What You Need to Know About Sizing Water Heaters
When purchasing a water heater, choose from tankless, solar, heat pump and traditional models. Here are some guidelines to consider when sizing each of these water heaters.
Sizing Tankless Water Heaters
With a tankless water heater, preheated water isn't stored in a tank. Instead, cold water water is immediately routed to a heater when hot water is needed, and the resulting hot water is sent directly to your faucet.
Tankless water heaters are rated by their flow rate -- this is the maximum amount of hot water they can produce, expressed as gallons per minute. To figure out what kind of flow rate you need, start by compiling a list of the hot-water appliances you expect people to use in the home simultaneously. For example, if it's likely that one person will be taking a shower while another is doing the dishes in the kitchen, factor in both the shower head and the kitchen faucet.
Next, figure out the combined flow rate of all the appliances on your list. This sum will be the level of flow rate your tankless water heater needs to provide. Using the previous example, if the flow rate on the kitchen faucet is 0.5 gallons per minute and the flow rate on the shower is 2 gallons per minute, you would need a water heater with a flow rate of at least 2.5 gallons per minute.
Sizing Solar Water Heaters
A solar water heater uses a roof-mounted solar panel to collect solar energy that is used to heat water. Like traditional water heaters, these heaters store water in a tank. Two factors need to be considered when sizing solar water heaters: the size of the solar panel's collector area and the size of the water storage tank.
The rule of thumb is to provide a collector area of about 20 square feet for the first two family members. For each additional family member, increase the collector area by 8 square feet in sunny parts of the country, or 12-14 square feet for more northern climes.
If your household includes up to three people, a 50- to 60-gallon storage tank should be adequate. For a home with three to four people, get a medium-sized storage tank that holds 80 gallons. If there are more than four people under your roof, purchase a large-volume storage tank. These tanks can hold 100 gallons or more.
Sizing Traditional and Heat Pump Water Heaters
Traditional and heat pump water heaters both heat and preheat hot water in a storage tank. To size these heaters, use the appliance's first hour rating; this is the number of gallons of hot water the heater can produce in an hour. This figure will be listed on the top left corner of the heater's Energy Guide label.
Choose a heater with a first hour rating that is within 1 or 2 gallons of your home's peak demand. To get a sense of the peak demand, use the following estimates:
- Shower: 10 gallons per hour
- Shaving: 2 gallons per hour
- Hand dishwashing or food preparation: 4 gallons per hour
- Automatic dishwasher: 6 gallons per hour
- Laundry: 7 gallons per hour
- 2 Showers (10 x 2 gallons): 20 gallons per hour
- 1 Hand dishwashing or food preparation: 4 gallons per hour
- Total gallons per hour: 24
Hiring a Plumber to Assist with Water Heater Sizing
Water heaters are typically installed by a plumber, who can assist with sizing the heater, as well. Choose a licensed plumber, since licensing indicates a certain level of competence. Select a plumber who's bonded and insured, which protects you if there is any property damage or injury while the water heater is being installed. The price of installation will depend on the type of water heater selected.