Radiators cold at the bottom — sludge, and what to do about it
Cold at the top and hot at the bottom means trapped air — bleed it. Cold at the bottom and hot at the top means magnetite sludge sitting in the radiator, and bleeding will do nothing at all.
Sludge is iron oxide from corroding radiators. It settles, blocks flow, ruins efficiency, wrecks pumps and diverter valves — and it will invalidate a new boiler's guarantee if it is not dealt with at install.
Try this first — free
Checks you can safely do yourself
Confirm it is not just air
Bleed the radiator with a key. If the top warms and the bottom stays cold, it is sludge.
Check your filter
If you have a magnetic filter fitted, look at how quickly it fills. A filter caked black within months means a heavily loaded system.
Balance the radiators
If one radiator is starved and the rest are fine, closing the lockshield slightly on the nearest radiators can restore flow — worth trying before spending anything.
Note the black water
If you drain a little water from a radiator and it comes out inky black rather than clear, you have your answer.
Never remove the boiler casing or touch gas components yourself — that's illegal as well as dangerous. Anything behind the front cover is Gas Safe work.
If that didn't fix it
What it usually turns out to be
Magnetite sludge build-up
The overwhelming majority of cases. A full powerflush restores circulation and efficiency.
£450 – £650
No inhibitor in the system
Inhibitor depletes every few years. Without it corrosion runs unchecked.
£99 – £150
No magnetic filter fitted
Every install we do gets one. Retrofitting is cheap insurance.
£180 – £260
Failing pump unable to push flow
Often a symptom of sludge rather than a separate fault.
£280 – £420
Badly balanced system
Free to correct if the water is clean.
£99
Prices inc VAT and include our £99 diagnostic. We confirm the price before any work starts — no surprises on the invoice.
Straight talk
When we'd tell you to replace instead
- The radiators are pinholed and weeping as well as blocked
- The boiler is old and a powerflush plus repairs approaches replacement cost
- You are replacing the boiler anyway — always flush at the same time
We carry full powerflushing equipment in-house, so it is our own kit and our own engineers rather than a subcontractor.
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Frequently asked questions
Do I really need a powerflush?
Only if the system water is actually loaded. We test before recommending it and we will tell you if a chemical clean and filter will do instead.
How much is a powerflush in Birmingham?
Typically £450 to £650 inc VAT depending on the number of radiators. Usually discounted when carried out with a new boiler install.
Will a powerflush fix cold radiators for good?
Combined with fresh inhibitor and a magnetic filter, yes — that is the combination that keeps it clean long term.
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