Heat Pump Water Heater Design Considerations

This is Part II of an expert best practices web guide to Heat Pump Water Heaters published by the ENERGY STAR Residential New Construction program. Read Part I of the guide.

Design for Efficient Operation

To ensure efficient operation, a Heat Pump Water Heater should be installed in a sufficiently large room, or be properly vented. Manufacturers typically require access to a minimum of 450 or 700 cubic feet of free air space where the water heater is installed, along with ample space to allow installation and service. An 8-ft by 12-ft room with an 8-ft ceiling, for example, provides sufficient volume. The installation location relative to other parts of the home (hot water uses and living areas) will affect efficiency and may impact residents’ comfort.

Be sure to consult the manufacturer’s installation guide for recommendations specific to your model. 

Sound

Heat Pump Water Heaters have a fan and compressor, both of which can make a modest amount of noise when the Heat Pump Water Heater is heating water. Heat Pump Water Heaters that meet ENERGY STAR Version 5.0 product specifications emit sound levels less than 55 dBA—about the level of a background conversation. (NOTE: 55 dBA is also the maximum sound rating level allowed by the ENERGY STAR NextGen program for HPWHs installed in occupiable space.)

Sound levels vary among products. Some Heat Pump Water Heaters have sound pressure ratings of 45 dBA—about the level of a quiet dishwasher—and even softer-sounding products are in development.

Avoid locating a Heat Pump Water Heater next to rooms where sound levels are more important to comfort, such as bedrooms and living areas. To further reduce the impact of noise, consider additional sound insulation for the installation room.

Cool Exhaust Air

Heat Pump Water Heaters exhaust air that has been cooled and dried. Avoid locating an Heat Pump Water Heater near areas where residents will be sensitive to cooler air temperatures. Infrequently occupied spaces where temperature variations are less likely to be bothersome—like hallways, garages, and utility rooms—are good choices. Locations with waste heat available also serve as excellent locations. In some areas, the cooler and drier air can be a benefit, for example, in a laundry room or near a home gym.

Typical Installation Locations

The local climate will often determine best locations to install a Heat Pump Water Heater. Be sure to consult the manufacturer’s installation guide for recommendations specific to your model and follow applicable local codes.

• Basements, including unconditioned ones, are often the best locations for Heat Pump Water Heaters in any climate.
• Garages are also a great option because they can provide ample volume. In warmer climates, where outdoor temperatures are usually above 50° F, uninsulated garages are acceptable; however, if the building is in a cooler climate, consider possible freezing conditions and follow standard location practices in your area.
• Interior rooms (e.g., utility, laundry, and IT rooms) are other common choices that work in any climate. A HPWH can benefit from waste heat produced by other equipment in the space.  
• Rooms outside the thermal envelope—such as attached sheds and utility rooms—also work in warm climates. In hot climates, they can increase energy efficiency compared to interior spaces.

Venting in Closets

In some homes or dwelling units, it is necessary to plan for the Heat Pump Water Heater to be placed in a confined space, such as a small mechanical closet. If the Heat Pump Water Heater must be installed in closets adjacent to rooms that are frequently occupied, vent the cooler exhaust air through a duct or transfer grille into an area where temperature is not an issue.

Refer to manufacturer requirements for minimum requirements for the amount of air needed, and choose one of the following options to achieve proper airflow.

Passive venting options:

Best practice is to provide a total minimum net-free area of 240 square inches or greater, with both high and low openings to allow air to circulate. This can be done through a fully louvered door, using both high and low transfer grilles, or a high transfer grille and a ¾”door undercut.

Active venting options (ducted):

• Duct Heat Pump Water Heater  intake air directly into the HPWH. To allow cool exhaust air to leave the space, install a large louver or transfer grille that provides at least 130 square inches of net free area, placed in a location near the Heat Pump Water Heater exhaust.
• Duct Heat Pump Water Heater exhaust out of the space. To allow warm air to enter the space, install a large louver or transfer grille that provides at least 130 square inches of net free area or at least a ¾” door undercut to allow air to enter the space. 
• Duct both intake air and exhaust air with balanced airflow.

For active venting, ducts must be short, unrestricted, and as straight as possible. Design ducting to vent exhaust air into a location where a cool air stream will have minimal impact on occupant comfort. Only duct to the outside if located in a warm climate and ducting both the inlet and outlet. Refer to manufacturer guidance for duct sizing and maximum distance requirements.

