If your radiators are cold at the bottom, the boiler is noisy or some rooms never quite warm up, someone has probably mentioned a power flush. Prices vary more than most people expect, so here is a straight explanation of what you should pay, what changes the figure, and when you do not need one at all.
For a standard house with a combi or system boiler, most heating engineers in the Bexley, Dartford and Bromley area charge somewhere between £300 and £600 for a full power flush. That usually covers a day's labour, the cleaning chemical, an inhibitor added at the end and a check of each radiator afterwards.
Larger homes cost more because the engineer is connecting to more radiators and running the machine for longer. Anything with 12 or more radiators, or a mix of old cast iron and modern panel radiators, can push the price towards £700 to £900. Very cheap quotes under £250 are worth questioning, as they often mean a quick chemical dose and a drain down rather than a proper machine flush.
The number of radiators is the biggest factor, but it is not the only one. The condition of the system matters just as much. A 1970s terrace in Welling or Sidcup with original steel pipework and no inhibitor for a decade will have far more black sludge than a five year old system, and it takes longer to clear.
Access is the other thing that catches people out. Radiators boxed in behind fitted units, microbore pipework with 8mm or 10mm runs, and heating in loft conversions all add time. If the engineer has to remove and replace radiator valves that no longer close properly, expect that to be added on. A good quote will list these as separate items rather than bundling everything into one vague figure.
A power flush is the right call when the symptoms point to sludge and corrosion debris. The classic signs are radiators that are warm at the top and cold across the bottom, brown or black water when you bleed them, a boiler that kettles or cuts out on overheat, and a pump that has failed more than once.
It also pays for itself when you are fitting a new boiler. Most manufacturers require the system to be cleaned before installation and will not honour the warranty if a heat exchanger blocks up with old debris. If your installer has not mentioned cleaning the system, ask why.
Not every cold radiator needs a flush. A single radiator that is cold at the top simply needs bleeding. A radiator that is cold all over often has a stuck thermostatic valve pin or a lockshield valve that has been closed off, both of which are five minute jobs.
If the system is only mildly dirty, a chemical clean is a cheaper middle ground. The engineer adds a cleaning agent, lets it circulate for a week or two, then drains and refills with inhibitor. This tends to cost £120 to £200 and works well on systems that have been reasonably maintained. An honest engineer will tell you which option fits your system rather than defaulting to the more expensive one.
Ask whether the price includes inhibitor, a magnetic filter clean or fit, and a check of each radiator afterwards. Ask how long the engineer expects to be on site, because a genuine power flush on an average house takes most of a day, not an hour. If you are told the system will be flushed but nobody mentions removing the pump or protecting the boiler, that is a sign the job will be done in a hurry.
It is also reasonable to ask what happens if the flush does not fully clear a radiator. Badly corroded radiators sometimes cannot be saved, and it is better to know the cost of replacing one before the work starts than to be surprised afterwards.
For a typical three bedroom house it takes between four and eight hours, depending on how many radiators there are and how heavy the sludge is. Very dirty systems can run into a second visit.
A properly controlled flush should not, but it can reveal weak spots that were already there, such as pinholes in a corroded radiator or a weeping joint. A good engineer will inspect the system first and warn you if anything looks fragile.
Most systems only need it once every five to ten years, and often not at all if inhibitor is topped up and a magnetic filter is fitted and cleaned at each annual service. Regular servicing is much cheaper than repeat flushing.
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