Size a tankless water heater from two numbers: the most hot water your household draws at once, in gallons per minute, and the temperature rise from your coldest incoming water to the outlet temperature you want. Pick a unit whose manufacturer flow table delivers that flow at that rise. Then confirm the house can feed it. Gas units need gas-line and venting capacity, and whole-house electric units commonly need 208 or 240 volts and 50 to 200 amps. The method below follows the U.S. Department of Energy’s Building America measure guideline on tankless water heaters.

Step 1: Add up peak flow

List the fixtures and appliances that could run at the same time, such as two showers and the dishwasher, and add their flow rates. DOE’s guideline gives these typical flow rates (from the ASPE Domestic Water Heating Design Manual):

Fixture or appliance Typical flow rate
Bathtub 2.4 to 4.0 gpm
Shower 2.5 gpm
Kitchen sink 2.5 gpm
Laundry sink 4.0 gpm
Dishwasher 1.9 gpm

Use the actual rating on your fixtures where you know it. A WaterSense showerhead, for example, flows no more than 2.0 gpm. Count the overlap your household really has, not every fixture in the house.

Step 2: Work out the temperature rise

Temperature rise is the outlet temperature minus the incoming water temperature. DOE’s guideline says the desired output is 120°F for most uses, and to use the minimum incoming temperature, which your water utility can provide. Winter inlet water decides the size. DOE’s own example: with 65°F incoming water and a 105°F shower, a 2.0-gpm shower needs a unit that delivers a 40°F rise at 2.0 gpm. With colder inlet water the same shower needs a bigger rise, and the same unit delivers less flow.

Step 3: Read the manufacturer’s flow table

Manufacturers publish tables showing how many gallons per minute a model delivers at each temperature rise. Find your peak flow at your maximum rise. If a model only meets it at its limit, step up a size or plan a separate point-of-use unit for a remote fixture.

Step 4: Check what the house has to supply

  • Gas units. DOE says to confirm the gas line can supply the unit’s Btu input, sized to the manufacturer’s recommendations and the plumbing code. In a retrofit, the gas line may need to be enlarged. Venting and combustion air also need to suit the model. Condensing units make condensate that must be drained properly.
  • Electric units. Most need 208 or 240 volts and 50 to 200 amps, may need multiple circuits, and sometimes a larger electrical service. DOE notes that in cold regions a whole-house electric unit can be costly to run. Electric point-of-use units work well for individual rooms and fixtures.
  • Cold-water supply. Tankless units typically need a 3/4-inch or 1-inch supply line, and an older house may have only a 1/2-inch main. Size it to the manufacturer’s specification.
  • Power outages. A tankless unit’s electronic controls don’t work during an outage, gas models included. DOE mentions a battery backup (UPS) sized for the unit as an option.

Step 5: Plan for hard water and flushing

Scale builds up in the heat exchanger, restricts flow and shortens the unit’s life. DOE says to flush on the manufacturer’s schedule, which can range from twice a year to once every three years. If supply hardness is above about 8 to 14 grains per gallon (136 to 239 ppm), manufacturers may require a water softener. Test your water before you buy, and leave service valves and clear space for flushing.

What to expect day to day

  • A short delay. DOE describes a 1- to 3-second lag between water starting to flow and the burner or elements firing, on top of the time hot water takes to travel through the pipes.
  • A minimum flow to start. Each unit needs about 0.4 to 0.9 gpm to fire, so a trickle from a faucet or a very low-flow fixture may not turn it on.
  • Brief temperature dips. Rapid on-and-off use can send a few seconds of cooler water, which DOE calls the “cold water sandwich.” It’s uncommon in steady use.
  • Service life. DOE’s guideline says tankless units are generally considered to last up to 20 years, compared with 10 to 15 years for tank types.

Quote checklist

Ask each installer to state, in writing:

  • the flow rate the unit delivers at your winter temperature rise;
  • gas line, venting, condensate, or electrical work, itemized;
  • supply pipe size and any upgrade;
  • isolation and flushing valves, and the flushing interval for that model;
  • permit, inspection and removal of the old heater.

Make sure every quote uses the same peak flow and inlet temperature.

Sources

This is desk research. The DOE guideline dates from 2012, so check current model specifications and local code before you buy.