When a combi boiler's radiators heat up normally but the taps run cold, the fault almost always lives in the domestic hot water side, not the whole boiler.
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Quick answer
A combination gas boiler that heats the home normally but produces no hot water at the taps usually has a problem specific to the domestic hot water path: a diverter valve stuck in the heating position, a scaled or blocked plate heat exchanger, a failed flow sensor or flow switch that should detect a tap opening, or a faulty hot water thermistor. Since the central heating side is clearly working, a technician can focus the diagnosis on the domestic hot water components rather than the whole system.
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A combination boiler heats both the home's radiators and the domestic hot water on demand from the same burner and heat exchanger, but it uses separate components to route and control each function. A diverter valve decides whether the boiler's output goes to the heating loop or to the plate heat exchanger that heats domestic water as it flows through, and a flow sensor detects when a tap is opened so the boiler knows to switch into hot water mode. When heating works but hot water does not, the burner and main heat exchanger are clearly functioning, which points the diagnosis squarely at the components dedicated to the domestic hot water path.
A diverter valve that is stuck, worn, or has a failed actuator can prevent the boiler from switching over to hot water mode even when a tap calls for it, so the unit stays in heating mode or does not respond at all when a tap opens. A scaled or partially blocked plate heat exchanger on the domestic hot water side is another frequent cause, particularly in areas with harder water, though Metro Vancouver's reservoir-supplied water is comparatively soft, which somewhat reduces (without eliminating) scale buildup compared to Fraser Valley wells or many other parts of the country; scale can still accumulate over years of use, especially on units that have never had a service flush.
A flow sensor or flow switch that has failed, or is fouled with debris, will not tell the control board that a tap has opened, so the boiler never triggers hot water mode even though everything else is functioning. Less commonly, a failed hot water thermistor can cause the control board to misread the water temperature and shut down hot water production as a protective measure.
Check whether the issue is at every tap in the house or just one, since a single cold tap points toward a plumbing or mixing valve issue at that fixture rather than the boiler. Note whether the boiler makes any sound or the display changes at all when you open a hot tap, since a complete lack of response points more toward the flow sensor, while a boiler that seems to try and then stops points more toward the diverter valve or a thermistor fault. Check hot water flow rate: a weak trickle rather than no flow at all can suggest a partially scaled plate heat exchanger rather than a fully stuck valve.
A diverter valve or flow sensor replacement is a routine, contained repair on most combi boilers. A scaled plate heat exchanger can often be professionally flushed rather than replaced outright, though a heavily scaled exchanger on an old unit may need replacement. None of these repairs typically justify replacing the whole boiler on their own, since the heating side is proven to be working; replacement becomes a more relevant conversation only if the unit is old enough that multiple components are failing in sequence, in which case a technician can walk through whether continued repair or a full combi replacement, ballparked at $7,500 to $15,000 or more depending on system size, makes more sense, not a quote.
Safety note
If troubleshooting reveals any gas smell, soot, or a burning odour near the boiler while investigating this issue, leave the home and call FortisBC at 1-800-663-9911 rather than continuing to inspect the unit yourself.
This pattern most often points to the diverter valve failing to switch over or a flow sensor not detecting the tap opening, since both would affect every tap equally while leaving the heating side unaffected.
If only one tap is affected, or if the hot water problem is intermittent in a pattern tied to specific fixtures, it is worth checking mixing valves and shutoff valves at that fixture before assuming it is the boiler.
Not necessarily; low pressure with some flow more often points to a partially scaled plate heat exchanger, while a complete absence of hot water more often points to the diverter valve or flow sensor.
This requires isolating the boiler, connecting a flushing pump, and using an appropriate descaling solution, which is a job for a licensed technician rather than a DIY task.
A stuck diverter valve or fouled flow sensor typically will not resolve itself, and a partially scaled heat exchanger tends to worsen gradually rather than suddenly, so it is worth scheduling a repair rather than waiting.
It varies by boiler model and accessibility, and is best estimated by the technician once the specific valve and unit are identified.
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