«May 2016 NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States ...»
The HPWHs in LaFayette were installed in the mechanical closets with doorways that were fitted with a door shoe to reduce noise and air transfer to the living space. Noise reduction was the primary reason to duct the HPWHs to the attic and not use a louvered door, because previous studies have reported noise as the major complaint from tenants who live with HPWHs (Chasar and Martin 2013). Figure 3 shows a schematic of the ducted HPWH installation in the mechanical closet. The ceiling of the mechanical closet has a transfer duct to the encapsulated attic to provide intake air (Figure 4, left); the HPWH’s exhaust is directly ducted to the attic (Figure 4, right). The distance between the ducts’ terminals is to be a minimum of 5 ft, and the different orientations of the ducts are to prevent cool exhaust air from recirculating into the intake duct.
Figure 4. Vertical intake transfer duct leads to a vent in the mechanical closet’s ceiling (left);
horizontal exhaust duct connected to 3-in. × 14-in. rectangular duct inside the wall cavity leading to the HPWH in closet (right) The test home (Figure 5) in Savannah, Georgia, is located in the Savannah Gardens community, a 44-acre site that was developed to meet the standards of the EarthCraft Communities 1 program (Community Housing Services Agency Inc. 2012). Heating degree days (base 65) for this site are approximately 1,700, and cooling degree days are approximately 2,500. The site is near the northern boundary of IECC Climate Zone 2. The three-bedroom, two-bathroom, all-electric test home has approximately 1,200 ft2 of conditioned space on an elevated slab foundation. The HPWH is installed in a spray-foam-encapsulated attic (~1,508 ft3) wherein a 10-ft duct terminates (Figure 6) to prevent air recirculation and provide insight about attic air conditions when ducted. The house adjacent to the test home is of similar size, dimension, and construction with an electric 50-gal A.O. Smith water heater (ECRT-52) in its encapsulated attic. The temperature and RH in the attic of this neighboring home were also monitored. The NCTH and neighboring home have ground-source heat pumps with a seasonal energy-efficiency ratio of
18.2 and COP of 3.7. HVAC ducts for every monitored site are located in encapsulated attics.
http://www.earthcraft.org/builders/programs/earthcraft-communities/ Figure 6. HPWH in encapsulated attic (left); unducted exhaust with 10-ft duct attached (right) 2 Experiment
2.1 Research Questions The following research questions/hypotheses were developed from the aforementioned research
foci and are answered in Section 4:
Do HPWHs installed in or connected to encapsulated attics perform differently in different climate zones?
Exterior conditions in IECC Climate Zone 2 are warmer and more humid annually than in IECC Climate Zone 4. It is not clear how these exterior conditions, attic-to-living-zone connectivity, and thermostat set points impact attic air temperatures and humidities in the two climates, which are determinants of HPWH COP.
How do the real-world COPs vary when they are subjected to different use patterns, and how do they compare to other field studies?
Real-world COPs are expected to vary depending on average tank temperature, ambient air temperature and humidity, local incoming water temperature, and water draw patterns.
Long-term average real-world COPs are expected to be similar to laboratory results under similar conditions.
Did the HPWH satisfy DHW demand in the efficiency operating mode?
The HPWH is expected to satisfy DHW demand in efficiency mode most of the time. To prevent the equipment from abuse, tenants of the duplexes in LaFayette do not have access to the HPWHs to change their operation or tank set points. A survey will be conducted of the residents of all 30 duplex units to document their satisfaction with their DHW supply. Power data of the HPWH in Savannah will reveal any changes to mode of operation.
How much does HPWH exhaust air affect temperature and RH conditions in the mechanical closet and attic space compared to alternative systems?
The exhaust/input air will affect temperature and RH conditions in any zone it draws or exhausts air to or from. Small mechanical closets are expected to experience greater temperature and RH variances during heat pump operation; these variances in attics are expected to be localized to the area of air exchange. Variations in closets are expected to influence COP values when the intake is not directly ducted.
