When radiators run cold at the bottom, the boiler kettles, or the heating is slow to warm up, the usual culprit is sludge - a build-up of rust and debris circulating through the system. A power flush clears it out, but it is not cheap, so it pays to know when it is genuinely needed and when something simpler will do.
A power flush is usually £300-£700 depending on the number of radiators. A simpler chemical flush is £150-£300, and a magnetic filter to prevent it happening again is £120-£200 fitted.
Typical UK Prices
| Job | Typical price |
|---|---|
| Small system (up to 6 radiators) | £300-£500 |
| Average system (7-10 radiators) | £400-£700 |
| Large system (10+ radiators) | £600-£1,000 |
| Chemical flush (lighter clean) | £150-£300 |
| Magnetic filter fitted | £120-£200 |
Signs Your System Needs One
- Radiators cold at the bottom but warm at the top - see radiator not heating up
- Dirty black or brown water when you bleed a radiator
- The boiler making banging or kettling noises
- Some radiators never get properly hot while others do
- Slow warm-up across the whole house
What a Power Flush Involves
A pump is connected to the system and pushes water and cleaning chemicals through at high velocity to dislodge the sludge, which is then drained away. The system is refilled with clean water and an inhibitor added to slow future corrosion. It is usually a half-day job, and a good engineer finishes by fitting or checking a magnetic filter so the debris is caught next time rather than settling back into the radiators.
Cheaper Alternatives First
- Bleed and balance
- If only the top of a radiator is cold, it is trapped air, not sludge - bleeding it is free. See how to bleed a radiator.
- Chemical flush
- A lighter clean without the high-pressure pump. Cheaper, and enough for a mildly sludged system.
- Inhibitor top-up
- Adding corrosion inhibitor to a reasonably clean system helps prevent sludge forming in the first place.
Is It Worth It Before a New Boiler?
Most installers insist on a flush when fitting a new boiler, and for good reason - pumping old sludge through a brand-new heat exchanger can damage it and may void the warranty. If you are replacing the boiler anyway, factor the flush into that quote rather than treating it as a separate job. On its own, a power flush makes most sense when you have clear sludge symptoms; throwing one at a healthy system rarely changes much.
If a flush does not fix cold radiators, the fault may be elsewhere - our boiler repair cost guide covers the alternatives, and the new boiler cost guide helps if the system is past its best. No heat at all right now? Get an emergency heating engineer on the way.