No Hot Water From Boiler: Causes & Fixes for BC Homes
Your boiler is running but the taps run cold. Different boiler types have different causes — here's how to diagnose it quickly and what the fix involves.
Combi Boiler vs. System Boiler: DHW Works Differently
Before diagnosing a no-hot-water problem, it helps to know your boiler type:
Combi (Combination) Boiler
A combi boiler provides both central heating and domestic hot water (DHW) from a single unit. Hot water is produced on demand — when you open a tap, the boiler fires and heats water through a plate heat exchanger before it reaches your fixture. There is no separate hot water tank. Most newer installations in Metro Vancouver condos and townhomes use combis.
System or Heat-Only Boiler
A system boiler heats a separate indirect hot water cylinder (tank). DHW is stored ready for use. Many older Vancouver detached homes and strata buildings use system boilers with 60–120 L tanks. If you run out of stored hot water, you wait for the tank to reheat.
Common Causes of No Hot Water
1. Diverter Valve Failure
The diverter valve in a combi boiler switches water flow between the central heating circuit and the DHW plate heat exchanger. When it fails — either by seizing in the heating position or not fully diverting — you lose hot water while heating continues normally (or vice versa). A stuck diverter valve is the single most common cause of no hot water on Navien, IBC, and Viessmann combi boilers in Metro Vancouver. Symptoms include heating that works fine while hot water is absent, or hot water that suddenly turns cold mid-shower. Diverter valve replacement is a half-day job for a licensed technician.
2. DHW Flow Sensor Failure
Combi boilers detect when a hot water tap is opened using a flow sensor (flow switch or turbine). If this sensor fails or becomes clogged with debris, the boiler doesn't know hot water has been requested and doesn't fire for DHW. You can sometimes identify this by the boiler displaying no response at all when you open a hot tap — no fan noise, no burner activity. Flow sensor replacement is a quick repair, usually under 45 minutes.
3. Limescale in the DHW Heat Exchanger
Metro Vancouver's water is relatively soft compared to some parts of BC, but areas supplied from the Seymour Reservoir system still carry enough minerals to cause gradual scaling of the DHW plate heat exchanger. Heavy scaling reduces heat transfer efficiency — you get lukewarm water rather than hot, and flow rate may also drop noticeably. Chemical descaling (powerflush) or heat exchanger replacement resolves this. Installing a phosphate dosing pot on the cold mains supply to the boiler prevents future buildup.
4. DHW Temperature Setting Too Low
System boilers with indirect cylinders have a separate thermostat or immersion heater controller for the hot water tank. If the DHW setpoint has been reduced (sometimes accidentally through a smart thermostat app update), water in the tank won't reach full temperature. On system boilers, check that the cylinder thermostat is set to at least 60°C — this is also the Legionella prevention temperature recommended by BC health authorities.
5. Zone Valve Serving DHW Circuit Has Seized
In system boilers, a motorised zone valve typically controls hot water flow to the indirect cylinder. A failed or seized zone valve means hot water from the boiler never reaches the tank. Other heating zones may work normally. The zone valve actuator can often be replaced without draining the system.
5-Minute Homeowner Diagnosis
- Does your central heating work? If heating works but no hot water — combi boiler diverter valve is the prime suspect.
- Does the boiler respond at all when you open a hot tap? If completely silent — flow sensor or controls issue. If it fires but water is still cold — DHW heat exchanger or diverter problem.
- Check the DHW temperature setting on your thermostat or boiler controls. Ensure it's set above 55°C.
- For system boilers: feel the indirect cylinder — if it's cold after the boiler has been running for an hour, the zone valve or wiring to the DHW circuit is the likely cause.
Most no-hot-water faults on combi boilers are resolved in a single visit. Our technicians carry common diverter valves and flow sensors for Navien, IBC, Viessmann, Rinnai, and Rheem models — the boilers most commonly installed in Metro Vancouver in the last 15 years.
No hot water? We get it fixed fast.
Call 604-359-1081 — Red Seal gas-fitters with same-day service across Metro Vancouver.