Propane tankless water heaters: what to know before you buy

In homes without natural gas service, a propane tankless water heater is often the most practical gas option. It delivers endless hot water like any tankless heater, but the fuel side needs more planning.

Propane vs natural gas models

• Different gas, different parts. Heaters are built for either natural gas or propane, and some can be converted with a manufacturer kit. Running a unit on the wrong gas causes combustion codes and is unsafe.
• More energy per cubic foot. Propane carries roughly two and a half times the energy of natural gas per cubic foot, so pipe sizing differs.
• Same maintenance. Descaling, filters and venting are the same.

Sizing the propane supply

A whole-house tankless heater can burn a large amount of gas at full fire. The propane supplier should confirm that the tank, regulators and gas line can deliver that demand together with your other propane appliances.

• Tank size. Larger tanks vaporize propane faster. A small tank may not keep up on a cold day.
• Cold weather. Propane vaporizes more slowly as temperatures drop, and a low tank level makes it worse. Ignition or flame failure codes on cold mornings often trace back to this.
• Regulators. Most homes use two-stage regulation; the supplier sets it up.

Common problems

• Ignition failure codes when the tank is low or the weather is very cold
• Temperature swings when several propane appliances run at once
• Combustion codes after a conversion that was not done exactly per the kit

Our tankless water heater installation cost guide covers typical prices, and the gas line installation cost guide covers line upgrades.

Propane vs electric tankless

In homes without natural gas, the other tankless option is electric. Whole-house electric tankless units need very large electrical capacity, often several dedicated high-amperage circuits, which can mean a panel upgrade. Propane units need a suitable tank and venting instead. Your climate matters too: in cold regions, where incoming water is very cold, gas units usually deliver more hot water per minute than electric ones.

Questions to ask your installer

• What flow rate will this unit deliver in winter at our incoming water temperature?
• Has the propane supplier confirmed the tank and regulators can supply the heater plus our other appliances?
• What venting will it need, and where will it terminate?
• Where will the condensate drain, if it is a condensing model?
• Will isolation valves be installed for future descaling?

Maintenance

Propane units need the same yearly care as natural gas units: descaling, cleaning the inlet filter, checking the venting and, on condensing models, the condensate line. See our guide to descale a tankless water heater.