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.
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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.
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Lucie Victoria
15 May 2025
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Excellent service, highly recommend
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James Mowat
30 April 2025
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Incredible service — problems fixed perfectly once again.
Lee, Paul and the team at COMGAS are fantastic. They arrived on time and got straight to work, quickly identifying the root cause of the plumbing and heating issues I was experiencing.
The problems were diagnosed swiftly, I was kept fully informed throughout, and both issues were fixed with impressive speed and precision.
Having worked in the industry for nearly 20 years, I know how rare it is to find reliable, trustworthy, and genuinely skilled professionals — let alone a team this good.
I joked about it at the time, but it’s true: I had the best shower I’ve ever had in this house after they finished the job — and I’ve lived here for 10 years!
I honestly couldn’t ask for more and can’t recommend COMGAS highly enough.
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Faz
31 March 2025
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ComGas replaced my boiler and water tank at short notice. So glad i used them. Thank you
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Laura Andrews
26 March 2025
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Lee came out to the school I work in to look at a problem with our heating, he diagnosed the issue and was able to fix it on the spot in a neat and tidy manor. Would highly recommend
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Chloe
26 March 2025
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I recently used Comgas for a gas and heating service, and I couldn't be more impressed! From start to finish, their team was professional, friendly, and highly knowledgeable. They quickly identified the issue with my heating system and explained everything in detail, offering both short- and long-term solutions.
The technician was punctual and took extra care to ensure everything was running smoothly. I really appreciated their attention to detail and how they made sure to clean up after the work was completed. My home is now warm and cozy, all thanks to their expert service. I highly recommend Comgas for anyone looking for reliable, efficient, and trustworthy gas and heating professionals!
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Bec Newey
8 July 2024
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Messaged Lee on a Sunday evening with request. Instant reply that evening and engineer came next day. Polite, professional and helpful. Thorough investigation and speedy resolution to a leak with sink in wc. Happy customer.
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P Duffy
2 April 2024
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Scott and the team have served me well over several years. Skilled craftsmen. Dependable careful and very cosy-effective. I highly recommend them.
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Cilla Trout
22 March 2024
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Definitely recommend friendly reliable service
Scott was helpful and reassuring regarding our boiler answered our concerns and explained the problem and best course of action thank you for your advice
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.
