Why does my boiler provide hot water but no heat?
When your hot taps run warm but the radiators stay stubbornly cold, it tells you something useful: the boiler is firing and making heat, but that heat is not reaching your central heating circuit. For Metro Vancouver and Fraser Valley homeowners, hot water but no heat is a common winter complaint, and several of its causes are quick to check.
This is essentially the mirror image of the heat-but-no-hot-water problem, and it points to the controls, valves, or pump that direct heat into the radiators. This guide walks through why a boiler gives hot water but no heat, what you can safely check yourself, and when to call a licensed, insured technician. HeatLand has diagnosed and repaired heating faults on all major boiler brands across the region since 1999.
Check the controls and pressure first
Start with the simplest causes, which are also the most common. Confirm the room thermostat is set above the current temperature and switched to heat mode, and check its batteries if it is a wireless type; a dead thermostat battery is a frequent reason radiators stay cold while hot water still works. Check the programmer or timer is actually set to call for heat right now, since schedules often slip after a power cut and the heating period may simply be off. Look at the pressure gauge; if it is below about 1 bar, low pressure can prevent the heating circuit from working, and topping up to 1.2 to 1.5 bar via the filling loop may restore it. Some boilers have separate controls or dials for heating and hot water, so make sure the heating side has not been switched off or turned down. These checks take only a few minutes and resolve a good share of hot-water-but-no-heat calls before any technician is needed.
Diverter valve, pump, and radiators
If the controls and pressure are fine but radiators stay cold, the cause is usually mechanical. In a combi boiler, a diverter valve that sticks toward the hot water position will send heat to your taps but not your radiators, the classic hot-water-but-no-heat fault. The circulating pump is another common culprit: if it has seized or failed, hot water can still be produced for taps but nothing is pushed around the heating loop, so radiators stay cold. Motorized zone valves that fail closed can shut off heating while leaving hot water unaffected. At the radiator level, widespread trapped air or heavy sludge can block circulation, though this usually causes partial rather than total heating loss. You can safely bleed radiators that are cold at the top and check that their valves are open, topping up pressure afterward. But diverter valve, pump, and zone valve faults sit inside the system and need professional diagnosis and repair.
When to call a licensed technician
If you have checked the thermostat, programmer, heating controls, and pressure, and bled the radiators, but the heating still will not come on while hot water works, it is time for a professional. A stuck diverter valve, a failed pump, or a faulty zone valve all require diagnosis and repair by a licensed technician, and on gas boilers this work must be carried out by a licensed gas fitter. HeatLand’s technicians are TSBC-certified and can quickly pinpoint whether the fault is the diverter valve, the pump, a valve, or the boiler’s controls, then repair it. Safety note: never open the boiler casing or attempt internal repairs yourself, and if you ever smell gas or notice signs of carbon monoxide such as headaches, dizziness, or nausea that ease when you leave the house, shut the boiler off, leave the home, 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
- Set the room thermostat above room temperature, in heat mode
- Replace thermostat batteries if wireless
- Confirm the programmer is calling for heat now
- Check the boiler’s heating side is switched on
- Top up pressure to 1.2 to 1.5 bar if low, and bleed radiators
- Call a technician if radiators stay cold but hot water works
| Cause | Check or fix | DIY or pro |
|---|---|---|
| Thermostat off or dead battery | Set to heat, replace battery | DIY |
| Programmer not calling for heat | Check schedule | DIY |
| Low pressure | Top up filling loop | DIY |
| Stuck diverter valve | Replace valve | Pro |
| Failed circulating pump | Diagnose and replace | Pro |
| Failed zone valve | Diagnose and replace | Pro |
Frequently Asked Questions
Why do my taps run hot but the radiators are cold?
It means the boiler is firing and making heat, but that heat is not reaching the heating circuit. Common causes are a thermostat or programmer set wrong, a stuck diverter valve favouring hot water, or a failed circulating pump. Check the controls and pressure first; if radiators stay cold, a licensed technician should check the diverter valve and pump.
Can a thermostat cause no heat but still allow hot water?
Yes. Hot water and heating are often controlled separately, so a thermostat set too low, switched off, or with a dead battery can leave radiators cold while hot water still works normally. This is one of the first and easiest things to check. Set the thermostat above room temperature in heat mode and replace batteries in wireless models before calling anyone.
Is a stuck diverter valve the cause?
It can be. In a combi boiler, a diverter valve stuck toward the hot water position sends heat to the taps but not the radiators, producing hot water with no heat. It is a common mechanical cause once the controls and pressure are ruled out. Because the valve is inside the boiler, a licensed gas fitter should diagnose and replace it.
What should I check before calling a technician?
Confirm the room thermostat is set to heat and above room temperature with working batteries, check the programmer is calling for heat now, make sure the boiler’s heating side is on, verify pressure is 1.2 to 1.5 bar, and bleed any radiators cold at the top. If heating still fails while hot water works, call a licensed technician to check the pump and diverter valve.
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.