When a radiator is not heating properly, where it is cold is the most useful clue you have. Before assuming the boiler is at fault, walk round the house with the heating running and note which radiators are cold and in which part.
Cold at the top, warm at the bottom
This is trapped air. Air rises to the top of the radiator and displaces the hot water. Bleeding the radiator with the heating switched off and the system cool releases it. Have a cloth and a small container ready, open the bleed valve slowly with the correct key, and close it as soon as water appears steadily.
Afterwards, check the boiler pressure — bleeding always lowers it, and you may need to top the system back up using the filling loop as described in your boiler manual. If air keeps returning, something is drawing it in and the system needs looking at properly.
Cold at the bottom, warm at the top
This usually means sludge. Corrosion debris settles in the lowest part of the radiator and blocks circulation through it. Bleeding will not help. Depending on how widespread it is, the answer may be removing and flushing individual radiators, a chemical clean, or a full power flush of the system.
Some rooms hot, others never warm
If the radiators nearest the boiler heat quickly while the furthest stay cool, the system almost certainly needs balancing. Balancing adjusts the lockshield valve on each radiator so hot water is shared correctly around the circuit rather than taking the easiest path. It is a methodical job with a thermometer, and it makes a genuine difference to comfort.
One radiator completely cold
- A thermostatic radiator valve with a seized pin — common on valves left closed all summer
- A lockshield valve that has been closed and never reopened
- A radiator heavily blocked with sludge
- An airlock in the pipework serving that radiator
All radiators lukewarm
When nothing gets properly hot, look at the system rather than individual radiators. A failing circulating pump, a motorised valve that is not opening fully, incorrect boiler flow temperature, or a control fault will all produce weak heat everywhere. Heavy system-wide sludge does the same.
Safe checks you can make
- Check every thermostatic valve is open and not stuck
- Make sure furniture and long curtains are not blocking radiators
- Note which radiators are cold and where, then keep the list for the engineer
- Check the boiler pressure gauge when the system is cold
- Bleed a radiator that is cold at the top, following your boiler manual
Do not remove radiators, drain the system or open the boiler casing unless you know exactly what you are doing. Anything involving the gas appliance itself must be left to a Gas Safe registered engineer.
When to call us
If bleeding makes no difference, radiators are cold at the bottom, the water coming out is black, or several rooms are underheated, give us a call. We diagnose whether the fault is air, balancing, a valve, the pump or the condition of the system water — and put it right.
Written by the engineering team at Newcastle Boiler Maintenance Ltd, Gas Safe registered heating engineers in Newcastle upon Tyne.
