London · Essex · Hertfordshire
Most people quoted for a power flush don't need one
A cold radiator usually means air, a stuck valve, or poor balancing — all of which cost a fraction of a £600 flush. Work out which you've got before you spend anything.
The 30-second version
- No Cold at the top? That's air. Bleed it — free.
- No One radiator cold all over? Almost always a valve.
- Yes Several cold at the bottom? That's sludge.
- Yes Black water when you bleed? That's magnetite.
- Yes Boiler banging or kettling? Worth acting on.
What it should cost
£350–£800 for most homes in 2026, driven by radiator count, with London at the top of the range. We list every cheaper alternative above the expensive one.
What sludge actually looks like
Black, gritty magnetite that settles where flow is slowest. The symptoms that mean it — and the six that people mistake for it.
Power flush or chemical flush?
Two different jobs at two different prices. On a lightly sludged system the cheaper one may be all you need.
Why a power flush site is telling you not to buy one
Because we're a matching service. Engineers pay us for enquiries, and an engineer will only keep paying if the enquiries turn into work. A stream of people who needed a radiator bled is worth nothing to them and nothing to us.
So the incentive runs the right way: get the diagnosis right first, and send on the enquiries where a flush is genuinely the answer. How we vet engineers and how we're paid sets out the whole arrangement, including its weaknesses.
When it is the right call
You've got established sludge
Several radiators cold across the bottom, dark water on bleeding, a boiler that kettles, and a system old enough to have never had inhibitor topped up. That's a real case, and a flush is considerably cheaper than the radiators or heat exchanger it saves.
You're having a new boiler fitted
Here it isn't optional. BS 7593:2019 requires the system cleaned before the new boiler goes in, a permanent filter fitted, and inhibitor dosed correctly — and manufacturers ask for evidence of it when assessing warranty claims. More on boiler quotes.
You're retrofitting a heat pump
Heat pumps run lower flow temperatures with no output in reserve, so they're far less tolerant of restricted circulation than a gas boiler. The cleanse standard is higher. Why it matters more here.
It's a commercial or communal system
Offices, blocks, schools, care homes and pubs — larger volumes, multiple zones, and usually a building that has to keep running while the work happens. Commercial work.
Get an honest answer on what your system needs
Tell us the symptoms. We'll pass it to a Gas Safe engineer covering your area who can tell you whether you need a flush — including if the answer is no.
Areas we cover
Around forty towns and boroughs across London, Essex and Hertfordshire. Each page covers the housing stock locally and what it tends to mean for heating systems there.