Chemical flush
A cleaning chemical is added to the system and circulated by the system's own pump, usually for anything from an hour to a few days depending on the product and how bad things are. The system is then drained, refilled and dosed with inhibitor.
Typical cost: £150 to £250.
Time on site: often two visits, an hour or two each.
Good for: light to moderate sludging, newer systems, maintenance before a problem develops, systems where high-pressure flushing carries a leak risk.
Limitation: it relies on the system's own circulation. Anywhere flow is already blocked, the chemical does not reach properly — which is exactly where the worst deposits are.
Power flush
A dedicated high-flow, low-pressure pump is connected to the system, usually at the pump head or across a radiator's tails. Water and cleaning chemicals are driven through at a much higher velocity than the system normally sees, with the flow reversed repeatedly and individual radiators isolated and flushed in turn to dislodge and carry out sediment. A magnetic capture device usually sits in the flush circuit to trap what comes out.
Typical cost: £350 to £800, higher in London and the South East.
Time on site: most domestic jobs are a single day; larger houses can run into a second.
Good for: genuinely sludged systems, cold-at-the-bottom radiators, pre-boiler-replacement cleaning, older systems that have never been treated.
Limitation: on very old systems, disturbing consolidated corrosion can expose a pinhole that the deposit was effectively plugging. This is not a reason to avoid a flush, but it is a reason to have an honest conversation about system age first.
Which one, in practice
| Situation | Usually the right call |
|---|---|
| System under 10 years, sealed, mild symptoms | Test inhibitor first; chemical flush if needed |
| Several radiators cold at the bottom | Power flush |
| Black water on bleeding, system 15+ years | Power flush |
| New boiler going in, system in decent condition | Chemical flush may satisfy BS 7593 |
| New boiler going in, system visibly sludged | Power flush |
| Very old steel system, known weeping joints | Chemical flush, low pressure, discussed honestly first |
| Cast iron column radiators, period property | Specialist job — lower pressure, longer time |
| Hard water area with scale as well as sludge | Either, but with a descaler in the mix |
What both should include
Whichever you have done, the job is not finished until:
- The correct dose of corrosion inhibitor is in, calculated for your system volume rather than guessed.
- A permanent in-line filter is fitted, if there is not one already.
- Whatever was letting oxygen in has been identified.
- You have something in writing saying what was done — you will want it if there is ever a boiler warranty claim.
An engineer who wants to flush your system but has nothing to say about any of the above is selling you a job rather than fixing a problem.
Not sure which you need?
That is the normal position, and it is not something to resolve from a website. Send us the symptoms and we will put you in front of a Gas Safe engineer covering your area who can look at it. If the answer is that you need neither, that is a perfectly good outcome.
Common questions
What is the difference between a power flush and a chemical flush?
A chemical flush circulates cleaning chemicals using the system's own pump. A power flush uses a separate high-flow pump to drive water through at much higher velocity, reversing the flow and isolating radiators individually to physically dislodge deposits. The power flush is more thorough and roughly twice the price.
Is a chemical flush good enough before a new boiler?
It can be. BS 7593:2019 requires the system to be cleaned before a new boiler is installed, not specifically that it is power flushed. On a system in reasonable condition a chemical flush can satisfy that. On a visibly sludged system it will not.