Gas Tankless Water Heaters

A gas tankless heater's flow depends on how far it has to lift the water. The box figure is usually measured at a small rise; at a 70-degree winter rise a 199,000 BTU condensing unit delivers roughly 5 to 5.5 GPM. The size planner works it out for your inlet water and fixtures.

Condensing units recover heat from the exhaust, run cooler flue gas and vent in PVC or polypropylene, but need a condensate drain and usually a neutralizer. Non-condensing units cost less and vent in the maker's stainless system; outdoor models need no vent pipe at all.

Input decides the gas line. A 199,000 BTU unit asks the line for about 190 cubic feet of natural gas an hour at full fire, often more than an older half-inch line can carry once a furnace and range are on it. The requirement calculator shows the load; a gas fitter sizes the pipe.

Start from your hot-water demand

Install facts to confirm first

• Size it for January. Use your winter inlet temperature, not the box's maximum flow. The size planner works out the flow each unit holds at your rise.
• Check the gas line before the unit. A 199,000 BTU unit burns about 192 cubic feet of natural gas an hour at full fire. A licensed gas fitter sizes the pipe from its length and the total load.

Full gas tankless water heater spec grid

Figures are the makers' own, quoted with their conditions on each product page. Prices change; each product page links to the current price.