ComGas carries out power flushing for central heating systems across Farnham, Guildford and the surrounding area, clearing magnetite and sludge that cause cold radiators, pump failures and boiler noise. We fit a system filter and dose inhibitor afterwards, and we'll tell you plainly if flushing isn't actually the answer to your problem.
A flush that isn't followed by a filter and inhibitor just delays the same problem.
Central heating systems accumulate magnetite — black iron oxide sludge — as the steel and iron components inside corrode slowly over years. Left alone, it settles at the bottom of radiators, restricts flow, and works its way into pumps and valves, where it causes premature failure.
Power flushing forces water and cleaning chemical through the system at a high flow rate to dislodge and remove this sludge, radiator by radiator. It's most effective as part of a wider approach that also includes a magnetic filter and inhibitor, because a flush on its own doesn't stop the corrosion that produced the sludge in the first place — it just clears out what's already there.
ComGas Services UK Ltd Gas Safe · OFTEC · REFCOM F-Gas · CHAS · Constructionline
How we approach power flushing
The right method, followed by the protection that stops it happening again.
We identify the actual cause before flushing, choose the method that suits how badly the system is fouled, and protect the system afterwards so the problem doesn't simply return.
Signs Your System Needs Flushing
Several common symptoms point towards sludge or scale inside the system, though they need checking against other possible causes before flushing is recommended.
Cold spots at the bottom of a radiator, with the top staying warm, typically indicate settled magnetite
A noisy boiler or banging, kettling pipes can indicate limescale or sludge restricting flow through the heat exchanger
Repeated pump or valve failures often trace back to debris circulating through the system
Black, gritty water when bleeding a radiator is a reliable sign that magnetite is present
Flushing Methods: Chemical, Power and Mains-Pressure Jet
There isn't one single flushing method — the right approach depends on how fouled the system actually is, established by inspection rather than assumed from the symptoms alone.
Chemical flush: a cleaning chemical dosed and circulated for a period, suited to lightly fouled systems
Power flush: a high-flow, low-pressure pump forces water and cleaning chemical through the system at speed, radiator by radiator, to dislodge sludge
Mains-pressure jet flushing: used on individual severely blocked radiators where a power flush pump alone won't clear them
The method is matched to the system's condition, not applied as a fixed default regardless of how fouled it is
A flush that isn't followed by inhibitor and a filter just buys time.
Power flushing clears out sludge that has already accumulated, but it doesn't change the underlying chemistry of the system. Steel radiators and iron components will continue to corrode slowly whenever they're in contact with untreated water, which is exactly the process that produced the sludge in the first place. A flush with nothing done afterwards simply resets the clock on the same problem.
Inhibitor dosed into the system after flushing slows that corrosion significantly, and a magnetic filter fitted to the return pipe catches whatever debris does form before it can circulate and cause damage. Together, they're what turn a flush from a temporary fix into a lasting one — which is why we treat both as part of the job rather than an optional extra.
Corrosion inside a heating system is ongoing, not a one-off event that a flush permanently resolves
Inhibitor dosed after flushing significantly slows further corrosion and scale formation
A magnetic filter catches debris before it can circulate and cause pump or valve damage
A filter needs periodic emptying — check it as part of annual servicing
Reviews from real ComGas customers, shown as submitted through Google.
Posted on Google
Nick
3 September 2026
Trustindex verifies that the original source of the review is Google.
Matt from ComGas Services UK Ltd was very professional, polite, friendly and quick installing a new shower in my bath (and at a better height for me) and a much easier press-in plug for my sink! He left the place spotless. This is the kind of service and installation people dream about - a 5 star service (see photos).
Posted on Google
Laura Peters
17 August 2026
Trustindex verifies that the original source of the review is Google.
I have used Comgas twice and both times have been very happy with the service. Very friendly and honest. The most recent time, the job turned out to be a lot bigger than they initially thought but it was no issue and Paul stayed the extra time to make sure the job was done properly! He was really tidy, took a lot of care with the work that was done and took the time to explain what he was doing and why. Pleased to have found a reliable heating company!
Posted on Google
James Ross
8 August 2026
Trustindex verifies that the original source of the review is Google.
