There are few scenarios more frustrating than discovering a radiator isn’t working during the coldest times of the year. Before calling an engineer, there are several common causes and simple fixes you can try yourself. This guide explains why radiators might not be working, separates single radiator faults from whole-system issues, and outlines the DIY steps needed to resolve them.
Why Is My Radiator Not Coming On?
If one or more radiators are not heating up properly, don’t panic. Many issues are simple and can be fixed without professional help. This guide categorises problems into single radiator faults and system-wide issues.
Why Is Only One Radiator Not Working?
If a single radiator isn’t heating, the issue is usually isolated to that unit.
Trapped Air
Symptom: Cold at the top, warmer at the bottom
Cause: Air rises and displaces hot water at the top of the radiator
How to Fix: Bleed the radiator using a bleed key until water appears, then tighten
Sludge Inside
Symptom: Cold at the bottom, hot at the top
Cause: Sludge (rust, dirt, debris) settles at the base and blocks water flow
How to Fix: Remove the radiator and flush it with a hosepipe until the water runs clear
System Leakage
Symptom: Leak on radiator surface, loose connection, or corrosion pinhole
How to Fix: Tighten the connection nut; temporary sealants work for pinholes, but severe corrosion requires radiator replacement
Faulty Radiator Valve
Symptom: The entire radiator stays cold
How to Fix: Remove the TRV head and check the metal pin; gently free it if stuck. Replace the valve if damaged.
Why Are All Radiators Not Heating Up?
If none of your radiators are warming, the issue lies with the central heating system.
Incorrect Boiler Pressure
Symptom: All radiators cold; boiler may show an error
How to Fix: Top up pressure to 1–2 bar using the filling loop
Boiler Error / Off
Symptom: Pilot light out or error code displayed
How to Fix: Reset according to the manual; if error persists, call an engineer
Faulty Diverter Valve
Symptom: Hot water at taps but radiators stay cold
How to Fix: Diverter valve stuck in tap only mode; contact a Gas Safe engineer
Sludge in the System
Symptom: Cold spots throughout; boiler kettling, banging noises
How to Fix: A full professional powerflush of the entire system is required
Are New Radiators Difficult to Heat Up?
New radiators not heating typically indicate installation issues or trapped air.
Bleed the Radiator
Air often enters during installation; bleed until water flows smoothly
Balance the Heating System
If other radiators heat first, adjust lockshield valves to even out water flow
Check Boiler Pressure
If pressure is above 2.5 bar, repressurise according to the boiler manual
What If I’ve Tried Everything?
If your radiator still won’t heat up after all DIY checks:
Contact a Gas Safe Registered Engineer
A professional can diagnose deeper issues such as diverter valve failure or hidden leaks.
Frequently Asked Questions
How can I stop radiators not heating up after summer?
Bleed all radiators before winter to remove trapped air.
What usually makes radiators not heat up after bleeding?
Often caused by a stuck TRV pin or sludge buildup.
What is the danger if a boiler is not serviced regularly?
Carbon monoxide risk and expensive repairs due to undetected faults.
What should you try if your radiators don’t heat up after bleeding?
Check TRV pins, boiler pressure, and cold-bottom sludge indicating need for flushing or balancing.
Conclusion
A cold radiator is usually caused by trapped air, sludge buildup, or low system pressure. By checking the radiator top for air, bottom for sludge, and boiler pressure, most issues can be resolved easily. If problems persist or a faulty diverter valve is suspected, contact a Gas Safe Registered engineer for a proper diagnosis and repair.