DO NOT:

• Do not duct only the Heat Pump Water Heater intake air or exhaust air to the outside. Doing so will create a pressure imbalance that will lead to air infiltration or exfiltration, increasing the load on the space heating and cooling systems. 
• Do not run any ducts between the garage and the Heat Pump Water Heater. This may bring exhaust fumes or other contaminants into the living space. 
• Do not vent the water heater exhaust air near a thermostat. The cooler exhaust air will provide a false reading to heating and cooling systems. 
• Do not duct both the Heat Pump Water Heater intake and exhaust air to the outside in cold-climate regions or locate the Heat Pump Water Heater outside in cold-climate regions. Intake air temperatures below approximately 40°F will trigger electric resistance elements and significantly reduce Heat Pump Water Heater efficiency.

Tank Sizing

Follow the local plumbing code’s minimum first-hour rating (FHR) requirements and manufacturer recommendations for tank sizing. For maximum efficiency, upsize the tank over the standard practice used for electric resistance or fossil fuel-fired water heaters. Heat Pump Water Heaters typically come with auxiliary resistance elements that can run to meet periods of high water demand when the unit is set to Hybrid Mode. Resistance operation is less efficient than running the heat pump alone. Upsizing the tank will minimize inefficient resistance heating, allowing the heat pump to do the majority of the water-heating work. The ENERGY STAR NextGen program requires minimum rated tank volumes based on the number of bedrooms, as shown in the table below.

• Tank Size Tip #1: Upsizing tank size increases the potential for thermal energy storage, which will allow a user to take maximum advantage of utility load-management programs or time-of-use electric rates.

• Take Size Tip #2: Upsize the tank if the occupants are likely to have high hot water draw periods, such as a household with teenage children or occupants with a preference for baths.

   

Tank Size Guidelines (Gallon Capacity)

Number of Bathrooms	1 to 1.5			2 to 2.5				3 to 3.5
Number of Bedrooms	1	2	3	2	3	4	5	3	4	5	6
FHR per Uniform Plumbing Code	38	49	49	49	62	62	74	62	74	74	74
ENERGY STAR NextGen Minimum Tank Size	36	45	59	45	59	72	72	59	72	72	72

Design Considerations for Multifamily Buildings

There is no one-size-fits all application option for heat pump water heaters in multifamily buildings. There are several design features that must be considered when planning to apply Heat Pump Water Heaters in multifamily buildings. The architect and mechanical designer must engage early in the design process to account for these needs. 

The best system for a specific building will depend on a variety of factors including climate, number of units, type and size of units, number of floors, presence of a parking garage, roof access, balcony presence/absence, and interior or exterior corridors. Heat Pump Water Heaters are most effectively integrated into multifamily buildings if considered from day one of the design process.

There are two primary domestic hot water system types for multifamily buildings: in-unit serving one dwelling unit per water heater or a central system serving all or part of the building. In-unit Heat Pump Water Heaters are typically located in a closet within the apartment or just outside it. A variation on this approach is using a single 80-gallon Heat Pump Water Heater to service two to four dwelling units.

Central systems heat and store hot water in a central location, such as a mechanical room, and use a recirculation loop to distribute the hot water. Unlike in-unit Heat Pump Water Heaters, with central systems there is no need to figure out how to keep the cool exhaust air from impacting occupant comfort. A downside to central systems is that they must pump water long distances, resulting in energy losses of 30 to 50%.

Venting Heat Pump Water Heaters into Corridors

Due to the smaller size of most apartments, for in-unit Heat Pump Water Heaters, exhausting cool air into the living space can potentially cause comfort problems, especially in heating dominated climates. One solution to prevent comfort issues is to locate the Heat Pump Water Heater in a (properly ventilated) closet off the corridor where heat can be harvested and cool air rejected. If the Heat Pump Water Heater is accessed from the corridor, code will typically require fire-rated assemblies between the Heat Pump Water Heater closet and the dwelling unit. These fire-rated assemblies will have the benefit of providing of additional sound proofing between the unit and the water heater.

When a heat pump water heater is actively operating (in heat pump mode), it generally is providing 2,500 to 5,000 Btus/hr of cooling. This will nominally increase the amount of heating needed for the space, and decrease the required cooling. Designers should account for this additional cool air when sizing heating and cooling systems when multiple Heat Pump Water Heaters reject cold air to the same space (e.g., a corridor).