«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 average water-heating coefficient of performance values ranged from 3.1 to 1.9 in heatpump-only mode. The survey results indicated overwhelming resident satisfaction with the DHW supply. The larger differences between incoming air temperature and humidity at Savannah compared to LaFayette reveal a linear relationship between incoming air temperature and HPWH performance. However, the smaller temperature and humidity differences between the four units in LaFayette revealed no discernable relationship. Other potential variables investigated for HPWH performance included inlet water temperature, number of heat pump operation events, total DHW demand, DHW demand during heat pump operation, and tank set point temperature.
Ducting strategies such as exhaust duct only, intake duct only, and exhaust and intake ducting had no effect on HPWH performance vis-à-vis pressure drop across the heat exchange coils.
However, the annual air temperature must be considered when the intake of an HPWH is being ducted.
The air conditioning provided by the HPWH affects only the temperature of the mechanical closet and attic space during the time the heat pump is operating. The HPWH’s air-conditioning impact on HVAC equipment loads is minimal when the intake and exhaust airstreams are connected to a sealed attic and not to the living space. The zone temperatures return to their previous levels shortly after the HPWH stops operating. An HPWH was not suitable as a replacement dehumidifier in the sealed attics, because daily peak moisture loads were reduced only if the heat pump operated during the morning. Exhaust ducts should be insulated to avoid condensation on the exterior; however, this increases the risks of condensation inside the duct near the HPWH, which is caused by significant temperature variations between the compressor and the duct and bulk moisture around the condenser. All ducts should be at least 8 in. in diameter to prevent airflow restriction on A.O. Smith Voltex HPWHs.
xi 1 Project Overview
1.1 Problem Statement This report presents the field-monitoring results of a ducted heat pump water heater (HPWH) study that includes five A.O. Smith Voltex HPWHs that were installed in two geographic locations in two climate zones. Both sites are affordable housing projects where Southface Energy Institute partnered with the local municipality’s housing departments to improve building
sustainability. The test plan was developed around the following research foci:
Evaluate ducting strategies for HPWHs in test sites in two climate zones. A test site in LaFayette, Georgia (International Energy Conservation Code [IECC] Climate Zone 4A), includes HPWHs in mechanical closets inside the thermal envelope with ducting to an encapsulated attic. The test site in Savannah, Georgia (IECC Climate Zone 2A), includes an HPWH that is directly inside an encapsulated attic in which ducts terminate.
Derive the coefficient of performance (COP) during real-world use patterns.
Evaluate the HPWH’s ability to satisfy domestic hot water (DHW) demand in the efficiency mode (heat pump only) with set points of 120°F and 150°F (due to an unexpected change in set point temperature at the Savannah site).
Determine the impact of HPWH exhaust air on temperature and relative humidity (RH) conditions in mechanical closets and attic space. Temperature and RH of a single-family home with a HPWH in the sealed attic are compared to a nearly identical house with an electric resistance storage water heater (ERSWH) in the attic.
Investigate the impacts of different HPWH ducting strategies on whole-house heating, cooling, and moisture loads.
Evaluate the cost-effectiveness of the HPWH water-heating strategy, especially in the context of affordable and rental housing.
1.2 Background HPWH technology was first patented in 1935 and commercialized in the 1950s, but the technology has recently been commercialized due to dramatically improved technology, economic viability, and the “green energy” movement (Shapiro and Puttagunta 2013). An HPWH uses an air-source heat pump as the main heat source and has electric resistance elements as auxiliary heaters. An HPWH also provides space cooling and dehumidification to its surroundings; it affects the heating, ventilating, and air-conditioning (HVAC) performance of the entire house if it is placed inside the conditioned space. The performance and side effects of HPWHs are dependent upon a number of variables, such as ambient air temperature, humidity, outgoing draw profile, and incoming water temperature. HPWHs have undergone extensive testing in laboratory settings to quantify their individual performance (Sparn et al. 2011).
