Short answer: Before a new boiler goes on, the heating system should be cleaned, refilled with inhibitor and fitted with a filter, as BS 7593 sets out. A power flush pumps water through at high flow to push sludge out. A chemical clean uses cleaner and the system's own circulation. Which suits depends on how dirty the system is.
A new boiler has narrow waterways and a sensitive heat exchanger. Put it on an old system full of sludge and it can start to block within months. That is why cleaning the system is part of a proper boiler installation, not an extra.
What sludge is and where it comes from
Sludge is mostly black iron oxide. It forms when steel radiators slowly corrode inside. Air getting into the system speeds it up. So does fresh water from frequent topping up, which also dilutes the inhibitor, the chemical meant to stop corrosion.
Signs of sludge:
- radiators cold at the bottom and warm at the top
- black water when you bleed a radiator
- a noisy boiler or pump
- radiators that take a long time to heat
Hard water adds scale
North and east London water is hard. Thames Water’s 2025 reports show averages of 268 mg/l in Hackney West, 276 mg/l in Highams Park and 273 mg/l in Tottenham South, all in its hard band of 200 to 300 mg/l. You can see the Highams Park report here.
In a heating system, scale builds up mainly when fresh water comes in. A system with a slow leak that has been topped up for years can have both sludge and scale. Our guide on why boiler pressure keeps dropping explains how to spot that.
What BS 7593 asks for
BS 7593 is the British Standard for treating water in domestic heating systems. Fernox summarises the 2019 version as three steps: clean the system, protect it with inhibitor, and fit a filter. For hard water areas of 200 ppm and above, it says a scale reducer should be fitted too. The Domestic Building Services Compliance Guide makes the same point about treating the feed water where hardness is over 200 ppm.
Power flush or chemical clean
| Method | How it works | When it suits |
|---|---|---|
| Power flush | A pump is connected to the system and pushes water and cleaner through at high flow, one radiator at a time, flushing sludge out | Heavily sludged systems, cold bottoms on many radiators |
| Chemical clean | Cleaner is added and circulated by the system’s own pump, then drained and flushed | Systems in fair condition, or alongside a new boiler where the old one is removed |
| Mains flush | Mains water is run through the system to drain | Light dirt, smaller systems |
There is no single right answer. On a very old system with thin, corroded radiators, a high flow power flush can expose weak spots and cause leaks. On a lightly dirty system it may be more than is needed. The installer should look at the water, the radiators and the age of the system before choosing.
What gets fitted afterwards
Cleaning is only part of it. After a clean, the system is refilled and protected.
Inhibitor. A chemical added to the heating water to slow corrosion and scale. Its level should be checked at each service.
Magnetic filter. A small canister on the pipe returning to the boiler. A magnet inside catches iron particles. An engineer cleans it out at the annual boiler service.
Scale reducer. Fitted on the cold mains feed, mainly to protect a combi’s hot water side. Given the local hardness figures, it is normally part of a new combi installation here.
Older London systems
North and east London has many systems that have been changed over decades. A Victorian terrace in Walthamstow or Leyton may have radiators of very different ages, some on pipes laid when the house first had central heating and some added with a later extension. Warner Estate half houses, where one terraced house holds two flats, often have two separate systems with different histories in the same building. Our Walthamstow area page covers this housing.
Converted houses can also have pipework running between flats, with valves that are hard to reach. Clean water helps a new boiler last, but it is worth asking the engineer about any radiators or valves they think are near the end of their life.
Warranty and records
Many boiler warranties expect the system to be cleaned and protected at installation, and to be serviced every year. Terms vary by manufacturer, so read yours. Keep the paperwork that shows what clean was done, what inhibitor was added, and that a filter was fitted. Our guide on boiler warranties and annual servicing explains why records matter.
Who does the work
Cleaning the water side of a heating system is not gas work in itself, but it is normally done as part of fitting or servicing a gas boiler. The boiler work must be done by a Gas Safe registered engineer. Ask to see the ID card, and check it on the Gas Safe Register. The right clean and its price are confirmed after a look at the system.
Quick questions
Do I need a power flush with a new boiler?
The system needs cleaning in some way. Whether that means a full power flush or a chemical clean depends on the condition of the radiators and the water. The installer should test and look before deciding.
What is a magnetic filter?
It is a small unit fitted on the pipe going back into the boiler. A magnet inside catches black iron sludge before it reaches the boiler. It is cleaned out at each service.
Will a power flush fix cold radiators?
If sludge is the cause, it often helps. If a radiator is cold because of a stuck valve, air or a balancing issue, flushing alone will not fix it.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.