Radiator cold on one side
Check the steps below first — if you\'re not confident, get it fixed safely today.
Find a Gas Safe engineer near you →Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
If the radiator is warm on the side where the flow pipe enters and cold on the other side, the most likely causes are partial sludge blockage or a flow restriction.
First, try bleeding the radiator — sometimes a partial airlock can cause this pattern.
Check the lockshield valve at the cold end — it may have been closed too far during balancing. Remove the plastic cap and open the lockshield valve a few more turns using an adjustable spanner.
If bleeding and adjusting the lockshield valve does not resolve it, there is likely sludge inside the radiator body blocking flow. You can try isolating and removing the radiator to flush it out with a hosepipe, or consider a power flush (£300–£600 for a whole system).
If multiple radiators show this problem, the whole system may be sludged and a full power flush by a heating engineer is recommended.
Not confident doing this yourself?
Post the job and we'll match you with vetted local engineers. Free, no obligation.
Find a Gas Safe registered engineer near you and get it fixed properly.
Was this guide helpful?