Why does my boiler provide heat but no hot water?
It is a common and confusing situation: your radiators are warming the house just fine, but when you turn on a hot tap, only cold water comes out. For Metro Vancouver and Fraser Valley homeowners with combi boilers especially, heat but no hot water usually points to one specific mechanical culprit inside the boiler.
Because the heating circuit is working, you know the boiler itself is firing and making heat, so the problem lies in how that heat is being directed to your domestic hot water. This guide explains why a boiler gives heat but no hot water, what the likely causes are, and when to call a licensed, insured technician. HeatLand services combi and system boilers of all major brands across the region and has done so since 1999.
The diverter valve: the most common cause
In a combi boiler, a diverter valve directs the flow of heated water between the central heating circuit and the domestic hot water circuit. When you turn on a hot tap, the valve is supposed to switch, prioritizing hot water over heating. If that valve sticks in the heating position, your radiators keep getting hot water but your taps do not, which produces exactly the symptom of heat but no hot water. A stuck or failing diverter valve is by far the most frequent cause of this problem. Over time the valve can seize with limescale or sludge, or its internal parts and motor can wear out. Because the diverter valve sits inside the boiler and its replacement involves draining and working within a sealed, gas-fired appliance, this is not a DIY repair. A licensed technician can confirm the diverter valve as the cause and replace it, restoring hot water without disrupting your heating.
Sensors, airlocks, and other causes
While the diverter valve is the usual suspect, a few other faults can cause heat without hot water. A faulty flow sensor or thermistor on the hot water circuit may fail to detect when you open a tap, so the boiler never switches to heat the water. An airlock in the hot water side can block flow. Low mains water pressure or a partially closed isolation valve on the cold feed can also starve the hot water circuit. On some systems, scale buildup in the heat exchanger reduces hot water output specifically. Because these causes overlap in their symptoms, proper diagnosis matters, guessing and replacing the wrong part wastes money. A licensed technician can test the sensors, check flow, and inspect the heat exchanger to pinpoint which fault is at play. If your boiler is a system or conventional type with a separate hot water cylinder rather than a combi, the cause may instead lie with the cylinder, its immersion element, or a zone valve.
When to call a technician and safety notes
Because the common causes of heat-but-no-hot-water are all internal boiler components, this is a job for a licensed technician rather than a homeowner repair. In British Columbia, work on gas boilers must be carried out by a licensed gas fitter, and HeatLand’s technicians are TSBC-certified for exactly this kind of diagnosis and repair. Before calling, you can do a couple of harmless checks: confirm the hot water setting or dial on the boiler has not been turned down, and make sure any cold-feed isolation valves are open. Safety note: never attempt to open the boiler casing or work on internal components yourself, and if you ever smell gas or suspect carbon monoxide, headaches, dizziness, or nausea that improve when you leave the home, shut the boiler off, leave, and call your gas utility and a technician immediately. HeatLand offers same-day and emergency service across Metro Vancouver and the Fraser Valley, with financing available for larger repairs.
Quick Checklist
- Confirm the boiler’s hot water dial has not been turned down
- Check cold-feed isolation valves are fully open
- Note that radiators working means the boiler is firing
- Do not open the boiler casing yourself
- Call a licensed technician to test the diverter valve and sensors
- Treat any gas smell as an emergency
| Cause | Boiler type | Fix |
|---|---|---|
| Stuck diverter valve | Combi | Replace valve (pro) |
| Faulty flow sensor or thermistor | Combi | Test and replace (pro) |
| Airlock or low cold-feed flow | Combi/system | Check and clear (pro) |
| Cylinder or immersion fault | System/conventional | Diagnose cylinder (pro) |
Frequently Asked Questions
Why do my radiators work but I have no hot water?
In a combi boiler this almost always means the diverter valve is stuck in the heating position and not switching flow to the hot water side when you open a tap. Since the radiators heat, the boiler is firing fine, the problem is directing that heat to your taps. A licensed technician can confirm and replace the diverter valve to restore hot water.
Can I fix a stuck diverter valve myself?
No. The diverter valve sits inside the boiler, and replacing it involves draining the system and working within a sealed, gas-fired appliance. In British Columbia, gas boiler work must be done by a licensed gas fitter. Attempting it yourself is unsafe and may void your warranty. A TSBC-certified technician can diagnose and replace the valve properly and safely.
Is heat but no hot water an emergency?
It is not usually dangerous, so it is not an emergency in the safety sense, but being without hot water is a real inconvenience worth resolving quickly. The exception is if you smell gas or suspect carbon monoxide, then shut off the boiler, leave, and call for help immediately. Otherwise, book a technician promptly to diagnose the diverter valve or sensor fault.
Could it be something other than the diverter valve?
Yes. A faulty flow sensor or thermistor, an airlock, low cold-feed flow, or scale in the heat exchanger can also cause heat without hot water. On system or conventional boilers with a separate cylinder, the fault may lie with the cylinder or immersion element. Because symptoms overlap, a technician should diagnose the exact cause rather than replacing parts by guesswork.
Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. This article is for general guidance only. If you smell gas, suspect a carbon monoxide leak, or are unsure about any electrical or gas component, stop troubleshooting immediately and call a licensed technician or 911.