Tankless Water Heater Repair in Southern Ontario
No hot water, lukewarm water, or an error code on the display. We diagnose and repair tankless water heaters across Southern Ontario, including Navien, Rinnai, Noritz and A. O. Smith units.
Most tankless faults are one of a small number of things, and several of them are not the heater at all. Before anything is replaced it is worth knowing which group your problem is in, because the difference between a $0 reset and a new heat exchanger is usually diagnosis, not luck.
What Usually Fails on a Tankless Water Heater
| What you are seeing | Usual cause | What we check first |
|---|---|---|
| No hot water at all, unit shows an ignition fault | Gas supply, gas pressure, or a frozen intake or exhaust in winter | Confirm gas is on and the line is sized for the unit, check inlet pressure, inspect the vent terminal for ice or blockage |
| Hot water for a minute, then cold | Flame loss, or flow too low to keep the burner lit | Check inlet gas pressure and the vent system, then minimum activation flow against the fixture being used |
| Lukewarm water that never gets hot | Undersized for the winter temperature rise, or scale on the heat exchanger | Measure incoming temperature and actual flow, then descale and re-measure before concluding the unit is too small |
| Temperature swings while showering | A fixture mixing cold in, low flow, or a failing flow sensor | Measure flow at the fixture, then test the sensor rather than replacing it on suspicion |
| Unit runs but output is dropping over months | Scale build-up, which is the normal failure path on hard water | Descale the heat exchanger and discuss water treatment, because it will return otherwise |
| Error code with the unit still running | On Rinnai this is usually the scale warning, which is the one code that lets the unit keep working | Flush the heat exchanger before it becomes a replacement |
| Water at the base of the unit | Condensate drain blocked or frozen, or a failed fitting | Clear or re-route the condensate path and check the neutraliser, then pressure-test the fittings |
Two of those seven are not repairs. A unit that was always marginal for an Ontario winter was sized wrong, and a unit losing output over months has scale, not a fault. We say so when that is the answer, because replacing parts does not fix either one.
Navien Tankless Water Heater Repair
Navien NPE and NPE-2 units are among the most common tankless heaters in Southern Ontario homes, and three codes account for most of the calls.
| Code | What it means | What it usually is |
|---|---|---|
| E003 | Ignition failure | Gas supply not open, gas line undersized for the unit, or air in the line. In an Ontario winter it can also be a frozen intake or exhaust rather than a gas problem at all |
| E012 | Flame loss | Inlet gas pressure dropping under load, often when the furnace fires at the same time on a shared line |
| E110 | Abnormal air pressure | An obstruction in the exhaust. Ice at the vent terminal is the common January version |
The frozen-unit case is worth knowing before you call anyone. Navien notes that an ignition or air-pressure code in cold weather can mean the unit is frozen rather than faulty, and that clearing the fault may be a reset once it has thawed. If your unit went down during a cold snap and the vent terminal is iced over, that is the first thing to look at.
The NPE-A2 models carry the ComfortFlow® recirculation pump and buffer tank inside the unit, which is a real advantage when it works and an extra component when it does not. A recirculation complaint on an A2 is a different diagnosis from the same complaint on a unit with an external loop.
Code meanings are from Navien’s own NPE-2 user manual, checked 1 October 2026: Navien documentation. Always work from the manual for your exact model.
Rinnai Tankless Water Heater Repair
Rinnai SENSEI® and earlier RU and RL series units use numeric diagnostic codes. Four cover most of what we are called out for.
| Code | What it means | What it usually is |
|---|---|---|
| 11 | No ignition | Gas not on at the unit, meter or cylinder; wrong gas type or pressure; an undersized gas line, meter or regulator; air in the line; dip switches set wrong; or the appliance not properly grounded |
| 12 | Flame failure | Inlet gas pressure, or a vent problem. Loose joints and vent terminals too close to the building can recirculate combustion gases back into the intake, which causes incomplete combustion and throws a 12 |
| 10 | Air supply or exhaust blockage | Build-up between the heat exchanger fins restricting airflow, or a condensate drain that is clogged or frozen and backing up into the trap, which shuts the unit down to stop it spilling |
| LC or 00 | Scale build-up in the heat exchanger | The only code that lets the unit keep running. It is a warning, not a failure, and it means flush now rather than replace later |
Code LC is the one to act on. It is the only Rinnai code that does not stop the unit, so it gets ignored for months while scale keeps building. By the time output drops enough to notice, a flush that would have taken one visit can have become a heat exchanger. If your display is showing LC or 00, that is a maintenance call today and a replacement avoided.
Code 10 catches people out in winter for a reason that has nothing to do with the heater: a condensate line run to an outside wall or an unheated space can freeze, back up into the trap, and shut the unit off. The heater is fine. The drain is not.
Code meanings and causes are from Rinnai’s published tankless diagnostic-code troubleshooting guide, checked 1 October 2026: Rinnai technical documentation. Work from the guide for your exact model.
Noritz Tankless Water Heater Repair
Noritz is the one brand of the four that still sells non-condensing units alongside condensing ones, and that changes what goes wrong. The NR66 and NR71 are non-condensing; the NRC661 is condensing.
On a non-condensing Noritz there is no condensate trap to freeze, so the winter failures we see are gas supply and venting rather than drainage. On the condensing NRC661 the condensate path matters as much as it does on any other condensing unit.
Ignition components are the most common wear item we replace on Noritz units, and scale is the usual reason a Noritz starts under-performing rather than failing outright. The approach is the same as any other brand: measure incoming temperature and real flow first, descale, then re-measure before deciding anything is broken.
If you are not sure which model you have, the label is on the front or the side of the cabinet. Read it to us over the phone and we will tell you what we are likely walking into.
