Facts about heat pump water heaters and if they are a good choice for your home.
As many as 40% of the homes in the United States use electricity to heat hot water for domestic use. Although heat pumps have traditionally been used for space heating and cooling are now also being used for electric water heating. They are about three times more efficient than electric resistance water heaters which mean they can produce the same amount of hot water for one-third the amount of electricity. A heat pump water heater is basically a standard electric water heater with a small compressor, evaporator coil and fan set on top. The units come equipped with electric resistance heaters for hot water heating during periods of high demand. The heat pump operates during off-peak usage times to maintain the water temperature and heat after times of use.
Stand-alone heat pump water heaters (HPWH) are different from heating and cooling heat pump systems that have integrated water-heating capability. You can add a desuperheater to your heat pump system that will take the waste heat from the compressor and heat water by way of an added heat exchanger. This article will describe the operation and maintenance considerations of stand-alone heat pump water heaters.
Some HPWH models only require 120V electricity, making them ideal for the "do-it-yourself" project. A HPWH will cost between $1,200 and $1,500 for the unit, which is about twice the cost of an electric resistance water heater. The typical pay back is 3 to 5 years and is dependent on usage and local utility rates. Installing the heater yourself will save between $300 and $700. Units will save between $200 and $275 on electric charges per year.
* Operating cost based on hot water needs for typical family of four and energy costs of 9.5¢/kWh for electricity, $1.40/therm for gas
There are four basic types of stand-alone HPWH. The heat pump can be integrated with or separate from the hot water storage tank, and cool exhaust air can be exhausted to the room or to the outside of the home. Because HPWH take heat from the surrounding room air, they cool and dehumidify the space they are in. This is a benefit during the cooling season and a drawback during the heating season. For this reason it is best that the HPWH be installed in larger rooms such as garages, utility rooms, or basements and not closets or laundry rooms since they require air circulation to work effectively. Dehumidification may also be useful if the unit is installed in a basement.
Diagram of Heat Pump Water Heater
A typical residential HPWH can heat 15 gallons of water per hour by 80°F, with a final storage temperature between 120°F and 140°F. At the same time, a HPWH provides some room cooling. During the heating season, this incidental cooling increases space heating needs.
Codes and Compliance
Heat Pump Water Heaters must conform to Underwriter’s Laboratories’ standard UL 1995.
In homes that have gas appliances or fireplaces, precautions should be taken to ensure backdrafting does not occur when using the exhaust air type heat pump water heaters. Consider relocating the HPWH to an area with adequate air circulation to avoid the use of exhaust. Alternatively, gas hot water heating appliances are more efficient than any electric model, so it would be more economical to install a gas water heater in lieu of a heat pump water heater.
Contact your local building official to determine if permits are required.
The first hour rating is the amount of hot water in gallons the heater can supply per hour, starting with a tank full of hot water. It depends on the tank capacity, source of heat, and the size of the burner or element. Product literature from a manufacturer should provide the first hour rating (FHR). Look for water heater models with a first hour rating that matches within a few gallons of your peak hour demand, the daily peak 1-hour hot water demand for your home.
Do the following to estimate your peak hour demand:
• Determine what time of day (morning, noon, evening) you use the most hot water in your home. Keep in mind the number of people living in your home.
• Use the worksheet below to estimate your maximum usage of hot water during this one hour of the day—this is your peak hour demand. Note: the worksheet does not estimate total daily hot water usage.
Adapted from Air Conditioning, Heating, and Refrigeration Institute.
Example: 3 Showers: 3 x 12 = 36 gallons
1 Shaving: 1 x 2 = 2 gallons
2 Hand washing: 2 x 4 = 8 gallons
Peak Hour Demand: 46 gallons
The installation is the same for a standard electric resistance water heater except for the addition of a condensate drain or pump and the location of the unit. As stated before, the room in which the heater is located must be large enough to support adequate air circulation, approximately 1000 cubic feet, a room 10 feet by 10 feet and 10 feet in height. Another option would be to add ventilation from adjacent rooms. A door with a louver or a sidewall grill open to an adjoining room is fine. Most units require about 8 to 10 inches of clearance above the unit.
Since the evaporator coil on the top of stand-alone water heaters or retrofit units cools the room air, condensation will form on the coil and most be drained away. Follow the manufacturers’ specifications, but you can either run the flexible tubing to a nearby drain, such as a laundry tub or drain or a sump pump pit. Condensate cannot be piped directly into a sewer line even with a trap installed. If no drain is nearby, you will need to install a condensate pump and run the discharge line to the nearest drain.
Since the units have an evaporator coil and fan, these are the primary components that require maintenance. All units come with a filter screen that is reusable and washable. Some filters are located on the side, while others are located on the top of the evaporator section. The filters should be checked and cleaned monthly.
The use of insulation blankets is not required on HPWH since they meet or exceed the National Appliance Energy Conservation Act standards with respect to insulation and standby loss requirements. They may also restrict the airflow around the unit. Pipe insulation is acceptable and recommended.
If a condensate pump is used, periodically check the pump to ensure proper operation. Every 3 months should be a fine. To check, pout about 2 cups of water into the receiver to activate the float. Make sure that the water is pumped out. Clogs will be visible if using clear vinyl tubing for the run to the drain. If you suspect a clog, remove the piping and flush it out outside with a garden hose.