Heat Pump Water Heaters

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Components of a heat pump water heater
Components of a heat pump water heater
Scope

Select heat pump water heaters as an energy-efficient alternative to electric-resistance water heaters.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

Description

A “heat pump” is a device that moves heat from one place to another. A HPWH moves heat from the surrounding space into the hot water tank. Most heat pumps operate as “hybrid” devices – i.e. they use the heat pump whenever possible, but built-in controls switch to conventional resistance heating when there are large hot water needs.  While a HPWH is an efficient unit on its own, it does impact the surrounding area so the final determination of viability is dependent on the site conditions.

A heat pump water heater (HPWH) is an efficient electric water heating option (Energy Factor ratings of greater than 2.0), but the actual operating efficiency of the unit is highly variable depending on several site specific factors. HPWHs are primarily designed as alternatives or replacements for standard, electric resistance water heater tanks (ERWHs), but they should also be evaluated when oil or propane are the water heating fuels. 

The HPWH products currently available in the market range in tank volume (gal), first-hour capacity ratings (gal), efficiency (EF), as well as physical size and weight. When selecting a water heater size, the first hour rating (FHR, listed in gallons) is probably the most important factor. The FHR is the amount of hot water that a water heater can deliver in a single hour (when starting with a full tank of hot water). The FHR should be equal to (or greater than) the highest volume of water typically used during any hour of normal operation in the home. While hot water use varies widely from home to home, some guidelines for first hour ratings in “typical” homes are shown in the table below. For more detailed analysis, see U.S. Department of Energy's "Sizing a New Water Heater" site which provides a worksheet to estimate target FHRs.

Guidelines for FHR ratings for "typical" homes.
Guidelines for FHR ratings (gallons) for “typical” homes. Source: Consumer Energy Center
Energy guide
Location of the FHR rating on an Energy Guide.

The efficiency of HPWHs is profoundly affected by the way hot water is used. When large quantities of hot water are used in clusters, HPWHs will revert to electric resistance mode, reducing the efficiency of the unit. A homeowner can reduce this effect by purchasing a larger HPWH, increasing the set point temperature, or changing behavior. By increasing the size and temperature of a HPWH, more hot water can be delivered at a given time before the resistance elements are needed. Spreading the water load over a longer period of time may also provide similar benefits and reduce standby losses.

HPWHs are not ideal for all applications; they remove heat from ambient air, generate more noise than other water heaters, and have special space and clearance requirements. Because they remove heat from the surrounding air, they impact the building loads of the home.  In colder climates especially, these impacts should be considered before installing a HPWH.

How to Locate and Install Heat Pump Water Heaters

  1. Select the best location for the HPWH. Determine the interaction with cooling and heating equipment. Should the unit be placed in conditioned space, semi-/unconditioned space, or in an attached garage? Hot climates will have a net cooling benefit, while cold climates will have a net heating penalty. Generally, homes in climate zones 1 and 2 have a net cooling benefit. For climate zones 1 and 2, installation in an attached garage or conditioned space may be appropriate.
  2. Select Location.
    Sufficient room volume (750 to 1000ft3) YES NO
    Adequate ambient temperature ( > 50ºF) YES NO
    There is sufficient space to meet clearance requirements YES NO
    Noise will not interfere with living spaces YES NO
    Condensate can be removed effectively YES NO
  3. Can the floor support the unit? If not, reinforce as necessary.
  4. Removal of older equipment (if applicable). In the case of existing home retrofits, follow accepted industry procedures and practices as listed in the Standard Work Specification (SWS):
    1. Remove old water heater and associated components
    2. Seal any unused chimney openings
    3. Remove unused oil tank, lines, and associated equipment.
  5. HPWH installation. Follow the guidelines for installation as listed in the Standard Work Specification (SWS) . These requirements are as follows:
    1. Repair any existing water leaks before installation.
    2. Install an emergency drain pan. Connect a ¾” drain line or larger to tapping on pan and run to drain or pump to daylight.
    3. Install temperature and pressure relief valve according to manufacturer specifications. Temperature and pressure relief valve discharge tube will terminate within 6” of the floor, or as prescribed by local code.
    4. Install di-electric unions according to manufacturer specifications.
    5. Discharge temperature will be set to not exceed 120° or as prescribed by local code.
    6. Commissioning will be in compliance with manufacturer specifications and relevant industry standards. The following will be checked once the system has been filled and purged:
      • Safety controls
      • Combustion safety and efficiency
      • Operational controls
      • Water leaks
      • Local code requirements
    7. Occupants will be educated on the safe and efficient operation and maintenance of the system, including:
      • Adjustment of water temperature and target temperature per local code
      •  Periodic drain and flush

In addition to the requirements outlined by the SWS, remember to install the following items for any water heater:

  • Place the unit on blocks
  • Install a condensate pump, if needed
  • Install heat traps to prevent thermosiphoning
  • Install tempering valves, if needed
  • Insulate hot water lines
Climate

A HPWH moves heat from the air surrounding the water heater into the storage tank, so a HPWH will have an impact on the space conditioning loads of the building in which it is located. When a HPWH is located in conditioned space, there is a very direct effect on space conditioning loads and possibly on comfort. In the summer, the HPWH will reduce the cooling load of the building; conversely, the heating load will be increased in the winter. A HPWH’s energy consumption and its impact on the space conditioning depend heavily on climate, home configuration, HPWH location, and the space conditioning systems used.  Regardless, the cooled air from the HPWH must be managed to ensure that there are no comfort issues for the occupants.

