BoilerFaultCodesRadiators cold at the bottom? It's sludge, not air.
Here's the test: put your hand on the radiator. Is the top cold and the bottom hot? That's trapped air — just bleed it. But if the top is hot and the bottom is cold, that's something else entirely: sludge. And bleeding won't fix it.
Bleed the radiator — free, takes 2 minutes, fixes it.
Needs flushing — £300–£700 job. Read on.
What is sludge?
Black sludge is a mix of rust, scale, and biological matter that builds up inside radiators over years. It settles at the bottom (gravity) and eventually blocks the water flow through the lower section of the radiator. The top still gets hot because water circulates above the blockage — but the bottom stays cold.
If you've ever bled a radiator and got black water instead of clear, that's sludge. It's corrosive to your boiler's heat exchanger and pump, so it's worth dealing with before those parts fail.
How sludge is dealt with — 3 options
Power flush
£350–£700An engineer connects a pump to your system and pushes cleaning chemicals and water through at high flow to dislodge sludge and flush it out. Takes 4–6 hours. Most thorough option — should clear most systems.
When: Worth doing if: several radiators affected, system over 8 years old, boiler running noisily.
Chemical flush
£150–£300Cleaning chemical added to the system, left to circulate for an hour or so, then drained. Cheaper and quicker than a power flush but less thorough — won't shift heavy sludge.
When: Worth doing if: only one or two radiators affected, you're servicing a fairly new system as maintenance.
Radiator flush (single rad)
£80–£150Engineer removes the affected radiator, takes it outside, and flushes it with a hose. Cheaper if just one radiator is sludged.
When: Worth doing if: only one radiator is affected and the rest of the system seems clean.
Prevent it coming back
After a flush, ask the engineer to add a central heating inhibitor (like Fernox F1 or Sentinel X100) and ideally fit a magnetic filter (Adey MagnaClean, Fernox TF1) on the return pipe. The filter catches new sludge before it settles in radiators.
Filter + inhibitor combo adds £150–£250 to the job but prevents most future sludge problems. Often part of a boiler cover plan or included in a new-boiler install.