Heating water with heat pumps is much more efficient than other water heating technologies. Using an electric heating element to keep a tank of water warm can be used as the baseline for efficiency. A poorly insulated tank constantly loses heat, so insulating the tank can increase overall water heating efficiency by 7% to 16%. Removing the tank and heating water on demand can increase efficiency by 24% to 34%. Upgrading to a heat pump water heater can increase efficiency by 300% to 400%—way more than other technologies. Electric heat pumps are so efficient that even direct solar heating is no longer competitive compared to using solar photovoltaic electricity to power a heat pump to heat water.

Two That Matter Most

To know how well you are meeting your need for hot water without wasting energy or creating unnecessary pollution, these two indicators matter most:

  • Whether you burn fuel to heat water.

  • How much energy you use.

Sustainable Practice: Heating 301

Heating water with heat pumps

Heat domestic water with electric heat pumps.

Equipment and Materials

  • Electric heat pump water heater

Steps

  1. Determine your demand for domestic hot water.

  2. Determine where to install an electric heat pump water heater.

  3. Select an electric heat pump water heater that will satisfy your demand and fit into the space where you will install it.

  4. Provide ventilation for the water heater.

  5. Provide drainage for the water heater.

  6. Provide electrical power to the water heater.

  7. Remove the old water heater (optional).

  8. Plumb in the new heat pump water heater.

Discussion

Heating domestic water is most efficiently done with a heat pump, which uses an electric pump to circulate refrigerant through heat exchangers. This pumps heat from a source, such as the air around the water heater, to the water inside it.

Air-source heat pumps extract heat from air; water-source heat pumps extract heat from water. Air-source heat pumps are the most common, found in refrigerators, air conditioners, and water heaters. Water-source heat pumps are rare in residential settings but can be used to provide cool water for air conditioning and hot water for showers and washing.

Heat pump water heaters that meet ENERGY STAR efficiency standards are eligible for a 30% federal income tax credit. Maine and other states also offer additional rebates for the purchase and installation of these efficient appliances.

Low-power heat-pump-only water heaters:

  • heat water slowly

  • can only operate in heat pump mode

  • have no other way to heat water besides the refrigerant loop

  • take 120 V electrical power

  • can often simply plug into an outlet

High-power hybrid heat pump water heaters:

  • can heat water quickly

  • can operate in heat pump or high-power mode 

  • have both a refrigerant loop and one or two high-power resistive electric elements

  • require 240 V (single phase) or 208 V (three phase) electrical power

  • require hard-wired dedicated circuits

Before heat pumps, the standard water heating technology burned fuel, such as coal, oil, propane, or natural gas, or used resistive electric elements to keep a tank of water warm. Those inefficient tank designs suffered from standby heat loss, as any heat that leaked out of the tank could not be put back in. Insulating the tank could increase efficiency by about 7% to 16% by reducing the rate of heat loss.

One way to solve the standby loss problem of water heating is to eliminate the tank. Tankless on-demand water heaters are 24% to 34% more efficient than inefficient tank designs because they don’t suffer standby losses.

Heat pump water heaters, by contrast, are 300% to 400% more efficient than inefficient tank designs. Most of the heat that leaks out of an efficient heat pump water heater is recaptured and put back into the tank by the heat pump. In operation, the tank is usually kept warmer than ambient air, so energy naturally flows from the tank to the air around it. A heat pump reverses the normal flow of energy. A fan draws in warm air through a heat exchanger, which extracts energy from the air and puts it back in the water.

When cold water is added to the tank of an air-source heat pump water heater, a fan must draw sufficient air through the heat pump so it can extract enough energy from the air to warm up the water. If a heat pump water heater is installed in a room lacking ventilation, the space will become a walk-in cooler, and the water heater's efficiency will drop.

The fan in a heat pump water heater draws in humid, warm air. When that air is cooled, it loses its ability to hold vapor, so water condenses in the heat exchanger. Most heat pump water heaters provide a line for this condensate to drain. The amount of condensate produced depends on the humidity of the air surrounding the water heater and the rate of water heating.

One of the main environmental concerns with electric heat pump water heaters is the type of refrigerant used. Common refrigerants such as norflurane (R134a) have greenhouse gas warming potentials much higher than carbon dioxide. In normal operation, the refrigerant remains in a pressurized tube. The risk is that the tube will develop a leak, causing the heat pump to stop working and allowing the refrigerant to escape into the atmosphere, where it can persist and trap heat.

As one do-it-yourself guide states, “Installing a new electric water heater may seem like an intimidating task, but with the right tools and knowledge, it can be accomplished by most homeowners.” However, before attempting this project you should check with your codes enforcement officer to determine if your local jurisdiction requires hiring a licensed plumber or electrician to do the work for you.

Definitions

  • Condensate: water that condenses from air during the operation of a heat pump

  • Domestic water: water used for washing and showering

  • Greenhouse gas warming potential: the potential of a gas to persist in the atmosphere and absorb radiation, thereby warming the surface of the Earth; carbon dioxide has a greenhouse gas warming potential of 1

  • Heat pump: a system that uses electricity to pump refrigerant in a loop through a heat exchanger to move heat from one place to another

  • Hybrid water heater: a heat-pump water heater that has both a refrigerant loop and a resistive heating element

  • Refrigerant: a substance which can be depressurized to absorb heat and pressurized to release heat

Troubleshooting

  1. Your utility closet doesn’t provide enough airspace for a heat pump water heater:

    1. Consider putting ventilation louvers in the door to your utility closet to allow air to flow in and out.

    2. Talk with your plumber about another location to install your water heater.

  2. Your electric service doesn’t provide enough amps for a 240 V electric water heater.

    1. Consider installing a low-power 120 V heat pump-only electric water heater.

    2. Upgrade your electrical service to provide more amps.

  3. You don’t have a place for condensate to drain near your water heater.

    1. Install a condensate pump to pump condensate to a drain.

  4. You aren’t sure what maintenance is required for a heat pump water heater.

    1. Read the manual.

    2. Plan to clean the air filter at least once a year.

    3. Plan to inspect the condensate drain hose and clean it out as necessary.

Strategies and Goals

  • Community

    • Demonstrate best practices

      • Inspire people in your community to heat water with heat pumps

  • Energy

    • Increase energy efficiency

      • Heat water more efficiently

Milestones

  • Decrease energy consumption

    • Measure: Fuel and utility bills

    • Method: Records

    • Time Period: Year

Limitations

  • Requires money to purchase a heat pump water heater.

  • Heat pump water heaters may be a different size or shape than the existing water heater.

  • Installing an electric heat pump water heater is slightly more complicated than installing a non-heat-pump electric water heating due to the airflow and drainage requirements. If you lack the skills or tools to do plumbing or electrical wiring, you may want to hire a plumber and an electrician to install the unit.

  • Some jurisdictions may require a licensed plumber to install the plumbing for a new water heater.

  • Some jurisdictions may require a licensed electrician to install the wiring for a new water heater.

Opportunities

  • Plumbing 101

    • Install water-wise plumbing fixtures

    • Use less hot water

  • Cleaning 103

    • Wash clothes in cold water

    • Use less hot water

References

Keywords

water heating, heat pumps, hybrid water heaters, electrification, efficiency