Do different HPWH ducting strategies affect whole-house heating, cooling, and moisture loads?
The HPWH installed in the encapsulated attic in Savannah is expected to reduce cooling and moisture loads compared to the similar neighboring home with an ERSWH in the encapsulated attic. Different duct strategies in LaFayette will have the greatest effect on attic air conditions. The whole-house heating, cooling, and moisture loads will vary depending on the area of air pathways between the attic and conditioned space. Different duct strategies will also directly influence COP values.
How well does Building Energy Optimiztion (BEopt™) software account for the interaction of the HPWH and total space heating, cooling, and moisture loads?
Ducting air to or from the encapsulated attic will lower the average annual attic temperature and provide dehumidification relief during the summer. HPWH operation is expected to decrease annual HVAC load in the hot-humid climate of Savannah. The impact on the HVAC load in LaFayette is expected to be less significant—and likely negligible—because it has fewer cooling degree days than Savannah.
2.2 Modeling Results BEopt E+2.3 models were constructed for the NCTH in Savannah, and of the three-bedroom and two-bedroom units of the typical duplexes in LaFayette, in accordance with the National Renewable Energy Laboratory’s Building America House Simulation Protocols (Engebrecht Metzger et al. 2012; Hendron and Engebrecht, 2010; Wilson et al. 2013). Each model was simulated with an ERSWH, HPWH, and a typical electric tankless water heater. The default specifications in BEopt were used for the ERSWH and tankless water heater; the HPWH specifications were from A.O. Smith’s documentation of its 60-gal Voltex (PHPT-60). In all cases, the water heaters were modeled as installed inside the sealed unfinished attic space, because BEopt cannot model the ducted case yet the HPWHs exchange air with the attics and not the living zones. All input specifications are listed in Table 1.
The site energy use was analyzed because these units are all-electric. Site energy for HVAC fan/pump, cooling, heating, and DHW is shown in Figure 7, Figure 8, and Table 2. The total values do not include energy loads from miscellaneous, ventilation fan, large appliances, or lights, because these values were not impacted by the water heater. The DHW site energy consumption decreased significantly for the two-bedroom and three-bedroom units in LaFayette and the NCTH in Savannah by 1,287 kWh/yr (57%), 1,512 kWh/yr (56%), and 1,536 kWh/yr (62%), respectively, compared to the ERSWH. The cooling site energy consumption of the three models decreased slightly by 59 kWh/yr (10%), 70 kWh/yr (9%), and 97 kWh/yr (11%), respectively, which indicates cooling energy consumption decreased because the HPWH could cool and dehumidify air. 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 HVAC fan/pump energy consumption stayed the same for both LaFayette models, but it decreased by 23 kWh/yr (6%) in Savannah, likely due to the ground-source heat pump. The models indicate the HPWH has net space-conditioning penalties in LaFayette of 64 kWh/yr and 77 kWh/yr, respectively. The HPWH had a net spaceconditioning decrease in Savannah of 9 kWh/yr, because it is a predominantly cooling climate (Climate Zone 2). An electric tankless water heater was also modeled in each dwelling. The tankless water heater reduced DHW site energy consumption marginally from the ERSWH models by 8%, 6%, and 6%, respectively, and consumed more than 3 times the energy as the HPWHs. The tankless water heater decreased the space-conditioning load by 26 kWh/yr and 24 kWh/yr in LaFayette and increased the load by 5 kWh in Savannah.
The models show the annual total home savings for a HPWH compared to an ERSWH was 1,222 kWh (28%), 1,436 kWh (27%), and 1,568 kWh (37%), respectively, which indicates an increase in savings with increased DHW consumption (because larger dwellings consume more DHW).
