A compact coil of metal, rather than a large storage tank, is what lets a tankless water heater turn cold water into hot water in the time it takes to travel through the unit.
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Quick answer
The heat exchanger in a tankless water heater is a coil or finned assembly, typically copper or stainless steel, that rapidly transfers heat from the burner's combustion into the water passing through it, which is what allows the unit to deliver hot water on demand without a storage tank. It fails mainly from mineral scale building up on the waterway side, condensate-related corrosion in condensing units if the condensate drain is not working properly, and, on units installed on an exterior wall without adequate freeze protection, cracking from freeze damage. Scale is the most common and most preventable of these failure modes.
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A tankless water heater does not store hot water; instead, it heats water as it passes through the unit, in the brief window between when a tap opens and when hot water reaches the fixture. The heat exchanger is the component that makes this possible: a coil or finned metal assembly, usually copper or stainless steel, positioned directly above the burner so that combustion heat transfers rapidly into the water flowing through the coil's internal passages.
Because there is no storage volume to smooth things out, the heat exchanger has to respond almost instantly as flow and demand change, which is part of why the flow sensor, gas valve, and heat exchanger all have to work together closely for the unit to deliver consistent temperature.
Mineral scale building up on the inside of the waterway passages is the most common long-term issue, since as water heats rapidly inside narrow copper or stainless passages, dissolved minerals can precipitate out and coat the interior surface. This scale insulates the metal from the water, forcing the unit to work harder to reach the same temperature and, over time, can restrict flow through the narrow passages enough to affect performance or trigger fault codes.
On condensing tankless units, which extract additional heat from the exhaust and produce acidic condensate as a byproduct, a condensate drain that is blocked or not draining properly can allow that acidic moisture to sit against the heat exchanger longer than intended, contributing to corrosion. On units installed on an exterior wall, which is common in some Metro Vancouver installations for existing exterior gas line routing, a hard freeze without adequate protection can cause water trapped in the heat exchanger to freeze and crack the metal, since even a Lower Mainland cold snap can be enough if the unit is exposed and unprotected.
| Symptom | Likely cause |
|---|---|
| Reduced hot water output or flow through the unit over time | Scale build-up restricting the internal waterway passages |
| Unit takes longer to reach set temperature than it used to | Scale insulating the heat exchanger from the water passing through it |
| Visible water leak from the unit, or corrosion-related staining | Corrosion from condensate exposure on a condensing model, or a cracked exchanger |
| No hot water at all after a hard freeze, with visible leaking on thaw | Freeze damage has cracked the heat exchanger |
A cracked heat exchanger, whether from freeze damage or corrosion, is generally not repairable and requires replacement, while scale-related performance loss is often addressed through descaling before it reaches that point.
A technician checks incoming and outgoing water temperature against the flow rate to assess whether the heat exchanger is transferring heat as efficiently as it should, inspects the condensate drain on condensing models for proper flow, and looks for any visible signs of leaking or corrosion at the exchanger and its fittings.
If scale is suspected, a descaling flush using the unit's built-in isolation valves is typically the first step, since this can restore performance without replacing anything. If the heat exchanger itself is cracked, corroded through, or otherwise physically damaged, replacement of the heat exchanger, or in some cases the whole unit depending on age and part availability, becomes the recommended path.
Periodic descaling as part of routine tankless maintenance is the most effective way to protect the heat exchanger over the unit's service life, particularly in homes that have not had any water treatment. For exterior-wall installations, ensuring proper freeze protection measures are in place and functioning is equally important given how quickly a cold snap can affect an exposed unit.
Any work on the gas line, gas control valve, burner, or venting of a natural gas appliance must be done by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas, hear a hissing sound near the appliance, or notice soot, a burnt smell, or headaches and nausea that ease when you leave the house, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else.
Safety note
Any work on the gas line, gas control valve, burner, or venting of a natural gas appliance must be done by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas, hear a hissing sound near the appliance, or notice soot, a burnt smell, or headaches and nausea that ease when you leave the house, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else.
Yes, in most cases a descaling flush restores performance without needing to replace the heat exchanger, provided the scale has not progressed to the point of physical damage.
The clearest sign is a visible water leak from the unit, often noticed after a hard freeze or over time from corrosion; a technician can confirm through inspection.
Yes, harder water increases scale formation, though Metro Vancouver's municipal reservoir water is relatively soft; areas on well water, such as parts of the Fraser Valley, see this issue more.
Yes, a cold snap can still freeze water in an exposed exterior-wall unit without adequate protection, even though prolonged deep freezes are less common here than in colder regions.
Generally no; a cracked heat exchanger typically requires full replacement of the component, or in some cases the whole unit, rather than a patch repair.
This depends on water quality and usage, and a technician can recommend a schedule based on your specific water conditions and how the unit is performing.
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