Last Updated: July 2, 2026 — Reviewed by Brian Hayes, Licensed Master Plumber
A tankless water heater is one of the most efficient ways to get hot water in a Durham Region home, but that efficiency depends on regular upkeep. Unlike a traditional tank that stores hot water, a tankless unit heats water on demand by passing it across a heat exchanger. That heat exchanger is exactly where mineral scale collects, and the local water supply here carries enough dissolved minerals to make annual maintenance a practical necessity rather than an optional extra.
At Hayes Plumbing, we service tankless heaters across Oshawa, Whitby, Ajax, Pickering, and Clarington. This guide walks through what tankless maintenance actually involves, how to spot the early warning signs, and what to expect from an annual service so your unit keeps delivering steady hot water through the coldest months of the year.
Why Tankless Units Need Regular Maintenance Here
Municipal water across Durham Region is drawn largely from Lake Ontario and treated to a moderate hardness, which means every litre carries dissolved calcium and magnesium. When that water is heated inside a tankless unit, those minerals precipitate out and cling to the heat exchanger as a chalky layer of scale. Over time the scale acts as insulation, forcing the burner to work harder, lengthening the time it takes to reach temperature, and eventually restricting flow through the exchanger.
Scale is the single biggest reason tankless heaters lose efficiency or fail early. A unit that is flushed on schedule can run reliably for well over a decade, while one that is never descaled can start throwing error codes within a few years. If your home has a water softener, scale builds more slowly, but the heat exchanger, inlet screen, and venting still need periodic attention.
Descaling and Flushing: The Core of Tankless Care
The most important maintenance task is a descaling flush, which circulates a mild acidic solution — typically food-grade white vinegar or a manufacturer-approved descaler — through the heat exchanger to dissolve accumulated minerals. Most tankless units installed with a proper service-valve kit make this straightforward: two isolation valves let us connect hoses and a small recirculating pump, close the unit off from the household supply, and run the solution through for roughly 45 minutes to an hour before rinsing with clean water.
For homes on Durham’s moderately hard municipal water, we generally recommend a flush once a year. Households with a softener can often stretch that interval, while homes on well water or an unusually hard supply may need it more often. If your unit was installed without isolation valves, we can add them so future flushes are quick and inexpensive. Skipping this step is the most common reason we get called out for a heater that has stopped keeping up with demand.
Cleaning the Inlet Filter and Inspecting the Venting
Beyond descaling, two components deserve regular attention. The first is the cold-water inlet screen, a small mesh that catches sand and grit before it reaches the heat exchanger. It clogs gradually, choking flow and triggering low-water faults. Cleaning it takes only a few minutes: shut off the supply, remove the screen, rinse away the debris, and reseat it with a fresh gasket if needed.
The second is the venting. Condensing tankless units produce acidic condensate and vent through PVC or specialized pipe, while non-condensing models use stainless-steel venting. We check that the intake and exhaust runs are clear of debris, nests, and lint, that the condensate drain and neutralizer are flowing freely, and that the outdoor termination points are not blocked. A partly obstructed vent can shut the unit down for safety, and on a gas model it is also a combustion-safety concern worth taking seriously.
Signs Your Tankless Heater Is Overdue for Service
Tankless units usually give warning before they fail outright. The most common early sign is water temperature that fluctuates — hot, then lukewarm mid-shower — which often points to scale narrowing the heat exchanger. Watch for these symptoms as well:
- Longer waits for hot water, or reduced flow at the tap
- A rumbling, knocking, or whistling sound while the unit fires
- Error or fault codes on the display, especially scale- or flow-related codes
- Visible mineral crusting around fittings, or a noticeable rise in your gas bill
- The unit cycling on and off without a steady draw
If you notice any of these, it is worth booking service before the unit stops entirely. For neutral background on choosing and maintaining an efficient water heater, review Natural Resources Canada water heater guidance, and then let us match that general advice to how your specific unit and household water are behaving.
Cold-Weather Care for Durham Region Winters
Our winters put extra demands on a tankless heater. Most modern units include built-in freeze protection that keeps the internal components above freezing as long as the unit stays powered, so we caution homeowners against unplugging or cutting power to an outdoor or garage-mounted unit during a cold snap. Where a unit vents outdoors, we make sure the intake and exhaust terminations sit well clear of expected snow depth so drifting does not block airflow.
The condensate drain on condensing models deserves special attention in the cold, because a line running through an unheated space can freeze and back the unit up. During a cold-season service we confirm the drain is routed and insulated properly. If you are leaving a Durham home empty over the winter, talk to us before you shut things down so the heater is protected the right way rather than left to freeze.
What Our Annual Tankless Service Includes
When we service a tankless heater, we work through a consistent checklist: a descaling flush of the heat exchanger, cleaning the inlet screen, inspecting and clearing the venting and condensate path, checking the burner and combustion on gas models, testing water temperature and flow, and reading any stored fault history from the control board. We also confirm the unit is still sized correctly for how your household uses hot water, since a change like adding a second bathroom or a soaker tub can shift demand.
Keeping a simple record of each service helps with warranty coverage too — many manufacturers expect proof of regular descaling to honour heat-exchanger warranties. We leave you with a note of what was done and when the next flush is due, so the schedule never slips.
Frequently Asked Questions
How often should a tankless water heater be flushed in Durham Region?
For most homes on Durham’s moderately hard municipal water, once a year is a sensible schedule. Homes with a water softener can often go a little longer, while well water or an unusually hard supply may call for a flush every six to eight months. If yours has never been descaled, that is the place to start.
Can I descale a tankless heater myself?
If your unit has isolation valves, a mechanically comfortable homeowner can run a vinegar flush. That said, a full service also covers the inlet screen, venting, combustion, and control-board diagnostics that a basic flush misses, so many owners prefer to have the whole system checked at once.
Why does my hot water go warm and then cold?
Fluctuating temperature is one of the classic symptoms of scale on the heat exchanger, though it can also point to a clogged inlet screen or a flow-sensor issue. A descaling flush resolves most cases; if it persists after that, we diagnose further.
Do tankless heaters freeze during Ontario winters?
Indoor units rarely freeze as long as they stay powered, thanks to built-in freeze protection. The bigger risks are an unpowered unit, a blocked outdoor vent, or a condensate line running through an unheated space — all things we check during a cold-season service.
Book a Tankless Flush Before Winter Demand Peaks
If your tankless heater is due for its annual descaling, or you have noticed fluctuating temperatures or a new error code, we can help. Call Hayes Plumbing at (905) 576-3043 or reach us through the contact page, and we will get your unit flushed, inspected, and ready to keep up with hot-water demand across the Durham Region.
