Costs
Power flush: what it costs and when it is worth it
A power flush costs £350 to £700 for a typical house, priced by the number of radiators. It is worth paying for when radiators are cold at the bottom or the water runs black, and not on a system that is already clean.
What a power flush is
A heating engineer connects a high flow pump to the system, usually in place of the circulating pump or across a radiator's connections, and drives water round the circuit far faster than the system's own pump can. Cleaning chemicals loosen the deposits, the flow direction is reversed at intervals, and each radiator is isolated and worked in turn so that all the pump's force goes through one at a time. A magnetic filter and a dump valve on the machine carry the dirty water out until it runs clear. Then the system is refilled, dosed with inhibitor and bled.
What it costs
The price is mainly set by the number of radiators, because each one is flushed individually.
| Work | Typical price |
|---|---|
| Power flush, typical house | £350 to £700 |
| Magnetic filter, supplied and fitted | £150 to £300 |
| Single radiator off the wall and flushed | £50 to £90 |
| Annual boiler service | £70 to £120 |
What moves a quote within that range:
- Radiator count, and whether any are very heavily fouled.
- Whether a magnetic filter, inhibitor and a water test are included or extra.
- Seized radiator valves that have to be replaced before each radiator can be isolated.
- Access to connect the machine, which is harder with a boiler in a loft or a cupboard full of pipes.
- A flush done at the same time as a boiler replacement usually costs less than a standalone visit.
Signs it is worth it
- Radiators cold along the bottom, more than one of them.
- Black or brown water from the bleed valves.
- A noisy pump, or a boiler that bangs and rumbles as it heats.
- A boiler locking out on overheating in cold weather.
- Radiators that take much longer to warm up than they used to.
Two or three of these together is a strong case. A single cold radiator is better treated on its own, off the wall. If the boiler is about to be replaced anyway, it makes sense to have the system cleaned as part of that job rather than separately beforehand.
When it is not worth it
On a system where the water is already clean, a power flush adds little. A water sample, which takes a minute to draw from a radiator, settles the question before any work is booked.
It also suits some systems more than others. Microbore pipework, the narrow 8mm and 10mm pipe common in houses from the 1970s, can block if a large amount of sludge is moved through it at once. And on an old system with ageing joints and valves, the cleaning chemicals and the extra flow can expose weeping fittings that the sludge had been sealing. An engineer will normally explain both risks before starting.
Chemical flush, power flush or neither
| Option | What it does | Suits |
|---|---|---|
| Chemical flush | Cleaner circulated by the system's own pump, then drained | Light fouling, new boiler on a sound system |
| Power flush | High flow pump works each radiator in turn | Heavy sludge, several cold radiators |
| Filter and inhibitor only | Catches what circulates and slows corrosion | Clean systems, as protection |
The industry reference for cleaning and treating heating water is BS 7593:2019, the code of practice for preparing, commissioning and maintaining domestic central heating systems. Boiler manufacturers commonly point to it in their installation instructions, which is why a new boiler is fitted with the system cleaned and treated.
Questions to ask before booking
Ask whether the quote is per radiator or for the whole house, and how many radiators it assumes. Ask whether the engineer will test the water first and show you the result. Ask what happens if a radiator valve will not close: whether it is replaced, and at what price. Ask whether a magnetic filter is included, and if one is already fitted, whether it will be cleaned. And ask how long the job will take, so that you are not left with the heating drained down overnight in winter.
What makes the result last
The flush is a one-off. What keeps the water clean is a magnetic filter on the return to the boiler, emptied at each service, and a correct dose of inhibitor, tested once a year and topped up whenever the system is drained. A system flushed and then left without either can be back where it started within a few years. Ask for the inhibitor brand and dose to be written on the invoice, and for a before and after sample to be shown to you.
Common questions
Can a power flush damage my heating system?
It can expose weak points. Joints and valves that were only sealed by sludge may start to weep once it is cleaned out. That is a reason to have the engineer inspect the system first, not a reason to leave a heavily fouled system alone.
How long does a power flush take?
Most of a day for a typical house, four to eight hours. A house with many radiators or very heavy sludge can run into a second day.
How often should a heating system be power flushed?
Not routinely. With a magnetic filter and inhibitor maintained, many systems never need a second one. Repeated flushing usually means the cause of the corrosion has not been dealt with.