F.75 Fault Code Explained: Causes and Fixes
February 2, 2026
If your taps run hot but your radiators stay cold, the boiler itself is usually working.
The issue is often within the heating circuit rather than the hot water system.
This is a common fault and can often be traced to controls, valves or circulation problems.
Most modern systems separate:
If one works and the other doesn’t, it usually points to a control or flow issue rather than total boiler failure.
Motorised valves control the flow of hot water to radiators.
If the valve fails:
This is one of the most common causes in system boilers.
If the pump is not circulating water through the heating system:
If the thermostat is faulty or incorrectly set:
Check that:
Air trapped in pipework can prevent circulation.
Signs include:
• Gurgling sounds
• Some radiators partially heating
• System slow to respond
• Bleeding radiators may help in minor cases.
Sludge build-up can restrict flow to radiators.
You may notice:
If hot water works normally, the issue is unlikely to be:
The boiler is firing — the heat just isn’t reaching the radiators.
You can:
If heating still does not work, professional diagnosis is recommended.
Contact a Gas Safe engineer if:
Correct diagnosis prevents unnecessary part replacement.
To reduce future problems:
If you have hot water but no heating, the issue is usually within the heating controls or circulation system.
It is rarely a complete boiler failure — but it does require proper diagnosis.
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.
