Power flush — what it is, when you need one, and what it costs

A power flush is a process where an engineer connects a high-flow pump to your central heating system and circulates a chemical cleaning solution at high velocity to remove sludge (black magnetite), scale, and corrosion deposits from the pipework, radiators, and heat exchanger.

Signs that your system may benefit from a power flush: some radiators are slow to heat or cold at the bottom despite bleeding, the system water (from bleeding) is very dark or black, the boiler is kettling or noisy, or the heating takes a long time to warm up the house. A new boiler being installed into an old system almost always requires a flush.

A power flush is carried out by a heating engineer. They connect the flush machine between two points on the system (typically where the pump sits), add chemicals (cleaner + descaler), and run the machine for 4–8 hours on a typical 5–8 radiator system, flushing each radiator individually with a combination of flow reversal and mechanical agitation.

At the end, the system is refilled with clean water and a full dose of central heating inhibitor (Fernox F1 or Sentinel X100) is added to prevent future corrosion. A magnetic filter is recommended at this point.

Cost depends on system size: a typical UK semi-detached with 6–8 radiators costs £400–£600; a larger detached with 10–14 radiators costs £600–£900. Be cautious of quotes significantly below this range — cutting the flush short is common and leaves sludge in the system.

If a power flush is not viable (very old or fragile pipework that might not survive the pressure), a chemical flush (adding a cleaner to the system and running it for several weeks before draining) is a less invasive but less thorough alternative at £100–£200.