Cold radiators, or radiators that are warm in some spots and cold in others, are one of the most common heating niggles — and often one of the easiest to improve. Here’s how to work out what’s going on and what you can safely sort yourself.
Cold at the top: trapped air
If a radiator is hot at the bottom but cold at the top, the usual cause is trapped air. Air rises to the top of the radiator and stops hot water filling that section. The fix is to bleed the radiator:
- Turn the heating off and let the radiators cool.
- Find the bleed valve (a small square nut at the top corner of the radiator) and use a radiator key.
- Hold a cloth underneath and turn the key slowly anticlockwise — no more than a quarter to half a turn.
- You’ll hear air hissing out. As soon as water starts to dribble out, close the valve again.
After bleeding, check your boiler pressure — bleeding can drop it, and you may need to top it up using the filling loop.
Cold at the bottom: sludge
If a radiator is warm at the top but cold along the bottom, trapped air isn’t the problem — it’s usually sludge. Over the years, rust and debris settle to the bottom of radiators and block the flow of hot water. Bleeding won’t fix this.
The solution is a power flush or chemical clean of the system, which is a job for a heating engineer. If several radiators have cold bottoms, or your system is generally sluggish, sludge build-up is the likely culprit.
Some radiators hot, others cold: balancing
If the radiators nearest the boiler get roastingly hot while those furthest away stay lukewarm, the system probably needs balancing. This means adjusting the small lockshield valves on each radiator so that hot water is shared evenly around the house, rather than taking the easy path through the nearest radiators.
Balancing is fiddly and best done by someone who does it regularly — it involves measuring temperature differences across each radiator and adjusting accordingly.
What you can safely do yourself
- Bleed radiators that are cold at the top
- Check and top up boiler pressure afterwards
- Turn radiator valves fully on in rooms you want heated
- Check that nothing (furniture, long curtains) is blocking the heat
When to call an engineer
It’s worth bringing someone in if:
- Radiators are cold at the bottom despite bleeding (sludge)
- Some radiators never get properly hot (balancing or flow issues)
- The whole system is slow to warm up
- You’re bleeding radiators repeatedly — frequent air can indicate a separate fault
Bettess Gas can sort sluggish heating, power flushing and balancing as part of our plumbing and heating work across Watton and the surrounding Norfolk area. Persistent cold radiators are often a sign your system would benefit from a proper look during its annual service.