The short version
- Cold top, warm bottom — trapped air, bleed it
- Warm top, cold bottom — sludge settled in the panel
- Cold all over while others are hot — valve or balancing issue
- Upstairs cold, downstairs hot — usually pump speed or balancing
Read the pattern first
Run the heating for twenty minutes, then check each radiator top, middle and bottom.
- Cold across the top only: air in the system — bleed and re-pressurise
- Cold band across the bottom: iron oxide sludge sitting in the panel
- Completely cold: lockshield closed, TRV pin stuck, or airlock in that branch
- One end cold: partial blockage or poor flow through that radiator
The fixes, cheapest first
Bleeding costs nothing. A stuck TRV pin can often be freed. Balancing the system redistributes flow so the far radiators get their share. Only when the panels are genuinely loaded with sludge does a flush become the right answer.
When it keeps coming back
Air that returns every few weeks means it's being drawn in — usually a small leak, a failing auto air vent or a pumping-over problem on an open-vented system. That's a fault to fix, not a bleed to repeat.
Common questions
How often should I bleed radiators?+
Once a year at the start of the heating season is normal. Much more than that suggests a fault.
Do I need to re-pressurise after bleeding?+
On a sealed system, usually yes — check the gauge and top up to the marked range.
Will a flush fix a single cold radiator?+
Not necessarily. If only one is affected, valves and balancing are the first suspects.
Can you balance the system for me?+
Yes, we balance flow across the whole system as part of remedial work.
Rooms not getting warm?
We'll test flow and temperature across every radiator.