An HPWH must be installed in a location with a certain volume of air to avoid recirculating cool exhaust air, which could reduce its efficiency. Historically, HPWH installation in conditioned spaces was challenging due to the confined spaces where electric or gas water heaters were often installed, such as closets and mechanical rooms. Duct kits enable HPWHs to be installed in small, confined spaces in new and existing buildings. They also present opportunities to move cooled and dehumidified air to optimize whole-house cooling and moisture loads, which reduce HVAC loads. Ducts can be configured such that cool, dehumidified air can be delivered to a living zone during the summer and exhausted to the outside during the winter. Sparn et al. (2013) proposed that ducts can also be configured to provide conditioned outdoor air in the living zone to minimize ventilation requirements. However, evaluation of ducted HPWH performance in real-world installations and their effects on home comfort, HVAC energy use, and controls is still in its infancy because data are lacking and few companies make it easy to duct HPWHs.
Southface adhered to the National Renewable Energy Laboratory’s Field Monitoring Protocol for HPWHs (Sparn et al. 2013) to collect valuable data about in-situ ducted HPWHs to add to the limited field-monitored data and serve as a reference point for the refinement of HPWH computational models.
The U.S. Department of Energy’s Building America research team Partnership for Home Innovation analyzed five 60-gal A.O. Smith Voltex HPWHs (model PHPT-60) that were installed in home attics that were encapsulated with open-cell spray polyurethane foam rooflines.
The HPWH units were designed to operate in four modes (which differ from newer Voltex
Efficiency. The most energy-efficient mode. This mode uses the heat pump alone to heat the water in the tank. The electric resistance elements are not used unless the ambient operating temperature is lower than 45°F or higher than 109°F to prevent damage to the refrigerant cycle components. The heat pump turns on when the tank temperature drops 9°F lower than the set point. Tank temperature is calculated through a weighted average of the two thermistor readings at the upper and lower parts of the tank, in which the upper thermistor reading accounts for 75% of the weight (Sparn et al. 2011).
Hybrid. This is the default, manufacturer-recommended setting. This mode uses the heat pump as the primary heating source. The upper heating element operates during large DHW draws when the average tank temperature drops 18°F lower than the set point.
Electric. The water heater functions as a conventional electric unit and relies solely on the electric resistance elements to heat the water in the tank.
Vacation. The controller adjusts the water temperature set point to 60°F and is recommended when no DHW draws are planned for a long period. This mode minimizes energy consumption and prevents the water heater from freezing during cold weather.
The efficiency mode was of primary interest in this study. The PHPT series can be ducted (maximum of 10 ft) to another zone when the free air volume of the occupied zone is lower than 750 ft3 (A.O. Smith 2011b, 2012). Inlet and Outlet Duct Kits, which are identical, are available from the manufacturer and enable multiple ducting configurations.
Two notable laboratory analyses have been conducted on the 80-gal A.O. Smith Voltex HPWH (model PHPT-80) (Larson and Bedney 2011; Sparn et al. 2011). Larson and Bedney and Sparn et al. used the same data set for their independent analyses. The differences between the PHPT-80 and PHPT-60 are limited to the tank volume and anode size (2 mm for PHPT-60; 3 mm for PHPT-80), as stated by A.O. Smith’s representative and literature (A.O. Smith 2011b). Across the studies, A.O. Smith 80-gal HPWHs had various draw profiles imposed under different conditions; the resulting COPs in efficiency mode ranged from 2.0 to 3.5 (Sparn et al. 2011) and in hybrid mode from 1.8 to 3.5 (Larson and Bedney 2011). None of the studies investigated the use of the manufacturer’s optional duct kits; however, two studies investigated the effect on COP of reduced airflow over the heat pump’s evaporation coil. Reducing airflow by blocking onethird of the intake grille caused an insignificant decrease (1%); a two-thirds blockage caused a decrease on COP values of ~6% to ~2% during the heating cycle (Sparn et al. 2011). The effect of ducting the HPWH was expected to be similar to the one-third blocked case due to the grille area blockage of the duct kits and the increased static pressure from the ducts.
Shapiro and Puttagunta (2013) conducted a Building America field-monitoring study on both the 60- and 80-gal A.O. Smith Voltex HPWHs and reported COP values of 2.1 for both model sizes.
