What size water heater do I need?

Quick sizing by household

Ohio State's guide gives these starting points for a storage tank:

• 1 or 2 people: 30 to 40 gallons
• 3 or 4 people: 40 to 50 gallons
• 5 or more: 50 to 80 gallons

CenterPoint Energy's guide adds a rule of thumb: start with a 30 gallon tank for two people with one bathroom and a washer, then add 3.5 gallons for each extra bathroom and 5 gallons for a dishwasher. These are starting points. Big tubs, rain showerheads and teenagers change the math.

The better way: your busiest hour

The first hour rating, or FHR, is how much hot water a heater can supply in an hour starting with a full hot tank. Rheem's guide to the EnergyGuide label shows where to find it. CenterPoint says to pick your household's busiest hour, add up the hot water used in it, and choose a heater with an FHR close to that total.

Its activity chart, from the Minnesota Blue Flame Gas Association, lists a shower at 3 gallons a minute, a clothes washer at 25 to 40 gallons a load, a dishwasher at 5 to 10 gallons, and a tub bath at 15 to 25 gallons. So two 5-minute showers and a dishwasher load in the same hour come to roughly 35 to 40 gallons, and you'd want an FHR around 40 or a little more. CenterPoint suggests leaving 10% to 15% to spare.

Why a 40 can beat a 50

Recovery matters as much as storage. CenterPoint notes that a gas heater with a bigger burner can heat incoming water faster, so a powerful 40 gallon tank can outperform a basic 50 gallon tank during back-to-back showers. Ohio State notes that gas heaters generally have a better FHR and recovery than electric ones, which lets them use smaller tanks. That's why two heaters with the same tank size can have very different first hour ratings.

Bigger isn't automatically better. CenterPoint says an oversized tank wastes energy keeping unused water hot, while an undersized one runs constantly and can wear out early.

Sizing a tankless water heater

A tankless heater has no stored hot water, so it's sized by flow. A Department of Energy Building America guide lists typical flows of 1.5 to 2.5 gallons per minute for a showerhead, 0.5 to 1.5 for a bathroom sink, 1.0 to 2.5 for a kitchen sink, and 2.0 to 4.0 for a bathtub. Add up the fixtures you run at the same time.

Then account for how much the unit has to warm the water. The guide says the maximum flow and temperature rise depend on the heater's capacity, and ENERGY STAR measures tankless flow ratings over a 67°F rise. Colder incoming water in winter means fewer gallons per minute at the same setting. In towns near NOAA's Bridgewater, Massachusetts station, which averages about 141 nights a year at or below freezing, that matters (see our Hanson page). Our tank vs. tankless guide covers the trade-offs.

Space and special cases

The heater has to fit where the old one was, with room for venting and service. In a manufactured home, the heater must also be listed for that use. Dacono, Colorado, where about one in five homes is a mobile or manufactured home, is a good example (see our Dacono page). If you keep running out of hot water with a heater that should be big enough, the problem may be the heater, not the size. See hot water runs out fast.

Local pages mentioned

• Hanson, MA
• Dacono, CO