I had an issue with my boiler and Matt came out to have a look the same day and managed to fix it. Great, professional service. Highly recommend.
Posted on Google
Ian Chevalier
11 June 2026
Trustindex verifies that the original source of the review is Google.
First time we've used this company and it won't be the last. Very professional and efficient in everything they do and Matt was excellent in his knowledge and efficiency when dealing with our gas and plumbing problems.
Posted on Google
Karen Flackett
20 May 2026
Trustindex verifies that the original source of the review is Google.
Excellent service. Matt came & fitted 2 taps & waste pipes he was polite & friendly & carried out the work efficiently & in a timely manner.
Posted on Google
Paul Norris
30 April 2026
Trustindex verifies that the original source of the review is Google.
Fantastic experience from the chaps at ComGas! Special thanks to Matt who removed an old boiler and replaced with a new energy efficient to satisfy the demands of a five bed house with a few bathrooms/baths and showers. The ComGas admin team and Lee were fantastic too on the booking and administration side of things!
Posted on Google
Julie Galvin
21 April 2026
Trustindex verifies that the original source of the review is Google.
Here at St Joseph's we have worked with Comgas for a number of years. They have completed big projects and undertake regular maintenance for our school. They price fairly, and their work is completed to a very high standard. All the engineers carry a DBS which makes life so much easier when trying to book an engineer visit.
On the rare occasion that we have an emergency I can trust that Lee will get one of his engineers to respond quickly and efficiently.
Nothing is too much trouble and no job is too small.
Posted on Google
Chesca Haley
16 April 2026
Trustindex verifies that the original source of the review is Google.
Lee and the team are amazing! So helpful, so knowledgeable and really look after their customers. Highly recommend.
Posted on Google
Sarah Rogers
6 April 2026
Trustindex verifies that the original source of the review is Google.
Absolutely amazing service, always a thorough job and very knowledgable. Really nice guys too. Lee has sorted numerous issues at work and around our house. We have recommended to friends and family who are also very happy
Posted on Google
Alison Molan
25 March 2026
Trustindex verifies that the original source of the review is Google.
Lovely engineers, Matt and Jack, were a pleasure to have in the house. Thorough, knowledgeable, tidy and polite. Would highly recommend.
Power flushing questions
Frequently asked questions.
If your radiators are cold at the bottom, your boiler is noisy, or you've had repeated pump or valve failures, get in touch and we'll assess whether flushing is actually the fix.
It's a process that forces water and a cleaning chemical through the heating system at a high flow rate and low pressure, dislodging sludge and debris from radiators and pipework so it can be flushed out, radiator by radiator.
How do I know if my radiators need flushing rather than bleeding?
Bleeding removes trapped air, usually indicated by warmth at the bottom and coolness at the top. Sludge does the opposite — cold at the bottom, warm at the top. If bleeding a radiator doesn't fix a cold patch, or the water that comes out is black and gritty, flushing is the more likely answer.
Will power flushing fix a boiler that keeps losing pressure?
Not on its own — pressure loss is usually caused by a leak or a faulty component such as an expansion vessel or pressure relief valve, not by sludge. We'd diagnose the actual cause of pressure loss before considering whether a flush is also needed.
Is a chemical flush enough, or do I need a full power flush?
It depends on how fouled the system is. A chemical flush circulated for a period can be sufficient for lightly affected systems, while heavier sludge generally needs the higher flow rate of a power flush to shift it effectively.
How often should a system filter be emptied?
Typically as part of an annual service, though a heavily sludged system may need it checked sooner after a flush while any remaining debris settles out. We check the filter each time we service the system.
Can flushing damage an old system?
It can, if pipework or radiators are already badly corroded and structurally weak — the flushing process can find a weak point that was previously being held together by the sludge inside it. We inspect the system's condition before recommending a full power flush.
Do I need to flush before fitting a new boiler?
In most cases, yes. Manufacturers generally require the existing system to be flushed clean before a new boiler is connected, since sludge from the old system can quickly damage a new heat exchanger.
What's the difference between a power flush and mains-pressure jet flushing?
A power flush treats the whole system at once using a dedicated pump. Mains-pressure jet flushing targets individual radiators using mains water pressure and is typically used where one or two radiators remain badly blocked after a general power flush.
