Tankless Gas Water Heaters: Which One to Buy

How big a tankless heater do you need?

Add up the hot water you use at once. A shower is about 2 gallons a minute, so two at once is about 4.

Then check the flow at your winter temperature rise. Water comes in colder in winter, so the heater must add more heat and can push less water. The DOE test rates flow at a 67°F rise (58°F in, 125°F out). Many boxes print the flow at a 35°F rise, which can be about twice as high.

Gas units are sized by burner: a 199,000 BTU unit needs a gas line and meter that can carry it, and an indoor unit needs its own vent.

Questions buyers ask

What size tankless water heater do I need?

Size on the certified flow at your winter rise. Two showers at once is about 4 gallons a minute; the gas units here certify 4.4 GPM at the 67°F test rise. The bigger number on the box is usually at a 35°F rise.

Is a gas tankless heater cheaper to run than a tank?

Usually somewhat, because it does not keep a tank of water hot all day. The certified number that compares them is UEF: higher costs less to run.

Can I install a tankless water heater myself?

A gas unit should go in with a licensed plumber: it needs the right gas line size, venting for an indoor unit, and often a permit.