How a power flush is actually done

Before anything is connected

A competent engineer starts by establishing what they are dealing with: system age and type, open vented or sealed, radiator count and material, pipework size, and where the problems actually are. Surface temperatures across each radiator — top, middle, bottom — map where the deposits sit. Bleeding a radiator onto a cloth shows what is circulating.

This is also where the risk conversation happens. On a very old steel system, flushing can expose a pinhole that corrosion had effectively sealed. That is not a reason to refuse the job, but you should hear about the possibility before work starts, not after.

1. Connecting the pump

The flushing unit is connected either at the circulating pump head or across the tails of a radiator. The boiler is isolated or run at a controlled low temperature depending on the method used and the system type. On an open vented system the header tank is isolated.

2. Initial circulation and chemical dosing

Water is circulated at high flow and low pressure — high velocity is what shifts sediment, not high pressure, which would simply stress the system. Cleaning chemical goes in and circulates through the whole system, typically warm, to break down deposits. A descaler goes in as well in hard water areas.

3. Radiator by radiator

This is the part that takes the time and the part that gets skipped. Each radiator is isolated in turn and flushed individually, with the flow reversed repeatedly, so that the full force of the pump goes through that one panel rather than taking the path of least resistance through the rest of the system. Stubborn radiators may be tapped along the bottom to break up consolidated sludge.

An engineer who connects a pump, circulates for an hour and leaves has not done this, and it is the difference between a flush and a rinse.

4. Flushing to clear

The system is flushed through with fresh water until what comes out runs clear. A magnetic capture device in the flush circuit traps the magnetite so it can be shown to you — and it is entirely reasonable to ask to see it.

5. Neutralising and refilling

Any residual cleaning chemical is neutralised and flushed out — leaving acidic cleaner in a system causes its own corrosion. The system is refilled with clean water.

6. Inhibitor, filter, and balancing

Corrosion inhibitor is added at the correct dose for the system volume. A magnetic filter is fitted if there is not one. The system is refilled to the right pressure, vented, and the radiators rebalanced — because flow characteristics change once the restrictions are gone.

7. What you should be left with

• Temperature readings before and after, so the improvement is demonstrated rather than asserted.
• A written record of what was done, what chemicals were used and at what dose. Keep this — a boiler manufacturer may ask for it.
• A clear statement of what inhibitor is in the system and when it should next be checked.
• Somewhere to go if a problem shows up in the following weeks.

How long it takes

A six to eight radiator house is generally a full working day. Fifteen radiators, or microbore pipework, or cast iron, can run into a second. Anyone offering a two-hour power flush on a ten radiator system is describing a different job from this one.