How to Choose and Maintain a Heat Pump Water Heater

By Lianna Schwalenberg

Heat pump water heaters take center stage on calls for energy management, carbon reduction legislation

Heat pump water heaters are targeted as a solution to energy management and carbon reduction called for in recent legislation. U.S. Congress along with the Department of Energy (DOE) finalized a range of energy-efficiency standards for residential domestic water heaters in April, which will require manufacturers to meet new and amended efficiency standards, required on and after May 2029, a mandate expected to save American households about $7.6 billion per year on utilities, and significantly cut energy waste and carbon pollution.

The current standards for water heaters, which are specified in the Code of Federal Regulations (CFR) in 10 CFR 430.32(d), have been in effect since 2010. One aspect of the upcoming changes is the slight increase in the Uniform Energy Factor (UEF), the measure of a water heat’s overall efficiency. The higher the UEF the more efficient the water heater is (10 CFR Part 430, Subpart B, Appendix E). For example, the UEF of a 55-gallon electric storage water heater with a medium draw pattern will increase from about 2.11 to 2.50. The DOE estimates that by 2029 more than half of all newly manufactured electric water heaters will use heat pump technology.

A basic understanding of this type of appliance will help industry professionals and consumers to navigate water heater choices and decide on solutions that best fit their needs.

 

Traditional Electric vs. Heat Pump

Heat pump water heaters are a sub-category of electric water heaters because they are powered completely by electricity. However, they have the added component of using the refrigeration cycle. Most heat pump water heaters are “hybrid,” meaning they have backup electric-resistance heating elements to keep up with high heating demand.

The CFR defines a water heater as a product that uses oil, gas, or electricity to heat potable water upon demand, and breaks water heaters down into three sub-categories: storagetype, instantaneous-type, and heat pump type. They define the latter as a product with “a maximum current rating of 24 amperes at a voltage no greater than 250 volts, which are designed to transfer thermal energy from one temperature level to a higher temperature level for the purpose of heating water, including all ancillary equipment such as fans, storage tanks, pumps, or controls necessary for the device to perform its function.” Basically, through the use of a compressor, expansion valve, two coils, and a fan, heat is absorbed from the surrounding space, creating hot water and, as a byproduct, a cooling effect of the surrounding space.

 

How the Heat Pump Water Heater Works

Heat pump water heaters work just like a heat pump air conditioner in heating mode. The compressor compresses gas refrigerant into the condensing coil where the refrigerant, at high pressure, changes from gas to liquid, transferring heat into the heat sink, which is water in the case of a water heater. The high-pressure liquid refrigerant then moves into the metering device where its pressure drops as it enters the evaporator coil. A fan draws air across the evaporator to cause the low-pressure refrigerant to change from a liquid to a gas, and the process starts all over again at the suction side of the compressor. All components are electric, including the metering device. Unlike a heat pump used for home heating and cooling, a heat pump water heater does not have a reversing valve. A reversing valve is not necessary because water heaters are expressly designed to add heat to water, not remove it.

The evaporator and fan are typically packaged with the water heater, which causes the surrounding space to feel cold. However, some manufacturers make “split system” heat pump water heaters which have outdoor units that look similar to a mini-split. With split systems, the compressor is outside, and it uses heat from the outside air to heat the water. According to Energy Star, split system heat pumps can operate in almost any climate, with the ability to extract heat from outdoor temperatures as cold as -25 degrees. At subfreezing temperatures, however, split heat pump water heaters will have longer run times and may have longer recovery times.

Visually, the configuration of a heat pump water heater is different from a standard water heater. The evaporator and fan are mounted on the top of the heater, with the cold-water inlet and hot-water outlet located on the side. Also on the side are two panels to access upper and lower heating elements and thermostats. Most heat pump water heaters have a digital display keypad with options to connect to the customer’s Internet and cell phone for customized control.

Some heat pump water heaters are pre-installed with a mixing valve, or it can be purchased as an accessory, at the water outlet to mix cold inlet water with the hot outlet to achieve more precise temperature control.

While the standard water temperature many heaters can achieve is 140 degrees, the DOE recommends 120 to save energy. To achieve this, most heat pump water heaters require installation in locations with at least 700 to 1,000 cubic feet of air space around the water heater.

Photo courtesy of Bradford White

Before Purchasing a Heat Pump Water Heater

Choosing a heat pump water heater is a similar process to choosing any water heater. Buyers should first determine the fuel source and whether they want it to be a storage tank style or instantaneous (tankless). Heat pump water heaters are the most efficient style of electric water heaters, and they can be storage style or tankless. To further improve cost-efficiency, other factors to consider include the desired First Hour Rating (a more useful metric than nominal capacity, how much hot water a full unit can deliver in the first hour of use) or GPM (gallons per minute) for instantaneous heaters and peak hour demand. The DOE has an online tool to help estimate how many gallons of hot water a home uses during peak demand. Buyers might also be interested in efficiency ratings, such as COP (coefficient of performance for heat pumps, typically 2.5-4.5) and the aforementioned UEF (typically 3.3-4.1 for heat pump water heaters).

 

Installation and Maintenance Considerations

To help with this cursory introduction to heat pump water heaters, Matt Oakes, owner of Modern HVACR, demonstrated how he maintains his Ruud Ultra Series Hybrid water heater, which he installed in his basement to convert his home to all-electric. Oakes said he is already seeing a return on his investment. For installation and maintenance, Oakes recommends checking with the manufacturer’s installation manual for specific instructions, but generally, a few things to consider while making installation decisions specific to heat pump-style heaters include the following:

• Condensate water trapping and removal.

• Space requirements for proper air flow and heat absorption – extremely small and cold spaces will not be conducive to heat exchange.

• Because heat pump water heaters have a fan, they tend to be slightly louder than a standard water heater.

• Whether to purchase a duct kit and where to exhaust the waste cooling.

 

Maintenance Considerations

Every manufacturer publishes their recommendations for maintenance periods and instructions in their installation manuals. Oakes strongly encouraged owners to educate themselves and call a trained professional when needed. Below are some procedures common among most manufacturers in periodically maintaining a heat pump water heater.

• Cleaning the washable filter.

• Flushing the condensate drain line.

• Draining or flushing the tank.

• Inspecting the anode rod, replacing only if necessary.

• Inspecting the heating elements, replacing only

if necessary.

• Exercising the temperature pressure relief valve by a professional.

Lianna Schwalenberg is an HVAC Service Technician at the K Company in Akron, Ohio. She specializes in residential service and commercial maintenance. For inquiries, she can be reached at lschwalenber@uwalumni.com.

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