Tankless water heater repair & maintenance: common problems and fixes

How a tankless water heater works (and why that matters for repairs)

A conventional water heater keeps 40–60 litres hot in a tank around the clock. A tankless unit heats water only when you ask for it: open a hot tap, a flow sensor detects the demand, the burner (or element) fires, and water is heated as it passes through a heat exchanger. No standby tank, no standby heat loss, and — in principle — endless hot water.

That on-demand design is why tankless units are so efficient, but it's also why they're more sensitive to two things: water flow and water chemistry. The heat exchanger has narrow passages, so even a small amount of mineral scale restricts flow and forces the unit to work harder and hotter. Understanding this one fact explains the majority of tankless service calls we run in Kanata. If you're still weighing the technology itself, our tankless vs. tank water heater guide compares them head to head.

The #1 issue in Ottawa: hard-water scale

Ottawa-area water — and especially the well water common in the rural west end — carries enough dissolved calcium and magnesium to leave scale on anything it's heated in. Inside a tankless heat exchanger, that scale builds up in the very passages the water has to flow through. As it accumulates, you get reduced flow, longer time to hot water, fluctuating temperatures, error codes, and eventually a unit that shuts itself down to protect the exchanger.

This is why descaling — periodically circulating a descaling solution through the unit to dissolve the buildup — is the cornerstone of tankless maintenance. In a soft-water region you might stretch it to every couple of years; in hard-water Ottawa, once a year is the right interval, and homes on particularly hard well water sometimes need it more often. Pairing a tankless unit with a water softener dramatically slows scaling; see our guide on water softeners vs. filters and the broader Kanata water hardness guide.

Common tankless problems and what they mean

Modern tankless units are good at telling you what's wrong — most display an error code when they fault. Below are the problems we see most often and what typically causes them. Many are straightforward repairs; the key is correct diagnosis rather than immediately condemning the unit.

No hot water from a tankless unit: first checks

When a tankless unit stops producing hot water entirely, run through the basics before assuming a major failure. Confirm the unit has power (and check its breaker), make sure the gas supply is on if it's a gas unit, and note any error code on the display — write it down, because it points the technician straight to the fault. Check that an inline water filter or the unit's inlet screen isn't clogged, since low flow can prevent the burner from firing.

If the unit is in an unheated space, also rule out a frozen line during a cold snap — something we see in Ottawa winters. For a broader walk-through that applies to both tank and tankless systems, our no hot water troubleshooting guide is a good companion. If the basics check out and you still have no hot water, it's time for service rather than guesswork.

What a professional tankless service includes

An annual tankless service is more than just a flush, though descaling is the centrepiece. A proper visit keeps the unit efficient, safe, and under warranty (many manufacturers require documented annual maintenance to honour the warranty). Here's what a thorough service looks like.

Tankless repair vs. replacement

Tankless units are designed to be repaired — most parts (igniters, flame rods, flow and temperature sensors, fans, and even heat exchangers) are replaceable. So unlike a leaking tank, where replacement is often the only sensible path, a faulty tankless unit is frequently worth fixing. The decision comes down to the unit's age, what's failed, and whether it's been maintained.

If your unit is under about 12 years old, has been descaled regularly, and the failed part is a sensor or igniter, repair is almost always the right call. Replacement starts to make sense when the heat exchanger itself has failed on an older, neglected unit, when repair costs approach the price of a new unit, or when the original was undersized and never kept up with the household's demand. Our broader repair vs. replace water heater guide walks through the same math.

What tankless repair and service cost

Routine maintenance is modest and pays for itself by protecting an expensive appliance and keeping it efficient. Repairs vary with the part: sensors and igniters are inexpensive, while a heat-exchanger replacement is the costliest repair and is usually the point where replacement enters the conversation. Here are realistic Ottawa-area ranges.

Whatever the job, ask for the error code reading and a clear repair-versus-replace comparison. Our tankless water heater service in Kanata covers annual descaling, diagnostics, and repair, and we'll always tell you when a repair no longer makes financial sense rather than selling you parts on a unit at the end of its life.

Getting the longest life from your tankless unit

The homes that get a full 15–20 years out of a tankless heater all do the same things: they descale annually, they address hard water at the source with a softener, and they don't ignore the first error code. Treat it like the precision appliance it is and it will reward you with two decades of endless hot water at a lower operating cost than a tank.

The homes that replace a tankless unit at 8 or 9 years almost always skipped maintenance and let scale do its damage. In hard-water Ottawa, that annual service isn't optional upkeep — it's the difference between a 10-year unit and a 20-year one. If you've moved into a home with an existing tankless unit and don't know its service history, a first descale-and-inspection visit is the smartest place to start.