Tankless water heater not heating: causes and what to check

No hot water at all

Noritz's troubleshooting guide lists gas supply interruptions, power problems, ignition failure, closed water valves and system errors as the usual causes. Its checks, in order:

1. Gas supply valve fully open. On propane, Rinnai adds, make sure the tank actually has gas.
2. Power on and breaker not tripped. A tankless gas unit still needs electricity for its controls, fan and igniter.
3. Water supply valves open.
4. Read the error code, then reset as the manufacturer directs. If the code comes back, call a technician.

On Rinnai units, code 11 is an ignition failure and code 12 means the flame went out. Rinnai's diagnostic manual starts both with the gas: valves open, propane in the tank, air purged from new gas lines, the right gas type and enough inlet pressure. It notes that a loose flame rod or electrode can also cause either code.

Hot at the shower, cold at the sink

A tankless heater uses a flow sensor to decide when to fire. Rinnai says its tankless gas water heaters need at least 0.4 to 0.6 gallons per minute through the hot side to ignite, and about 0.26 gallons per minute to stay on. A trickle from a bathroom faucet, or a low-flow aerator, may never reach that. Open the tap further and see if it lights.

When there's no error code at all, Rinnai's manual lists more flow-side causes: a clogged inlet water filter, hot and cold lines reversed, a crossover somewhere in the plumbing or a fixture that lets cold water into the hot line, or a flow sensor turbine that doesn't spin freely.

Lukewarm, or hot then cold

Noritz lists mineral scale, restricted flow, the temperature setting and several fixtures running at once as causes of water that swings between hot and cold. Its fixes: check the set temperature, clean or replace clogged water filters, descale the heat exchanger if maintenance is overdue, and try one fixture at a time to see whether you're asking for more hot water than the unit can make.

Scale is the big one in hard water areas. Rinnai's units display LC (or LC0 to LC9) when scale is building up in the heat exchanger, and it's the only code that lets the heater keep running. The fix is flushing the heat exchanger, and Rinnai says hard water must be treated to prevent it coming back. Coeur d'Alene, Idaho averaged 8.25 grains per gallon in its 2025 water report, which it rates as hard (see our Coeur d'Alene page), and Nampa reports up to 7.08 grains per gallon (see our Nampa page).

Not enough gas

Rinnai's manual says that if the inlet gas pressure is below what the rating plate lists, the gas system may be undersized. On natural gas it allows no more than a 0.3-inch water column drop with every gas appliance in the building at high fire, and 0.5 inch on propane. The problem can be the pipe size, the regulator, the meter, the propane tank or debris in the line. If the water only goes lukewarm while other gas appliances are running, the supply is the first thing to check.

Shuts off mid-shower

Noritz says unexpected shutdowns usually come from safety systems: a blocked vent, overheating protection, gas supply problems or dirty combustion parts. Its checks: look for obstructions at the vent termination, make sure there's airflow around the unit and inspect the gas connections. Rinnai's troubleshooting steps for airflow faults add checking the fan and looking for blockage in the heat exchanger.

When to call

Gas pressure tests, flame rods, igniters and flow sensors are jobs for a technician. Rinnai's own manual warns you must be qualified to service gas systems before going further. Our tankless water heater repair page covers what we see most. If scale is the cause, see tank vs tankless for how upkeep differs.

Local pages mentioned

• Nampa, ID
• Coeur d'Alene, ID