Best Tankless Water Heater by Fuel, Flow and Home Size

Add the GPM of the fixtures you run at once and multiply by your temperature rise. Two 2.0 GPM showers plus a 1.5 GPM faucet is 5.5 GPM, and at a 70 F rise that needs about 5.5 x 500 x 70 = 192,500 BTU of delivered heat. That is why 180,000 to 199,000 BTU gas units are the standard choice for a family home in a cold climate.

Heating 64 gallons a day by 60 F takes about 32,000 BTU, or 9.4 kWh, per day. On electric tankless at 17 cents per kWh that is about $580 a year; on gas at 95% efficiency and $1.50 per therm it is about $185 a year. A heat pump tank with a 3.75 UEF would use about 910 kWh, roughly $155 a year at the same rate.

It is most worth it in homes that use moderate amounts of hot water. The US Department of Energy says tankless heaters are 24% to 34% more efficient than a storage tank in homes using 41 gallons or less a day, and 8% to 14% more efficient at 86 gallons a day. They also free up floor space and never run out mid-shower, but installation usually costs more than a tank swap.

The Department of Energy puts the life expectancy of most tankless water heaters at more than 20 years, against 10 to 15 years for a storage tank. Warranties reflect that: Rinnai and Rheem both list a 15-year heat exchanger warranty on the residential gas models here (Rinnai requires registration within 90 days).

Gas delivers more flow: a 199,000 BTU gas unit gives about 5.4 GPM at a 70 F rise, while a 27 kW electric unit gives about 2.6 GPM at the same rise. Electric needs no venting or gas line but draws 75 to 112.5 amps at 240V. In cold-water regions, electric whole-house units usually cover only one or two fixtures at once.

Divide the kW by 240 volts to get amps. An 18 kW unit draws 75 A and uses two 40 A double-pole breakers, a 24 kW unit draws 100 A on two 50 A breakers, and a 27 kW unit draws 112.5 A on three 40 A breakers. Stiebel Eltron recommends at least 150 A service for its 24 kW Tempra.

Rinnai and Rheem both call for professional installation of their gas units, and most areas require a permit. Gas models often need a larger gas line than the 40,000 BTU tank they replace, plus a new vent. Electric units need new 240V circuits sized by an electrician.

On electricity, the heat pump tank wins by a wide margin. A 3.75 UEF heat pump uses about a quarter of the energy of an electric tankless unit that runs near 0.98 efficiency, and it runs on a 30 A circuit. It needs about 1,000 cubic feet of space that stays between 40 and 90 F, per the Department of Energy.

Yes, it needs descaling. Most makers call for flushing the heat exchanger with a descaling solution about once a year, and more often with hard water, which builds scale fastest. Rinnai's RX series has a Quick Flush feature, and a water softener ahead of the heater cuts scale.

No. Gas and propane tankless heaters need 120V power for the fan, igniter and controls, so they stop when the power goes out. A small generator or portable power station on the heater's outlet keeps it running; an electric tankless unit drawing 18 kW or more is beyond any portable backup source.