What a power flush actually does
Over years, central heating water corrodes the inside of radiators and produces black iron oxide sludge. It settles at the bottom of radiators, leaving them cold in the middle, and circulates through the boiler, where it wears the pump and clogs the heat exchanger. A power flush connects a pump to the system and pushes cleaning chemicals through at high flow, radiator by radiator, until the water runs clear.
A chemical flush is the gentler version: cleaner goes into the system, circulates for a period, and is drained down without the machine. It costs less, often around £100 to £200 as part of other work, and for lightly soiled systems it does the job.
The signs a system actually needs it
Cold patches at the bottom-middle of radiators while the tops get hot is the classic one, because sludge settles low. Others: radiators needing frequent bleeding, dirty brown-black water when you bleed them, kettling noises from the boiler, and a pump that runs hot or has failed before.
If radiators are cold at the top rather than the middle, that is air, not sludge, and bleeding them is free. If one radiator is cold everywhere, that is usually a stuck valve. A power flush fixes neither, which is why the symptom matters more than the sales pitch.
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Fitted alongside a new boiler, cleaning the system properly is close to non-negotiable: manufacturers commonly require evidence of a cleaned system and an inhibitor for the full warranty, and installers quote a flush as a line item on the job. What is worth checking on a boiler quote is which kind of flush is included, because a chemical clean and a power flush are different jobs at different prices.
Push back when a power flush is offered as a cure for a symptom it does not treat, or on a very old system with weeping joints, where high-flow cleaning can turn seeps into leaks. And after any flush, the lasting protection is cheap: inhibitor in the water and a magnetic filter on the return, which together are why a well-treated system may never need flushing again.
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