Before you start: Tankless water heaters are combustion appliances. Shut the unit down for propane odor, exhaust problems, error codes that keep returning, flame rollout, soot or overheating.

A Girard tankless water heater heats water only while water is flowing. That makes diagnosis different: shower flow, faucet mixing, LP pressure and inlet temperature all affect what comes out of the tap.

Use enough hot-water flow

Tankless heaters need minimum flow to turn on and stay on. If the shower head is restricted, the faucet is barely open, or a filter is clogged, the unit may cycle or never fire consistently.

Stop mixing like a house shower

Many RV tankless systems work best when the heater temperature is set closer to the desired shower temperature and the hot side is opened enough to maintain flow. Heavy cold-water mixing can reduce hot flow through the heater and cause cycling.

Check propane supply under demand

A stove flame proves there is some propane, not that the regulator can support the water heater under load. Weak LP pressure can create ignition failure, temperature swings or shutdowns.

Check 12V power and error codes

The control board, fan and ignition system need stable 12V power. Record any display code before resetting. Low voltage, poor grounds and loose connectors can cause odd behavior.

Account for cold inlet water

In cold weather, tankless units may need lower flow to reach the target temperature. If the inlet water is very cold and flow is high, the water may be warm but not hot even when the heater is working.

Tools, difficulty and likely cost

  • Difficulty: Beginner for observation, cleaning and reset checks; professional for live 120V, propane pressure, sealed refrigeration or internal control testing.
  • Useful tools: Display/error-code photo, Water-flow check, Propane level, 12V voltage reading.
  • Likely cost: Flow and setting corrections are usually free; LP pressure, fan, burner or control-board diagnosis requires service.

Related RV Solver pages

FAQ

Why does my Girard tankless water heater go hot then cold?

Common causes include low or unstable water flow, too much cold-water mixing, LP pressure problems, low 12V voltage or a control/error condition.

Does a tankless RV water heater need a certain flow rate?

Yes. Most tankless heaters need minimum flow to activate and stay lit. Restrictions can cause cycling.

Why is it worse in cold weather?

Cold inlet water requires more heat rise. Lowering flow and setting the heater correctly can help, but gas supply and unit capacity still matter.

Still narrowing it down?

The guided troubleshooter walks through the symptom in a safe order and points you toward the right system.

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