For HPWHs installed in unconditioned or “semi-conditioned” spaces such as basements, the space conditioning impacts are harder to determine. These spaces act as buffer spaces, so the heat transferred from the space into the storage tank is not necessarily transferred from the conditioned spaces. Air infiltration into the buffer space, ground coupling, solar gains, and heat transfer from mechanical equipment can all affect heat transfer between the buffer space and conditioned space. The transient effects of these heat transfer processes means that the HPWH space conditioning impacts are potentially reduced. Research is ongoing to better quantify the space conditioning impacts of HPWHs.

Research has determined that while the cooling/dehumidification benefits and higher COPs of an HPWH are more advantageous in the hot-humid climate, incoming water temperature tends to be higher. The incoming water temperature can be 85°F or higher for portions of the year. This means less water heating is required, which can minimize the cost benefit of the HPWH in hot-humid climate zones.

The technology, when applied appropriately, is still a better option than alternative electric/propane/oil water heating solutions, but overall savings expectations should be more conservative to account for space conditioning impacts and variations of performance from rating conditions.

The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide. 

IECC climate zone map
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC).

Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC). 

 

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Federal Water Heater Standard

The new Federal water heater standard, which takes effect in 2015, mandates EFs around 2.0 for all new electric storage water heaters with capacities greater than 55 gallons (Federal Register 2010). This regulation will effectively mandate the use of HPWHs in applications with large hot water demands and where electricity will be used to heat water.

ENERGY STAR Water Heater Key Product Criteria

To be ENERGY STAR labeled, electric water heaters must have an Energy Factor ≥ 2.0 and a First-Hour Rating ≥ 50 gallons per hour.  Based on the available electric water heating products in the market today, this can only be achieved by HPWHs. In addition, the electric water heaters must have a warranty ≥ 6 years on sealed system and comply with UL 174 and UL 1995 safety requirements.

DOE Zero Energy Ready Home Notes

The DOE Zero Energy Ready Home National Program Requirements, Exhibit 2, states water heaters should be ENERGY STAR certified. Homes following the Prescriptive Path must meet this requirement. Homes following the Performance Path must meet the performance level of a target home built to the requirements shown in Exhibits 1 and 2.

Homes equipped with an ENERGY STAR qualified whole home gas tankless water heater or an ENERGY STAR qualified heat pump water heater are exempt from the solar water heating requirements of the Renewable Energy Ready Checklist of the Zero Energy Ready Home National Program Requirements.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 1, Item 4) Hot water delivery systems (distributed and central) shall meet efficient design requirements or water heaters and fixtures shall meet efficiency criteria. 

Exhibit 2 DOE Zero Energy Ready Home Target Home.
The U.S. Department of Energy’s Zero Energy Ready Home program allows builders to choose a prescriptive or performance path. The DOE Zero Energy Ready Home prescriptive path requires builders to meet or exceed the minimum HVAC efficiencies listed in Exhibit 2 of the National Program Requirements (Rev 07), as shown below. The DOE Zero Energy Ready Home performance path allows builders to select a custom combination of measures for each home that is equivalent in performance to the minimum HERS index of a modeled target home that meets the requirements of Exhibit 2 as well as the mandatory requirements of Zero Energy Ready Home Exhibit 1.

Exhibit 2, Water Heater) ENERGY STAR levels for the system Energy Factor, as follows:

  • Gas/propane systems of ≤ 55 gallons, EF = 0.67
  • Gas/propane systems of > 55 gallons, EF = 0.77
  • Electric systems in detached dwellings, EF = 2.0
  • Electric systems in attached dwellings, EF = 1.5
  • For heating oil water heaters use EF = 0.60 

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies
Author(s)
Colon,
Martin
Organization(s)
BA-PIRC
Publication Date
Description
The Florida Solar Energy Center (FSEC) has completed a fourth year-long evaluation on residential hot water heating systems in a laboratory environment (east central Florida, hot-humid climate). This report contains a summary of research activities regarding the evaluation of two residential...
References and Resources*
Author(s)
Shapiro,
Puttagunta
Organization(s)
CARB,
Steven Winter Associates,
SWA
Publication Date
Description
Research study evaluating 14 heat pump water heaters, attempting to provide publically available field data on new HPWHs by monitoring the performance of three recently released products .
Author(s)
Shapiro,
Puttagunta,
Owens
Organization(s)
CARB,
National Renewable Energy Laboratory,
Steven Winter Associates,
SWA
Publication Date
Description
Document with evaluation of 14 heat pump water heaters (including three recently released HPWH products) installed in existing homes in the northeast region of the United States.
Author(s)
Steven Winter Associates
Organization(s)
Steven Winter Associates,
SWA
Publication Date
Description
Brochure providing consumer information about heat pump water heaters.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Home Innovations Research Labs, lead for the Partnership for Home Innovation (PHI), a DOE Building America Research Team

Building Science Measures
Building Science-to-Sales Translator

Heat Pump Water Heater = High-Efficiency Heat Pump Water Heater

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Technical Description

According to the Energy Information Administration, water heaters can account for 19% to 32% of a homeowner's utility bill. One option to reduce the amount of energy needed for water heating is to install a heat pump water heater. Although they use electricity, they can be two to three times more energy efficient than conventional electric-resistance water heaters. Heat pump water heaters operate on the same principle as the whole-house air source heat pumps, which move heat with electric compressors and pumps. However, instead of heating and cooling homes, they move heat from the surrounding space into the water tank.

High-Efficiency Heat Pump Water Heater
Sales Message

High-efficiency heat pump water heaters minimize wasted energy producing hot water. What this means to you is all the hot water you want at lower cost. Isn’t it time homes used advanced technology components?

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