Things to Consider
Larger tank sizes are recommended in households with more than two occupants as they provide a buffer in times of high demand, avoiding the need for electric resistance heating and resulting in more efficient operation.
To properly size a storage water heater, use the water heater's first hour rating. The first hour rating is the number of gallons of hot water the heater can supply per hour starting with a tank full of hot water. The number is determined by the tank capacity, source of heat (burner or element), and the size of the burner or element.
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For more information on heat pump water heaters, view the course Heat Pump Water Heater Basics. As always, be sure to consult manufacturer-specific information that pertains to the product you are installing.
For heat pump water heaters, look for the rating either on the label or in the product literature from a manufacturer. Look for water heater models with a first hour rating that matches within one or two gallons gallons of the peak hour demand -- the daily peak one-hour hot water demand for the home.
To estimate the peak hour demand:
Determine what time of day (morning, noon, evening) the occupants use the most hot water in the home. Keep in mind the number of people living in the home. Then estimate the amount of hot water used during that time. As a rough rule of thumb, a shower uses about 10 gallons of hot water, shaving uses 2, food prep or hand washing dishes uses roughly 4 gallons, running the dishwasher uses 6, and running the clothes washer uses 7 gallons.*
*Water use information is from the Energy.Gov Sizing a New Water Heater discussion and is based on information from the Federal Energy Management Program Energy Cost Calculator.
The Right Location
Conditioned Space: It is not advisable to place HPWHs in conditioned space without exploring the potential impact. In locations with potential cooling benefit, HPWHs may be placed in conditioned space, although special consideration must be used when deciding the best location for the HPWH. The cooling from these units may lead to over-cooling of the space because the operation of the unit is not controlled by a thermostat. Follow these guidelines when choosing a proper location for the HPWH:
- Never place the unit close to a thermostat, as this may result in improper heating or cooling of the home.
- Never place the unit near the kitchen. Oils from cooking can ruin the unit.
- Place the unit in a location that is not sensitive to colder temperatures.
- Make sure to meet the manufacturer's space requirements. Do not place the unit in a closet unless the closet door has a louvered door. Even with a louvered door, locating the HPWH in a closet will likely reduce overall performance of the unit.
- Many units are also taller than conventional water heaters so be sure you have enough vertical clearance.
- Noise may be an issue because the HPWH uses a compressor and fan to move air through the unit. Do not place the unit near bedrooms or other noise-sensitive locations.
Unconditioned Space: In most U.S. climates, HPWHs can be placed in unconditioned or semi-conditioned spaces. The most common semi-/unconditioned locations are garages and basements. Crawlspaces are generally too short for HPWHs, and HPWHs are typically not recommended to be placed in attics (because of potential for water leaks, weight of unit, ambient temperature that fluctuates outside the heat pump's operating range, etc.).
The primary consideration for unconditioned spaces is the size of the room and the ambient temperature of the space. The minimum temperature of the space should not be below 50°F. Operation of the HPWH will result in a significant drop in ambient temperature, and such a drop may result in an air temperature below the minimum recommended operating temperature in heat pump mode. HPWHs with electric resistance modes may be placed in colder basements, but the HPWH will not operate at its rated efficiency during colder months. In more temperate climates, the attached garage can be a suitable location as long as plumbing lines are not too long and can be properly insulated. In colder climates, however, the basement will likely have the highest temperature of all unconditioned spaces.