A way to reason about cold radiators from where the cold is, pointing to air, sludge or a flow problem.
A common call-out is some radiators not getting hot. The temperature pattern tells you what to check first, not which part has failed.
Cold across the top
Cold across the top with the bottom warm is the classic sign of trapped air. Air rises and collects at the high point, so hot water cannot fill the top of the panel. Venting (bleeding) the radiator usually clears it. Turn the heating off and let the radiator cool before opening the bleed valve. Catch any drips and close the valve once water flows steadily. Afterwards, check the sealed-system pressure and restore it only as the appliance instructions require. Air that keeps coming back points to a separate cause, such as a leak drawing in air or a system that was not properly filled.
Cold across the bottom
A persistently cold area along the bottom after warm-up can indicate deposits restricting the waterways, but check the flow and valve conditions before deciding that sludge is the cause. A radiator normally has a temperature gradient while giving up heat. If contamination is confirmed, clean and flush by the system and treatment instructions, then restore the specified inhibitor protection.
One radiator cold all over
If every other radiator heats well and one stays cold, look locally first:
- Check its valves: a closed or seized valve, a TRV with a stuck pin, or a lockshield wound shut.
- Look for trapped air or a local blockage.
- If local checks do not resolve it, assess system balancing and pump performance. Other radiators heating shows that heat reaches part of the circuit, not that every branch gets enough flow.
A system-wide pattern
If easier branches heat well while others remain cool, check balancing, valve operation, air and the pump’s available duty. The hydraulically disadvantaged branch need not be the most distant. Rebalance where the measurements show an imbalance; a confirmed blockage or failed part needs its own remedy.
Sludge and the magnetic filter
Iron-oxide sludge (magnetite) forms as ferrous components corrode. A magnetic filter captures magnetic particles and needs inspection and cleaning at the specified intervals. A full canister does not always block the flow completely: it depends on its condition and design. Filtration adds to cleaning and chemical protection; it does not replace them.
Exam cues
- Cold top with a warmer bottom: check for trapped air and vent as appropriate.
- Persistent cold patches below: investigate deposits and restricted flow before choosing a cleaning method.
- One radiator cold all over: check its valves and branch first.
- Uneven heating across the system: compare flows with the design and check balance, restrictions and pump duty.
Use the temperature pattern, valve checks, system pressure and water condition to confirm the cause. Take temperature readings without risking contact with hot surfaces, and let the radiator cool before any bleeding. Repair the confirmed fault and check that circulation and heating have recovered.