With an average electricity cost of $0.11/kWh 2, HPWHs can save $134 to $172/yr compared to According to the U.S. Energy Information Administration’s 2015 year-to-date residential electricity cost data for Georgia.
an ERSWH. The National Appliance Energy Conservation Act increased the efficiency requirements of electric water heaters in April 2015. Tank volumes greater than 50 gal must now meet efficiency standards that are achieved only by HPWHs. A 60-gal HPWH would cost $2,100 to purchase and install ducts; two 30-gal ERSWH would cost $500. However, the confined spaces in LaFayette would not have enough area to install two 30-gal tanks side by side, so a shelf would have to be installed at additional cost. The HPWH would have a simple payback period of 9.3–11.9 years, which is within the useful life expectancy.
2.3 Monitoring Approach Long-term monitoring was conducted for 6–9 months after the NCTH dwellings were occupied.
Flow and temperature sensors were installed on the HPWH plumbing systems, which were used to calculate the energy transferred to the water. Temperature and RH sensors were installed at the HPWH air intake and exhaust streams. Five sensors were installed in each NCTH attic to monitor temperature and RH at each of the four wall orientations and the attic center. The home adjacent to the NCTH in Savannah, Georgia, had monitors deployed in the attic as a reference point for an attic with an ERSWH. Both homes in Savannah were also monitored for living zone temperature and RH. Circuit monitors were installed to collect HPWH fan, compressor, and total power consumption in watts. All monitoring equipment recorded data at 1-min intervals. The collected data were used to compute COPs and to analyze temperature and RH variations, water consumption patterns of HPWHs, and energy balances to investigate the effect on wholebuilding heating, cooling, and moisture loads.
2.4 Monitoring Equipment
Monitoring for this project involved the following equipment (Table 3):
Figure 9 shows an image of the heat pump with the intake grille/filter removed to show the location of the components, the installed Vaisala Probe, and circuit transducers on the HPWH’s main, compressor, and fan power lines. Images of the installed water flow meter and thermocouples (Figure 27) and the heat pump intake side with the grille/filter (Figure 28) are shown in Section 3.8.
Figure 9. Monitoring equipment installed on the air intake side of the heat pump
2.5 Monitoring Status Table 4 shows a timeline that summarizes all the monitoring equipment and occupancy activity of both sites. All equipment is installed and has transmitted data to Southface’s server since April 11, 2014, in LaFayette, Georgia, and July 9, 2014, in Savannah, Georgia.
Table 4. Timeline of Monitoring Equipment and Dwelling Occupancy Activity.
Units A, B, C, and D Are in Lafayette and Units E and F Are in Savannah.
3 Performance Results
3.1 Analysis The COP of the HPWHs, which were operated in efficiency mode only, were evaluated in accordance with NREL’s HPWH Field Monitoring Protocol (Sparn et al. 2013). Equation 1 was
used to compute the COP:
= =, (1)
V draws is the volume of the water drawn C p,water is the specific heat of water is the difference between the outlet (T out ) and inlet (T in ) water temperature draws W input is the electrical energy consumed by the HPWH
Data were analyzed during time frames when each unit met the following criteria:
No obvious signs of missing data due to equipment malfunction such as:
o Long periods during which the power consumption data were 0 W with significant DHW draws. In a few instances the power monitoring equipment froze, stopped delivering data, and rebooted automatically.
o When flow meter data indicated consumption of several hundred gallons over short, unrealistic stretches of time. This was a rare occurrence; the impeller may have stopped in a resonance frequency position.
Periods during which the data did not meet the above criteria were disregarded. Table 5 reports the number of monitored days that were included in the analysis in addition to the number of adults and children who occupied each unit. Units in LaFayette are hereafter referred to as Units A, B, C, and D; the unit in Savannah is referred to as Unit E. Increased number of occupants correlated well with increased DHW consumption; however, the ages of the children were unknown. LaFayette D was occupied by two sets of tenants during the monitoring period. The first set occupied the unit from April 11, 2014 to July 7, 2014. Then the unit remained unoccupied until September 30, 2014, when the second set of tenants took occupancy.