Radiators cold at the bottom and a boiler that rumbles? The system needs cleaning, not bleeding

A flushing pump, a machine roughly the size of a large suitcase, is connected to the heating circuit, usually across the pump position on the boiler or across the tails of one radiator. It circulates the system water through its own reservoir at high velocity, with cleaning chemical added to break down the sludge and scale, and a strong magnet in the flow to trap the iron oxide. The direction of flow is reversed at intervals, because deposits that will not shift one way often move the other. Each radiator is isolated and flushed on its own, with a rubber mallet used on the panel to dislodge what is settled. When the water from every radiator runs clear, the chemical is flushed out with fresh water, the system is refilled and inhibitor is added.

Radiators cold at the bottom or in the middle while the top is hot. Radiators that need bleeding constantly. Black or brown water when a radiator is bled. A boiler that kettles: a rumbling, banging or whistling noise as it heats. A pump that is noisy or has failed more than once. Radiators that leak at the seam. A system that takes far longer to heat the house than it used to. The magnetic filter full of black sludge at every service. And, on any system more than a few years old, a boiler about to be replaced, because a new heat exchanger blocked with old sludge in its first winter is an avoidable and expensive way to void a warranty.

The engineer first checks the system is suitable: the age and type of pipework, the condition of the radiators, and whether any valve or joint looks likely to fail under the flush. That honesty matters, because a flush can expose a weakness that sludge was sealing, and a system with pinholed steel pipe under a concrete floor is better treated with a gentler chemical clean than a full flush. Then the pump is connected, the chemical circulated, each radiator flushed in turn, the boiler heat exchanger flushed, the water tested for clarity and for the dissolved solids that show the clean is complete, and the system refilled, dosed and pressure checked. A typical house takes most of a day.

Radiators give out their full output again, so rooms heat faster and the boiler runs for less time. The boiler's heat exchanger transfers heat properly, so it stops kettling and runs at its design efficiency. The pump is no longer grinding magnetite through its bearings. The corrosion that makes the sludge slows right down once the inhibitor is at the correct dose in clean water. And the pinholes and seam leaks that were on their way stop developing, because the mud that was eating the steel is gone.

A power flush is priced by the number of radiators, agreed in writing before the visit. Inhibitor and the cleaning chemical are included. A magnetic filter, where one is recommended and there is not one already, is quoted as a separate line so you can see the cost. If the survey on the day finds the system is not suitable for a full flush, the alternative is explained and priced before anything is connected, and you decide.

Flushes are booked across London and the surrounding towns and take a full day slot, so they are scheduled rather than squeezed in. A flush before a boiler replacement is done on the first day of the installation. Landlords with several properties can have the flushes grouped by area.

Because the survey comes first, and a system that should not be flushed is told so. Because each radiator is flushed individually rather than the whole system being circulated for an hour and called done. Because the water is tested at the end, not judged by eye. And because the same engineers install boilers and repair heating systems, so a flush is one part of keeping the system working, not a product sold on its own.