Tankless Water Heater Not Igniting

Not igniting, or not staying lit?

These are different faults and they lead to different pages, so it is worth thirty seconds to tell them apart.

• It never lights. You open a hot tap, the fan spins up, you hear the igniter clicking, and no flame arrives. That is this page.
• It lights and then drops out. You get hot water for a few seconds or a few minutes and then it goes cold, often mid-shower. That is flame loss — same components, different failure, and the manufacturers give it its own code number.
• Nothing happens at all — no fan, no click, no display. That is not an ignition problem; start at no hot water at all.

Before anything else

If you smell gas at any point: leave the building and call your gas utility from outside. Do not restart the unit, do not open panels, and do not use light switches or a phone inside the building.

A tankless heater that will not light is a gas appliance that has decided not to light. Repeatedly pressing reset to force it is not troubleshooting — every manufacturer’s ignition checklist ends in gas pressure measurements and combustion checks that need instruments and a licence. Two failed attempts is the point to stop and call someone.

The four things that have to line up

1. Flow. A tankless unit will not fire below a minimum flow rate, so a half-open valve, a clogged inlet filter, or a restricted aerator can leave you with a heater that is working perfectly and never lights. This is the one people skip, and it is the only one on this list that is fully yours to fix. Open the tap wider and see whether the burner catches; clean the inlet water filter.

2. Gas. Is gas reaching the building at all? Try a range burner or another gas appliance — if nothing gas-fired runs, the problem is upstream of the heater and not worth diagnosing at the unit. Check the main gas supply valve at the heater is open, handle in line with the pipe: this is Navien’s own first instruction for an ignition failure, and Rinnai’s too. On propane, check the tank gauge. An empty or undersized propane tank in cold weather is one of the most common causes of this entire symptom.

3. Spark. The igniter has to produce one, and the ignition circuit has to be getting the right voltage. Not a homeowner check — it means unplugging the igniter lead on a live gas appliance.

4. A flame the unit can see. Even when gas lights, a flame rod coated in carbon, loose in its bracket, or wired to the wrong position will report no flame and the unit will shut down as if it never lit. Rinnai names a loose flame rod — a “floating flame rod” — as a direct cause of both an ignition code and a flame-failure code on its older units.

What you can safely check

1. Open the hot tap fully and confirm you are getting a real flow, not a trickle.
2. Clean the inlet water filter if it has not been done recently.
3. Confirm another gas appliance in the house works.
4. On propane, check the tank level.
5. Confirm the gas and water isolation valves at the unit are open — a valve closed during other work and never reopened is a common cause after a plumbing or boiler visit.
6. Go outside and look at the vent terminal. Snow, a nest, leaves, or a dryer vent blowing into it will stop combustion, and on some units that shows as an ignition fault rather than a vent code.

That is the whole of the homeowner-safe list. Everything past it — inlet gas pressure, manifold pressure, the igniter, the flame rod, the gas valve — is professional work on a gas appliance.

Cold weather makes this worse

Ignition failures cluster in the first hard freeze of the year, for reasons that have nothing to do with the heater: propane tanks that were fine in October cannot keep up with demand in January, gas systems shared with a furnace see pressure drops when everything fires at once, and vent terminals get blocked by snow.

If your unit is reporting that it could not fire while running freeze protection, treat it as urgent rather than routine. Rinnai gives that its own number on the older generations — code 02 — and the reason to act quickly is not the ignition fault but what follows it: a heat exchanger that cannot protect itself from freezing is a far more expensive repair.

If your display shows a code

Go straight to it. The code turns “it will not light” into one specific circuit, and the answer differs by brand and by series — the codes below are the ignition faults each manufacturer documents.