When taps run hot but the house stays cold, the burner and heat exchanger are clearly working, so the fault usually sits in the heating-side controls.
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Quick answer
A combination gas boiler that produces hot water normally but will not heat the home usually has a fault specific to the central heating circuit: a failed or seized circulator pump, a stuck zone valve or diverter valve not switching to heating mode, a broken thermostat or wiring to it, low system pressure tripping a safety lockout on the heating side, or a failed room thermostat call not reaching the boiler. Since domestic hot water clearly works, the burner and main heat exchanger are functioning, narrowing the diagnosis to the heating circuit.
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A combi boiler shares one burner and one primary heat exchanger between domestic hot water and central heating, but each function has its own dedicated components downstream: a circulator pump moves water through the heating loop, a diverter valve routes the boiler's output to the radiators when heating is called for, and the thermostat and its wiring or wireless link tell the boiler when heating should run. If hot water at the tap works normally, the burner and main heat exchanger are proven functional, so a technician can focus on the heating-specific chain of components rather than treating it as a whole-boiler failure.
A circulator pump that has seized, lost power, or failed electrically will not move water through the radiators even if the boiler itself is firing correctly, which some boilers will detect and use to shut down the heating call, while others may continue firing briefly without circulation. A thermostat that has a dead battery, a wiring fault, or has lost its pairing with a wireless receiver simply never sends the call for heat to the boiler, so from the boiler's perspective nothing is wrong because it was never asked to run.
A diverter valve stuck in the hot water position, the mirror image of the more commonly discussed stuck-in-heating scenario, will keep routing output to the plate heat exchanger and never send it to the radiators. Low system pressure on the heating side, whether from a slow leak or an unaddressed air problem, can trip a low-pressure lockout specific to the heating function on many boilers while leaving domestic hot water, which does not depend on that sealed pressurized loop, completely unaffected.
Check the thermostat display for a low battery indicator or blank screen, and confirm it is set to call for heat rather than only showing the current temperature. Listen near the boiler when the thermostat is calling for heat: a humming or running sound with no warmth reaching the radiators can indicate a pump that is running but not moving water effectively, sometimes from a seized impeller that spins without pumping, while total silence points more toward a pump that has no power at all or a call that never reached the boiler. Check the system pressure gauge against the boiler's normal operating range printed near the gauge or in the manual.
A circulator pump replacement, a thermostat repair or replacement, and a diverter valve repair are all routine, contained jobs on most combi installations. A low-pressure lockout traced to a small leak is addressed by finding and repairing that leak rather than just repeatedly topping up pressure. As with the reverse fault pattern, none of these typically justify boiler replacement on their own since the burner and heat exchanger are proven to work; a technician only raises replacement as an option if the unit is old and has a pattern of unrelated component failures piling up over time.
Safety note
If you notice any gas smell, unusual burning odour, or soot while checking this issue, leave the home immediately and call FortisBC at 1-800-663-9911 rather than continuing to investigate yourself.
Yes, a dead thermostat battery or a lost wireless connection to the boiler's receiver means the boiler is simply never told to call for heat, which can look identical to a boiler fault from the homeowner's side.
Domestic hot water and central heating share the burner and main heat exchanger but use separate downstream components like the circulator pump, diverter valve, and thermostat wiring, so a fault in one of those affects only the heating side.
Topping up pressure to the manufacturer's recommended level is often something homeowners can do following the boiler manual, but if pressure keeps dropping, that points to a leak that needs a technician to locate and repair.
A pump that hums but does not seem to move water may be seized or airbound, while a pump with no sound or vibration at all may have no power reaching it. A technician can test this definitively with a meter.
If your system has multiple zones, one stuck zone valve typically only affects that zone; a whole-house loss of heat with working hot water points more toward the pump, diverter valve, or thermostat call rather than a single zone valve.
Only if the thermostat itself, its wiring, or its wireless pairing is the actual fault. A technician verifies the call for heat is reaching the boiler before recommending a thermostat replacement.
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