Gas Tankless Water Heater

A Gas Tankless Water Heater lights only when water moves. A flow sensor detects a tap opening, the controller modulates the burner anywhere from about 15,000 to 199,000 BTU per hour, and water passes once through a finned copper or stainless heat exchanger before it reaches you. Nothing is stored, so the standby loss of keeping 50 gallons hot all night disappears, and the wall unit occupies roughly the space of a carry-on bag. It needs 120 volt power for the fan and controls, which means no hot water in an outage.

Flow, not gallons, is the sizing number. Divide burner output by 500 times the temperature rise you need: a 199,000 BTU condensing unit puts out close to 7.5 gallons per minute lifting 70 F water to 120 F, and under 5 gallons per minute lifting 40 F water in a northern winter. Two showers at 2 gallons per minute plus a kitchen faucet at 1.5 already ask for 5.5, so a cold climate quietly costs you a fixture. Condensing models pull a second heat exchanger through the exhaust, reach a UEF above 0.90 and vent in plastic pipe. Non condensing models sit nearer 0.82 and need stainless steel category III venting.

Compare it against a Gas Storage Tank Water Heater honestly. Tankless costs two to three times more installed, because a 199,000 BTU burner is roughly five times a tank burner and the existing half inch gas line usually has to grow to three quarter inch or larger depending on run length and meter capacity. It cannot share the furnace flue. Condensate from a condensing model is acidic near pH 3, and some codes require a neutralizer before it enters a drain. A Heat Pump Water Heater beats it on running cost in most electricity markets; tankless beats it on endless supply and floor space.

Two habits keep one alive for 15 to 20 years. Insist on isolation valves with service ports at installation, then circulate 4 gallons of white vinegar through the exchanger for 45 minutes once a year on hard water, or every two to three years on soft. Above about 7 grains per gallon of hardness, some makers require treatment to hold the warranty. Expect a cold water sandwich too: shut a tap and reopen it within a minute, and the slug of water sitting in the exchanger arrives cool.