Tankless Water Heaters - Gas

Gas tankless water heaters from 120,000 to over 200,000 Btu/h, in natural gas, propane and convertible, condensing and non-condensing.

A tankless heater has no storage, so it is sized by flow at a given temperature rise, not by gallons. The same unit that delivers 6 gpm in Texas delivers barely 3.5 gpm in Minnesota, because incoming groundwater is 30°F colder and the burner has a fixed maximum output. Size from your local winter inlet temperature and the fixtures that must run at once — two showers and a dishwasher is a different unit than one bathroom.

Condensing versus non-condensing decides the installation, not just the fuel bill. A 90%+ condensing unit extracts enough heat to condense flue gas, so it vents in PVC or polypropylene and needs a condensate drain with neutralization, since the condensate is acidic. Non-condensing units need stainless category III venting. Neither vents into an existing atmospheric flue.

Gas line sizing is the constraint that stops most retrofits: a 199,000 Btu/h unit typically needs 3/4" and often a meter upgrade. Check supply before ordering.

Brands include Rheem, Navien and Noritz.

Popular in this collection: Navien NHW700-180AI-NG Compact Tankless with Recirculation Pump.

Frequently asked questions

What size gas tankless water heater do I need?

Size by flow and temperature rise, not by gallons. Add up the fixtures that will run at the same time and check the unit's flow at your winter temperature rise. As a rule of thumb, a 199,000 Btu/h condensing unit delivers roughly 5 gpm at a 70°F rise.

Condensing or non-condensing?

Condensing units (90%+ efficient) vent in PVC, CPVC or polypropylene and need a condensate drain with a neutralizer. Non-condensing units need stainless steel Category III venting. Neither can use an existing atmospheric flue.

Can I switch between natural gas and propane?

Some models are field-convertible with the manufacturer's kit, and others are sold as separate natural gas and propane versions. Check the fuel type in the product title before ordering.

Shop by fuel or brand: propane tankless water heaters, Navien, Rheem and Noritz. Install with tankless valve kits, and see our guide to sizing a tankless water heater.