Quick answer
Tankless water heater repair in the GTA typically costs $150–$800, plus a $90–$150 diagnostic visit that is credited toward the repair. Igniters, flame rods and flow sensors sit at the low end; fan motors and control boards at the high end. A $150–$300 descale flush clears many overheating faults. A failed heat exchanger is the exception: check the manufacturer's warranty before paying for one.
Tankless water heater repair cost is set by which part failed, and the unit usually tells you: nearly every fault puts a code on the display. Most repairs land in the $150–$800 range. The $90–$150 diagnostic comes first and is folded into the repair, so you do not pay twice to find out what is wrong.
Tankless water heater repair price ranges
- Diagnostic visit: $90–$150, credited toward the repair if you proceed.
- Descale flush for scale-related faults: $150–$300.
- Part replacements such as an igniter, flame rod, flow sensor, valve, fan or control board: $150–$800 depending on the part and the model.
- Heat exchanger replacement: quoted in writing after the warranty is checked, and weighed against a new tankless installation at $3,500–$6,500.
- Evenings, weekends and holidays: no surcharge at ZK Mechanical.
Ranges for other work are on our pricing page.
Common tankless repairs and what each one involves
- Igniter and flame rod: the igniter sparks the burner and the flame rod proves the flame is lit. Both foul with combustion residue. Often they can be cleaned and regapped; replacement means opening the burner chamber and renewing its gasket. Low end of the range.
- Flow sensor: a small turbine on the cold inlet that tells the board water is moving. Scale or grit jams it, and the unit fires late or not at all. The job is a drain-down and a swap. Low to middle.
- Water-adjustment valve: a motorized valve that throttles the flow to hold the set temperature. When it sticks you get temperature swings or weak flow. Middle of the range.
- Fan motor: the combustion fan wears at the bearings or clogs with lint. Replacement includes resealing the air path and a combustion check afterward. Middle to high.
- Control board: boards fail from power surges, moisture or age. The part is most of the bill, and the new board has to be set up for the model and gas type. High end.
- Heat exchanger: a scaled exchanger can usually be recovered with a flush. One that leaks has to be replaced, and it is the most labour and the most expensive part in the unit.
Why the error code sets the diagnosis before any price
A price quoted over the phone without the code is a guess. The code names the step in the firing sequence that failed, which narrows the parts list before a technician opens the cover.
- Navien E003, ignition failure: igniter, flame rod, gas supply or venting. Our Navien E003 page walks through it.
- Navien E016, heat exchanger overheating: most often scale or restricted flow.
- Rinnai 11, no ignition, and Rinnai 12, flame failure: gas supply, igniter, flame rod or a venting problem.
- Rinnai LC, scale build-up in the heat exchanger: the unit is asking for a flush. See Rinnai code LC.
Codes vary by model, and the label inside the front panel is the authority. Write the code down before resetting, along with what was running when it appeared.
When a descale alone clears the fault
On hard Halton and Hamilton water, overheating and scale codes are often fixed by the $150–$300 flush with no parts at all. That is the likely outcome when the unit is a few years old, has never been flushed, and shows E016 or LC along with rumbling or fluctuating temperature. A technician confirms it by comparing inlet temperature, outlet temperature and flow before and after the flush. If the code returns straight after a proper flush, a sensor or the exchanger itself is the problem.
Heat exchanger failure is the repair-or-replace line
Before paying for a heat exchanger, check the manufacturer's warranty. Residential heat exchanger coverage is often 10 to 15 years, but usually for the part only: labour coverage is much shorter, and registration, installation by a licensed contractor and water quality can all be conditions. Have the model, serial number and install date ready. Our Navien tankless repair page notes what to look for on that brand.
The rule we use at the kitchen table has three parts:
- Exchanger part still under warranty: repair it. You pay labour, not the part.
- Out of warranty, with the unit past about 12 years or the quote above half the cost of a new unit: replace it.
- Any other failed part on a unit under about 12 years old: repair it. Igniters, sensors, fans and boards are normal wear items.
- Rented unit: the rental company pays for repairs, so call them first.
Is it safe to wait on a tankless repair?
A unit that has locked out on a code is safe to leave off: the lockout is the safety system working. Do not keep resetting a unit that bangs on ignition, leaks water from the cabinet or smells of gas. Shut it off and book the visit.
ZK Mechanical services Navien, Rinnai and Bosch tankless heaters with written pricing before work starts. What the visit includes is on our tankless water heater repair page; send the model and the error code through the contact page so the technician arrives knowing where to start.