A combi boiler that used to keep up with the household and suddenly cannot is usually dealing with a specific, findable cause rather than simply being worn out.
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Quick answer
A Noritz combination boiler that is not producing enough hot water is most often affected by scale buildup on the heat exchanger reducing its transfer efficiency, a flow restriction such as a partially clogged aerator or filter, competing demand from multiple fixtures running at once, or a failing component such as a diverter valve or flow sensor. The fix depends heavily on which of these is actually happening, so a technician traces the specific bottleneck rather than assuming the unit is simply undersized for the home.
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If a combi boiler has always struggled to keep up with two showers running at once, that may genuinely reflect its output capacity relative to household demand, which is a sizing conversation rather than a fault. But if a unit that used to handle the household fine has become noticeably worse over weeks or months, something has changed, whether that is scale buildup, a developing mechanical fault, or a new fixture added to the home, and that is a diagnostic question rather than a sizing one.
| Pattern | Likely cause | What to mention |
|---|---|---|
| Hot water runs out or turns lukewarm only when two fixtures run together | Flow-based capacity limit | Which fixtures, and whether it improves when only one runs |
| Gradual decline over months with no clear trigger | Scale buildup on the heat exchanger | Roughly how long the decline has been happening |
| Water temperature fluctuates or swings unexpectedly | Diverter valve or flow sensor issue | Whether it happens on every use or only sometimes |
| Noticeably worse in winter than summer | Colder incoming water requiring more heating per litre | Whether the pattern is seasonal or has been constant |
Diagnosis starts with confirming the actual flow rate and temperature rise the unit is achieving under real conditions, then checking the inlet filter and flow sensor for restriction, testing the diverter valve's operation, and inspecting the heat exchanger for scale. Metro Vancouver's municipal water is comparatively soft, which slows scale formation relative to harder-water regions, but homes on well water in parts of the Fraser Valley, or units that have gone many years without descaling, are more likely candidates.
If the underlying issue turns out to be genuine capacity relative to household demand rather than a fault, the technician can walk through what a larger unit or a different configuration would look like, though that is a planning conversation rather than a repair.
Safety note
Any diagnosis or repair on a gas-fired boiler must be carried out by a Technical Safety BC licensed gas fitter, since the combustion, venting, and control systems are all connected. If you ever smell gas, hear a hissing sound near a gas line, or notice soot or a burning odour near the unit, leave the home immediately and call FortisBC at 1-800-663-9911 from a safe location.
A combi boiler heats water on demand at a fixed maximum flow rate, so when two fixtures draw at once, the available hot water per fixture is reduced. This is a capacity characteristic rather than a fault unless it is new behaviour.
Yes. Scale coating the heat exchanger reduces how efficiently heat transfers into the water, which can show up as lower output temperature or reduced flow over time, especially on units that have never been descaled.
That pattern is a reasonable indicator of a diverter valve issue, since that component is what switches the unit between space heating and domestic hot water modes, but a technician should confirm it directly.
Yes. Incoming municipal water is colder in winter, so the unit has to raise the temperature by a larger amount for the same output temperature, which can make a marginal system feel worse specifically in colder months.
Only after confirming the current unit is not simply suffering from a fixable fault like scale or a failing component. Sizing up is worth considering if the unit is functioning correctly but genuinely cannot meet household demand.
There is no universal interval, since it depends on water hardness and usage, but including a check as part of an annual service visit is a reasonable habit, particularly for units that have run several years without one.
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