The 80-gal unit did not use the electric resistance elements; 11% of the 60-gal unit’s energy consumption was from the electric resistance elements. They also reported an efficiency reduction of 16% (COP 1.76) for installations in confined spaces. This study focuses on different duct configurations for HPWHs installed in confined spaces to determine the optimal configuration and potentially avoid such an efficiency reduction.
The Electric Power Research Institute also conducted field monitoring of more than 145 HPWHs; however, the results have been anonymized to hide model/manufacturer information.
The COP calculations were also reported as weighted monthly averages; values ranged from 1.1 to 1.9 (Amarnath and Bush 2012). Ecotope and the Northwest Energy Efficiency Alliance (2015) studied 50 HPWHs in myriad conditions and reported that the average annual COP varied between 1.6 and 2.4. The results from this project will be compared to previous field-monitored studies vis-à-vis trends in operation and performance.
The HPWHs were installed at each site by local plumbing tradesmen whose HPWH training consisted of a 30-min installation video that was provided by A.O. Smith. The ducts were installed by local HVAC tradesmen and commissioned by Southface for an extra cost of $250– $300/unit at the LaFayette site. The only installation issue to date that was specific to HPWHs was that the plumber installed a condensation drainpipe in only one of the two condensation drains, which caused a leak into the drain pan.
1.3 Exhaust Duct Kit The exhaust duct kit (Figure 1) supplied by the manufacturer was installed easily using a #2 Phillips screwdriver, wire cutters (to cut the flex duct), and scissors. Once the kit was screwed to the heat pump, a short run of 8-in. flex duct connected the kit to the permanent metal duct in LaFayette. A.O. Smith exhaust duct kits have a relief vent below the round duct connection to prevent damage to the heat pump from excessive ducting pressure increases. This study investigated the relief vent’s effects on intake air temperature and humidity and to determine if sealing the vent to force all the exhaust air to the attic had any impacts. Due to the size of the mechanical closets in LaFayette, operating the HPWHs completely unducted was against the manufacturer’s recommendation and warranty. It was also not of interest because a previous Building America study reported a performance decrease of 16% when installed in confined spaces, and preliminary results showed ducted HPWHs performed comparably to unducted HPWHs in field studies (Shapiro and Puttagunta 2013).
Figure 1. Exhaust duct kit supplied by A.O. Smith
1.4 Introduction to Project Sites With the support of the U.S. Department of Energy’s Building America program, Southface partnered with the LaFayette Housing Authority (LHA) and the Housing Department of the City of Savannah to assist with the design of energy-efficient residential dwellings in their respective locales. The all-electric New Construction Test Homes (NCTHs) at both sites were completed in the beginning of 2013; however, they were not occupied until early 2014. The LaFayette site has 30 60-gal A.O. Smith Voltex HPWHs; the Savannah site has one house with an HPWH installed.
Each HPWH intakes and exhausts air from an open-cell, spray-polyurethane-foam-encapsulated attic. Bill Hosken, national sales manager for A.O. Smith, was involved since the early stages of both projects and provided insight and critical details from A.O. Smith’s research and development team.
LHA hired the architecture firm Lord Aeck Sargent to design 30 sustainable and affordable housing units that sought Gold certification in Leadership in Energy & Environmental Design in 15 duplexes. These units were built on two sites (1.3 geographic miles apart) in LaFayette, Georgia, that included 16 units (8 duplexes) at one site and 14 units (7 duplexes) at the other.
LaFayette is situated in the northwestern corner of the state, approximately 30 miles due south of Chattanooga, Tennessee. Heating degree days (base 65) for this site are approximately 3,200, and cooling degree days are approximately 1,700 for the mixed-humid climate (IECC Climate Zone 4). Figure 2 shows the three-bedroom/two-bedroom floor plan of 14 duplex buildings; the 15th building is a two-bedroom/two-bedroom duplex that is designed to meet Uniform Federal Accessibility Standards. Two buildings (four units) at the same location were chosen for longterm monitoring of HPWH variables of interest. Duplex units in LaFayette have variable-speed, split-system, air-source heat pumps with a seasonal energy-efficiency ratio of 14 and a heating season performance factor rating of 8.