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Water Heaters · Tankless RepairTankless Water Heater Repair & Descaling
Tankless water heater repair deals with faults a tank never had: codes on the front panel, hot water that drops out mid-shower, and a heat exchanger closing up with scale. Most calls come down to a descale, a plugged inlet screen or a fouled flame rod. A descale flush runs $150–$300, and a diagnostic visit is $90–$150.
Scale is the leading cause of tankless failure in this part of Ontario, and it is a slow one. Every litre passing through a condensing heat exchanger leaves a little calcium behind, the passages narrow, water velocity through them rises, and the unit starts overheating and modulating oddly. The symptoms arrive in order: slightly weaker flow at the tap, then temperature swinging, then a lockout code. Our note on hard water and tankless units across Hamilton and Halton explains why local supply makes this a scheduled maintenance item, not a rare event.
A descale is a straightforward service when the unit has isolation valves. We connect a pump across the service ports, circulate descaling solution through the exchanger for the better part of an hour, then flush it and clean the inlet water filter screen — a fine mesh that catches sediment and is a common cause of low flow after a watermain repair on your street. Where the original installer left the isolation valves off, which happens on rushed conversions and some builder installs, we fit them, and every future service gets cheaper. The routine is covered in tankless water heater maintenance.
Not everything is a fault. The cold-water sandwich — hot, then a slug of cool, then hot again when you turn the tap back on shortly after using it — is how these units behave and is not something to repair. So is the minimum activation flow: a heavily aerated bathroom faucet at a trickle may not draw enough water to fire the burner at all, which reads as no hot water at one fixture and fine everywhere else. Why a tankless goes hot then cold separates the normal from the broken.
The genuine faults we replace most often are flame rods and igniters, water flow sensors and thermistors, and condensate components — including the neutralizer cartridge on condensing models, which turns to a solid plug when nobody replaces the media. Winter adds one more: a sidewall intake or exhaust terminal drifted over with lake-effect snow will shut the unit down on a pressure fault, and it is a five-minute fix once you know to look. If the panel is showing a code, our Navien and Rinnai repair pages tell you what it points to.
Call for tankless service when
- An error code is showing on the front panel or the bathroom remote controller
- Hot water starts, drops out for twenty seconds, then returns
- The flow has quietly dropped off and the water no longer gets as hot as it used to
- The unit has never been descaled and has been running on local water for years
- Water is dripping from the bottom of the cabinet or from the condensate line
How it works
The code on the panel plus the model number off the label narrows it down before we arrive, so the right part is on the truck instead of on order.
Combustion and gas pressure checked, flame rod and inlet screen cleaned, venting inspected, and the exchanger flushed if scale is what is behind the symptom.
Isolation valves fitted if they are missing, a service label with the date on the cabinet, and a realistic descaling interval for your water rather than a generic one.
Brands: We service Navien, Rinnai and Bosch tankless water heaters, with the same TSSA-licensed gas technicians who install them.
Common questions
How often should a tankless be descaled?
Once a year on untreated local water. With a working softener you can usually stretch it to every second year. Skipping it entirely is the single most reliable way to shorten the life of an expensive heat exchanger.
Is my unit worth repairing?
Usually, yes — these are built to be serviced, and most parts are available. The judgement call comes when the heat exchanger itself is scaled through or leaking on an older unit; at that point we put the repair number beside a replacement number and let you compare.
Can I descale it myself?
If it has isolation valves, a submersible pump, a bucket and descaling solution, it is a job a comfortable homeowner can do. What you cannot do without instruments is verify gas pressure and combustion afterward, and those are the readings that catch a developing problem.
Why does one tap run cold when the rest are fine?
Almost always minimum flow. A low-flow aerator or a partially opened valve may not move enough water to trigger the burner. Open the tap further and see if it fires — if it does, the unit is working exactly as designed.
Does the annual service actually pay for itself?
On the water here, yes. A descale is a modest scheduled cost against a heat exchanger that is a large fraction of the price of a new unit, and a serviced unit holds its output rather than slowly delivering less. Our maintenance plans fold it into a schedule so it does not get forgotten.