Here’s What You Need to Know
Why You’re Getting Hot Water but No Heating
If your Viessmann boiler gives you plenty of hot water but your radiators stay cold, you’re not alone.
This is a common fault that often points to a stuck diverter valve or a faulty flow sensor (also known as a flow switch).
When the flow sensor becomes faulty, it can trick the boiler into thinking hot water is still being demanded — leaving the heating side inactive.
Quick DIY Checks Before Calling an Engineer
You can perform a few safety checks yourself before booking a service call.
1. Isolate the Cold Feed
Turn off the cold-water isolation valve beneath the boiler.
This stops any dripping taps from giving false readings to the flow sensor.
2. Check the Hot Tap
Make sure all hot taps are fully turned off.
Even a slow drip can keep the boiler “locked” in hot-water mode.
3. Test Heating Mode
Switch your heating on, then turn the hot-water temperature dial down to zero.
If the boiler fires up and your radiators begin to warm, the flow sensor is likely faulty and needs replacing.
When to Call a Professional
If these steps don’t restore heating, or if you’re unsure about performing the checks, it’s time to call a Gas Safe registered engineer.
The problem could involve:
- A stuck diverter valve
- Pump or motorised-valve failure
- Sensor wiring faults or PCB communication errors
Attempting internal repairs without Gas Safe registration is dangerous and illegal in the UK.
Professional Help from C4UK
At C4UK, we’ve repaired hundreds of Viessmann boilers across Surrey and London, quickly diagnosing and fixing heating and hot-water issues.
Our qualified engineers handle everything from simple sensor replacements to full system servicing.
If your Viessmann boiler still produces hot water but no heating, it may be due to a faulty diverter valve or flow sensor.
For additional manufacturer advice, see Viessmann’s official troubleshooting guide.
Contact Us Today
Don’t wait in the cold — book a qualified engineer now.
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- 📞 Call: 07869 520044 or +44 7596 076700
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