Quick answer
If your tankless water heater is frozen, do not try to fire it. Turn it off at the controller, close the gas valve and the cold-water valve, and leave it plugged in so the built-in freeze heaters keep working. Warm the room and pipes slowly, then check the heat exchanger and fittings for splits before the gas goes back on. Any drip means a service call.
What you should do if your tankless water heater is frozen comes down to one rule: thaw first, inspect second, fire last. Water expands by about 9% as it turns to ice, and the thin copper or stainless tubing inside a tankless heat exchanger is built to move heat, not to resist that pressure. A split often stays hidden until the ice melts. That is why the order of the steps matters more than the speed.
What to do right now, in order
- Switch the unit off at its controller but leave it plugged in. The electric freeze-protection heaters inside the cabinet run whenever the unit has power, whether it is switched on or not.
- Close the gas shut-off below the unit. The handle sits crosswise to the pipe when it is closed.
- Close the cold-water isolation valve feeding the heater, then open a hot tap somewhere in the house. The open tap relieves pressure as ice melts and tells you the moment flow comes back.
- Warm the space, not the metal. A portable heater set a metre or two back, or a hair dryer moved along the exposed pipes, does the job; a torch or heat gun can melt the plastic vent, cook the gaskets and start a fire inside a wall.
- Start at the tap end of a frozen pipe and work back toward the heater so meltwater has somewhere to go.
- When water runs at the hot tap again, open the cold isolation valve slowly with the gas still off and look for drips before anything else happens.
Why a frozen tankless should not be fired
A fully frozen unit usually will not light anyway, because no water moves past the flow sensor and the control board never calls for flame. The risk comes at partial thaw. If the burner lights against an ice plug, the trapped water can flash toward steam, and if the exchanger has already split, the first flow sprays water across the burner, the fan and the circuit board.
After the thaw, inspect before you restore gas: the heat exchanger coil, the inlet and outlet fittings, the isolation valves, the pressure relief valve and the floor of the cabinet. Run a dry paper towel along each joint. Manufacturers generally exclude freeze damage from the warranty, so a split exchanger is paid for out of pocket; check your own warranty card for the wording.
Where tankless water heaters usually freeze
- The condensate line. A condensing tankless makes water as it runs, and a drain line routed through a cold garage or out through a wall freezes shut, backs up into the trap and locks the unit out. On Rinnai condensing models that shows as code 25; codes vary by model, and the label inside the cover wins. The furnace version of the same problem is in what to do if a furnace condensate line is frozen.
- Water lines on an exterior wall. The pipes above and below the unit hold still water against cold brick or a garage wall, so they freeze before the heater does. You will often have full cold flow at the taps and nothing on the hot side.
- Cold air through the vent while the unit has no power. In an outage the freeze heaters are dead, and wind, a running dryer or a range hood can pull outdoor air down the intake and straight across the heat exchanger. This is how units freeze inside a house that still feels warm.
An ice storm that knocks the power out can set up all of these at once. If an outage is the cause, why a tankless will not restart after a power outage covers the restart sequence once the unit is thawed and dry.
How to stop a tankless freezing again
- Keep it powered all winter. Do not unplug the unit or switch its breaker off when you leave for a week; the freeze heaters need the outlet.
- Add drain-down valves if outages are common where you live. These are solenoid valves that spring open when the power fails and empty the heat exchanger without anyone being home. Rural properties in Caledon, Campbellville and Flamborough, where the well pump stops in the same outage, are the usual candidates.
- Insulate the hot, cold and condensate lines, and add thermostat-controlled heat cable where a line has to cross a garage or crawl space.
- Have the vent termination checked. Intake and exhaust need the clearances and slope that the installation manual and the CSA B149.1 gas code call for, and an intake facing the prevailing wind on an exposed wall is worth shielding or relocating.
- For a property left empty in winter, drain the unit by the manual: gas and water off, isolation-valve drain caps open, inlet filter out.
What a freeze repair costs, and when to call
Call before restoring gas if you find any of these:
- Water dripping from the cabinet or any fitting after the thaw.
- An error code that returns after one power cycle.
- A hissing or spitting sound when the water valve opens.
- A gas smell at any point. Leave the house and call Enbridge Gas at 1-866-763-5427 from outside.
A diagnostic visit is $90–$150 and is folded into the repair if you go ahead. A thawed condensate line, a replaced fitting or a new valve falls inside the usual $150–$800 repair band. A split heat exchanger is priced on a written quote because the part differs by model, and on an older unit we will put that number beside a new tankless at $3,500–$6,500 installed so you can compare.
ZK Mechanical services Navien, Rinnai and Bosch units from the Burlington shop, and a frozen heater with water on the floor is an emergency call on the 24/7 line with no after-hours surcharge. See tankless water heater repair for what the visit covers, then call (647) 801-1252 or send the details through the contact page.