Boiler Working — But Radiators Stay Cold?
If your taps produce hot water but your radiators remain cold, the boiler is firing correctly.
The problem usually lies within the heating circuit, controls or circulation system.
This is a common issue and is often repairable without full boiler replacement.
How Heating & Hot Water Are Controlled
In most systems:
- The boiler heats water
- Separate circuits distribute heat
- Controls decide where hot water flows
If hot water works but heating does not, the issue is usually within the central heating control or flow system.
Common Causes
1. Faulty Motorised Valve (Diverter or Zone Valve)
In system or conventional boilers, a motorised valve directs hot water to either:
The radiators
The hot water cylinder
If this valve sticks or fails:
- Hot water may still function
- Heating will not activate
- Valve failure is a frequent cause.
2. Circulation Pump Issue
The pump pushes hot water through radiators.
If the pump is weak or faulty:
- The boiler may fire
- Hot water may work
- Radiators stay cold
- You may hear humming, clicking or unusual noises.
3. Thermostat or Programmer Fault
Incorrect settings or a faulty thermostat can prevent heating from switching on.
Check:
- Heating schedule is active
- Room temperature setting is high enough
- Thermostat batteries are working
- Programmer is not set to “hot water only”
- Control faults are common and often overlooked.
4. Airlock in Heating Circuit
Air trapped in pipework can prevent circulation.
Signs include:
- Gurgling noises
- Some radiators heating partially
- Slow warm-up times
- Bleeding radiators may resolve minor air issues.
5. Sludge Build-Up
If radiators are:
- Cold at the bottom
- Warm only at the top
- Heating unevenly
- Sludge may be restricting flow.
System cleaning may be required if contamination is confirmed
What It Is Unlikely To Be
If hot water works correctly, the problem is unlikely to be:
- Gas supply interruption
- Complete ignition failure
- Major PCB failure
- The boiler is functioning — the issue is distribution.
When to Call a Gas Safe Engineer
Professional diagnosis is recommended if:
- Heating refuses to activate
- The system repeatedly locks out
- Pressure drops frequently
- Controls are unresponsive
- Pump or valve noises occur
- Correct testing ensures only the faulty component is replaced.
Preventing Heating Circuit Faults
To reduce the risk of future problems:
- Service the boiler annually
- Maintain stable system pressure
- Install a magnetic filter
- Address minor faults early
- Flush the system if sludge develops
- Proper maintenance improves heating reliability.
Final Advice
If you have hot water but no heating, the issue is usually within the heating circuit — not total boiler failure.
Professional diagnosis restores circulation and correct control operation.
Need a Local Boiler or Gas Engineer?
If you require professional heating or gas services, our Gas Safe registered engineers are available across Watford and surrounding areas. Contact us for reliable support and expert advice.
