned off for a long period before. The increased set point reduced the COP from 3.1 to 2.0 and the total DHW consumption by 21.1 gal/day.
How much does HPWH exhaust air affect temperature and RH conditions in the mechanical closet and attic space compared to alternative systems?
The air conditioning provided by the HPWH affects the temperature of the mechanical closet and attic space only during the time the heat pump is operating. Shortly after the heat pump stops operating, the values return to previous levels. Encapsulated attic peak humidity levels can be reduced if the heat pump operates during the first half of the day compared to when it operates later in the day or compared to an alternative DHW system.
Do different HPWH ducting strategies affect whole-house heating, cooling, and moisture loads?
Different HPWH ducting strategies did not have a major impact on whole-house heating, cooling, and moisture loads. The attic temperatures decreased during operation of the HPWH but returned to their preoperating condition within a few minutes after the HPWH stopped. The air conditioning of the HPWH did not appear to have any effect on the living zone air conditions.
Do different HPWH ducting strategies affect HPWH performance?
Different ducting strategies had no impact on COP. The intake air temperature increased slightly but not enough to increase COP.
How well does BEopt account for the interaction between the HPWH and total space-heating, cooling, and moisture loads?
BEopt shows that the HPWH slightly decreased the cooling site energy consumption of the three models by 59 kWh/yr (10%), 70 kWh/yr (9%), and 97 kWh/yr (11%), respectively, for the two-bedroom and three-bedroom LaFayette models and the home in Savannah. The heating site energy consumption increased by 123 kWh/yr (11%), 147 kWh/yr (11%), and 88 kWh/yr (11%), respectively, to compensate for the HPWH cooling air during the heating season. The attic temperatures measured during HPWH operation regularly decreased only if the sun was setting and the temperatures were already decreasing. If the sun was rising, causing attic temperatures to increase, HPWH operation would not necessarily decrease the attic temperature. This leaves some doubt about whether the HPWH will impact space-conditioning loads.
References A.O. Smith. (2011a). Ducting Kit Installation Instruction for Hybrid Electric Heat Pump Water Heaters. Retrieved from http://www.hotwater.com/Resources/Literature/InstructionManuals/Residential-Electric/HPWH-Inlet/Outlet-Duct-Instructions-(323471)/ A.O. Smith. (2011b). Service Handbook-Residential Hybrid Electric Heat Pump Water Heater for Models: PHPT-60 and PHPT-80. Ashland City, TN.
A.O. Smith. (2012). Installation Instructions and Use & Care Guide.
Amarnath, A., & Bush, J. (2012). Heat Pump Water Heaters: Field Evaluation of New Residential Products (pp. 24–36). ACEEE Summer Study on Energy Efficiency in Buildings.
Retrieved from http://www.aceee.org/files/proceedings/2012/data/papers/0193-000013.pdf Chasar, D., & Martin, E. (2013). Efficient Multifamily Homes in a Hot-Humid Climate by Atlantic Housing Partners.
Community Housing Services Agency Inc. (2012). Savannah Gardens. Retrieved from http://chsadevelopment.org/ Ecotope Inc, & Northwest Energy Efficiency Alliance. (2015). Heat Pump Water Heater Model Validation Study.
Engebrecht Metzger, C., Wilson, E., & Horowitz, S. (2012). Addendum to the Building America House Simulation Protocols. Golden, CO. Retrieved from http://www.nrel.gov/docs/fy13osti/57450.pdf Hendron, R., & Engebrecht, C. (2010). Building America House Simulation Protocols. Golden, CO. Retrieved from http://www.nrel.gov/docs/fy11osti/49246.pdf Larson, B., & Bedney, K. (2011). Interim Report and Preliminary Assessment of AO Smith Voltex PHPT-80 Hybrid Heat Pump Water Heater. Bonneville Power Administration.
Shapiro, C., & Puttagunta, S. (2013). Field Performance of Heat Pump Water Heaters in the Northeast. Retrieved from http://www.carb-swa.com/Collateral/Documents/CARBSWA/Research/58115_sw 2013-08-07.pdf
Sparn, B., Hudon, K., & Christensen, D. (2011). Laboratory performance evaluation of residential integrated heat pump water heaters. Contract. Retrieved from http://www.nrel.gov/docs/fy11osti/52635.pdf Wilson, E., Engebrecht Metzger, C., Horowitz, S., & Hendron, R. (2013). Building America House Simulation Protocols. Golden,CO.
For more information, visit: energy.gov/eere buildingamerica.gov DOE/GO-102016-4753 May 2016