A. O. Smith Tankless Water Heater Repair
The A. O. Smith Adapt®+ units with X3® scale prevention are a different service proposition from the rest, because the thing that normally degrades a tankless heater is the thing they are built to resist. A. O. Smith states that no annual descaling is required on these models.
That genuinely removes the most common failure path. It does not remove the others: gas supply, venting, condensate and the flow sensor all behave the same as on any condensing tankless unit, and routine servicing is still required even where descaling is not.
So when an Adapt+ under-performs, scale is lower down our list than it would be on another brand, and gas and venting move up. Starting in the right place is most of the diagnosis.
A. O. Smith states that X3 extends the unit’s life up to three times compared with a tankless model without it, and its 15-year limited warranty covers scale-related failures. Models and installation details are on our A. O. Smith tankless water heater page.
We also install these units. If repair stops making sense, the tankless water heater installation page covers the brands we fit and how we size them.
The Annual Descale, and Why Hard Water Decides Your Repair Bill
Scale is the single biggest factor in what a tankless water heater costs you over its life, and much of Southern Ontario is on hard water. Every repair conversation here eventually comes back to it.
A tankless unit heats water as it passes through a narrow heat exchanger. Hard water leaves mineral deposits on exactly those surfaces, which restricts flow, insulates the exchanger from the burner, and makes the unit work harder for less hot water. The symptom is not a failure. It is a slow decline that people put up with for a year before calling.
So the annual service is not a formality:
- Descale the heat exchanger. Yearly on most units. A. O. Smith’s X3 models are the exception and do not require it.
- Check the venting and the condensate path. On a condensing unit this is where winter failures start, not in the heater.
- Verify gas pressure under load. A gas line that was adequate when the tankless went in can be marginal once something else was added to it.
- Clean the inlet filter and check the flow sensor. Cheap parts that cause expensive-looking symptoms.
If your unit has isolation valves, the descale is a short visit. If it does not, it is a plumbing job first. Valves cost very little at installation and a lot afterwards, which is worth knowing if you are having a unit put in rather than repaired.
Where hard water is the real problem, treating the water is cheaper than replacing heat exchangers. A water softener protects the unit you already paid for. We will tell you if that is the honest answer rather than selling you a yearly descale forever.
Or, when the unit is due for replacement anyway, replace it with one that does not need descaling: an A. O. Smith Adapt+ with X3 scale prevention.
Repair or Replace
Tankless units last roughly 20 years against 10 to 12 for a tank, so the arithmetic usually favours repair for far longer than people expect. A few cases where it does not:
- The heat exchanger has failed on an older unit. This is the expensive part. On a unit past about fifteen years, replacement is normally the better value.
- It was never sized for the house. No repair fixes a unit that cannot meet the winter temperature rise. That is a sizing problem and the fix is a correctly sized unit, which is why we measure before quoting either way.
- Parts are no longer available. On discontinued models this decides it for you.
Everything else is usually worth repairing, including most ignition, sensor, venting and condensate faults. We would rather tell you the unit has years left than sell you a new one, and the measurements are what settle it rather than an opinion at the door.
If replacement is genuinely the answer, see tankless water heater installation. For tank units, water heater repair and water heater installation cover those.
Tankless Water Heater Repair Across Southern Ontario
We repair and service tankless water heaters across Southern Ontario from our base in Concord. Water hardness varies a good deal between these areas, and it is the main reason two identical units in two towns age differently.
Ajax · Barrie · Brampton · Brantford · East Gwillimbury · Guelph · Hamilton · Kawartha Lakes · Kingston · Kitchener · London · Markham · Mississauga · Niagara · Oakville · Orangeville · Oshawa · Ottawa · Peterborough · Richmond Hill · Smiths Falls · St. Catharines · Sudbury · Vaughan · Waterloo · Welland
Not sure whether we reach you? Call (289) 409-9929 and we will tell you honestly.
Tankless Water Heater Repair: Common Questions
Why does my tankless water heater have no hot water?
Most often gas supply, a lost flame, or in winter a frozen intake or exhaust rather than a fault in the heater. Check that the gas is on and look at the vent terminal outside for ice before assuming the unit has failed. An error code on the display narrows it considerably.
What does the LC code mean on a Rinnai tankless water heater?
Scale build-up in the heat exchanger. It is the only Rinnai code that lets the unit keep running, which is why it gets ignored. It means flush the heat exchanger now. Left long enough, a one-visit descale becomes a heat exchanger replacement.
What does error code E003 mean on a Navien tankless water heater?
Ignition failure. Usually the gas supply is not open, the gas line is undersized for the unit, or there is air in the line. In cold weather it can also mean the unit is frozen rather than faulty, in which case it may simply need a reset once thawed.
How often should a tankless water heater be serviced?
Once a year for most units: descale the heat exchanger, check the venting and condensate path, and verify gas pressure under load. A. O. Smith X3 models do not require annual descaling. Isolation valves fitted at installation keep the visit short.
Is it worth repairing a tankless water heater or should I replace it?
Usually worth repairing. These units last around 20 years, so most ignition, sensor, venting and condensate faults are economic to fix. Replacement makes sense when the heat exchanger has failed on an older unit, parts are discontinued, or the unit was never sized for the house.
Do you repair Navien, Rinnai, Noritz and A. O. Smith tankless water heaters?
Yes, all four, across Southern Ontario, including units another company installed. Tell us the brand and the model number from the label on the cabinet, plus any code on the display, and we can usually tell you what we are walking into before we arrive.
Book a Tankless Repair
Tell us the brand, the model number and any code on the display, and we will tell you what it is likely to be before anyone comes out.
Or send us the details and we will come back to you.
