Tankless water heater: no hot water

Q: Tankless water heater: no hot water

In Canada, when a tankless (on-demand) water heater stops making hot water, the first thing to check is the display: most units show an error/fault code that names the problem, and a single reset often clears a one-time fault. On a gas unit, confirm the gas is on and other gas appliances work. A clogged inlet screen or low flow below the unit's activation threshold is a common, owner-cleanable cause; in hard-water areas, scale buildup calls for a descale/flush. If the unit won't ignite or you suspect a flame-sensor or heat-exchanger fault, that is licensed work — a gas unit routes to a gas-fitter. If you smell gas near a gas unit, stop, leave, and call your gas utility from outside.

A tankless unit that stops making hot water is usually telling you why — most start with a code on the display and an easy first check, not a failed heater.

tuneWhat are you noticing?

Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.

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Start here

In Canadian homes, read the display first. Most tankless units show an error/fault code that names the problem; note it, then power-cycle the unit once (off at the switch or breaker for a minute, back on). A one-time fault often clears. If the same code returns, look it up in the manual — it points you straight at the cause. On a gas unit, if you smell gas at any point, stop and call your gas utility.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order

Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

warning Hot water and a small spillThe water in the unit can be hot, and opening the inlet screen lets a little out — keep a bucket and a towel handy and let the unit cool if it's just been running. This is a low-risk job on the water side only; do not open any gas or combustion component. If the unit keeps faulting after a clean screen, hand it to a pro rather than forcing it.

Run a hot tap fully open and listen: if the unit fires only at full flow, or a flow/ignition code appears at low flow, the inlet screen or a throttled valve is the prime suspect.
Close the cold-water isolation valve under the unit, then unscrew the inlet water filter/screen (put a bucket underneath — a little water will spill).
Rinse the screen clean of grit and scale, reseat it, reopen the isolation valve, and retest. While you're there, confirm all inlet valves are fully open and check faucet aerators for blockage.

buildadjustable-wrenchbuildbucket schedule15 min

verifiedWhy this one goes to a pro

A tankless unit that won't ignite, won't hold a flame, or is throwing a heat-exchanger fault involves live gas, combustion, and venting — get any of it wrong and you risk a gas leak or carbon monoxide. On a gas unit this is work for a licensed gas-fitter (and on an electric unit, an appliance tech or electrician for the heating side). Leave the covers on and book a pro.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

handymanHow to fix it

With the unit powered off and cool, connect a submersible pump and hoses to the service (isolation) valves, circulate a descaling solution (white vinegar or a food-safe descaler) through the heat exchanger for the time in your manual, then flush with clean water and restore service.

Turn the unit off (switch or breaker) and shut the hot and cold isolation valves so no household water flows through during the flush. Let it cool.
Attach hoses from a submersible pump sitting in a bucket to the cold service port, and a return hose from the hot service port back into the bucket. Open only the two service (purge) valves.
Fill the bucket with descaling solution — food-safe descaler or plain white vinegar per your manual — and run the pump to circulate it through the heat exchanger for the time the manual specifies (commonly 45–60 minutes).
Drain the solution, refill the bucket with clean water and flush for several minutes, then close the service valves, remove the hoses, reopen the hot and cold isolation valves, and restore power. Run a hot tap to confirm output.

schedule75 min savings$25–60

Tankless descaling / flush kitPump, hoses, and bucket; use a food-safe descaler or plain white vinegar — check your manual for the approved solution Tankless service valve (isolation) kitOnly if your unit has no service valves; must match the unit's connections — a plumber's job to install

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: licensed professionals only
Descaling/flushing on the water side is generally owner-permitted; the ignition/flame-sensor/heat-exchanger and any gas or venting work is licensed-only (gas-fitter). Confirm the provincial rule.

sort

Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.