A cold or barely warm radiator is frustrating, especially in winter. Before calling an engineer, it's worth understanding the possible causes — some you can fix yourself, others need professional attention.
Cold at the top, warm at the bottom
**Diagnosis:** Trapped air.
**Fix:** Bleed the radiator using a bleed key. Turn the key anti-clockwise at the bleed valve (top corner) until you hear hissing, then close when water appears. Check boiler pressure afterward.
Cold at the bottom, warm at the top
**Diagnosis:** Sludge build-up (magnetite).
Black sludge settles at the bottom of radiators where flow is slowest. It insulates the bottom section from heat.
**Fix:** This usually requires a power flush to resolve properly. A plumber can sometimes remove and flush the radiator individually if only one is affected.
Entire radiator cold, others are hot
**Diagnosis:** Closed lockshield valve, faulty TRV, or air-locked section.
Check the TRV (thermostatic radiator valve — the valve with numbers on, usually on the inlet) is not turned to zero or frost setting. Also check the lockshield valve on the other end is open (it should be open more than a full turn from closed).
If both valves are open and the radiator is cold while others heat normally, the TRV may be stuck. Remove the TRV head — if the pin is stuck down, the valve is closed. Tap it gently with a hammer handle to free it.
All radiators cold, boiler firing
**Diagnosis:** Boiler pump failure, diverter valve issue (combi), or low pressure.
Check your boiler pressure — if below 1 bar, re-pressurise. If pressure is correct but no radiators heat, the circulation pump or diverter valve (for combi boilers) may have failed. Call an engineer.
Some radiators heat, others don't
**Diagnosis:** System imbalance.
Central heating systems need to be 'balanced' so that heat is distributed evenly. Radiators close to the boiler receive flow preferentially unless lockshield valves are adjusted to restrict flow to nearer radiators. This is a job for a heating engineer.
NBS Plumbing & Heating — heating diagnostics and repairs across Sutton Coldfield and Birmingham — **07507 133204**.