No Hot Water from a Tankless Water Heater

Start with the display

If there is a code on the controller, stop reading general advice and go straight to that code — it narrows the problem from “the heater” to one circuit or one component. Use the search box at the top of this page, or pick your unit from the brand pages.

No code and no hot water is a genuinely different problem, and the rest of this page is about that case.

Is anything reaching the heater?

Power. A tankless heater is a computer with a burner attached. Check the breaker and, where fitted, the GFCI outlet it is plugged into. A blank controller usually means no power rather than a dead unit.

Gas. Does anything else gas-fired in the house work — a range, a furnace? If not, the problem is upstream of the heater. On propane, check the tank. If you smell gas at any point: leave the building and call your gas utility from outside.

Water flow. This is the one people miss. A tankless heater does not fire below a minimum flow rate, so a partially closed valve, a clogged inlet filter, or a sediment-blocked aerator can leave you with a unit that is working perfectly and never lights. Manufacturers’ own no-code troubleshooting starts here: check the flow rate, clean the inlet water filter, confirm the isolation valves are fully open.

The plumbing traps

Two classics, both of which look exactly like a broken heater:

• Hot and cold reversed. Common after any plumbing work, and permanent until someone corrects it.
• A crossover. A failed mixing valve, a bad check valve, or a single- handle faucet with a worn cartridge lets cold water bleed into the hot side. The heater fires normally and the water still arrives lukewarm. The test: shut off the cold supply to the heater, then open a hot tap. If water keeps running, you have a crossover somewhere in the house, not a heater fault.

When to stop and call someone

• The unit shows a code involving gas, ignition, exhaust, or overheating
• You smell gas — leave first, call from outside
• The unit lights and shuts down repeatedly
• You have checked power, gas, and flow and it still will not fire

Repeatedly resetting a gas appliance that has decided not to light is not troubleshooting. Two failed attempts is the point to stop.