ComGas installs and replaces radiators across Farnham, Guildford and the surrounding area, sizing each one to the room's actual heat loss rather than the radiator being removed. Valves are chosen and set correctly, the system is balanced on completion, and radiators can be resized now if a heat pump is a future possibility.
A replacement radiator should be sized for the room, not the gap on the wall.
The easiest way to replace a radiator is to fit the same size back in the same place. It's also the easiest way to end up with a room that never quite gets warm, because the original radiator may have been undersized for the room in the first place, or the room's heat loss has changed since — new windows, added insulation, or a loft conversion next door.
We calculate heat loss room by room before recommending a size, so the replacement (or new) radiator is matched to what the room actually needs at the flow temperature the system runs at. Where a room is a genuine outlier — a cold conservatory link, a room over a garage — we say so and size around it, rather than fitting a standard radiator and leaving the problem in place.
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How we approach radiator work
Sizing, valves, moves and replacements — all worked out properly first.
Every radiator job starts with the room's heat loss and the system it sits on, so sizing, valve choice and positioning are all decided correctly before anything is fitted.
Radiator Sizing & Heat Loss
Radiator output is matched to a proper heat loss calculation for the room, not to the size of the radiator being taken out or a rule of thumb per square metre.
Heat loss calculated per room from wall, floor, roof and window construction — not from the outgoing radiator's size
Radiator output checked in kW at the flow temperature the system actually runs at, not a generic catalogue figure
Undersized radiators are one of the most common reasons a room never quite reaches temperature
Radiator length, height and position reviewed where furniture, doors or skirting heating pipes constrain the options
The right valve, correctly set, is what allows a heating system to control each room individually and heat evenly rather than favouring whichever radiator is closest to the boiler.
Thermostatic radiator valves (TRVs) fitted to each radiator except the room holding the main thermostat, so rooms can be controlled independently
Lockshield valve set during commissioning to balance flow around the whole system, not left at its factory default
Angled, straight or corner valve bodies chosen to suit the existing pipework and keep visible pipe runs to a minimum
A correctly balanced system reaches temperature at a similar rate in every room rather than leaving some radiators lukewarm
Radiators are moved, added or replaced as part of redecoration, an extension, or simply because one has failed, with the pipework altered to suit rather than the layout being forced around it.
Radiators relocated to a new wall during redecoration or a room reconfiguration, with pipework rerouted accordingly
Additional radiators added to extend an existing system where the boiler and pipework have the spare capacity
Leaking or failed radiators replaced like-for-like or resized where the room's heat loss has changed since it was fitted
System drained, refilled and fully vented on completion to avoid airlocks and the cold spots they cause
LST, Designer Radiators and Heat-Pump-Ready Sizing
Low surface temperature and designer radiators are sized against the same heat loss calculation as any other radiator, and sizing changes again if a heat pump is a genuine future possibility.
Low surface temperature (LST) radiators, with a protective outer casing, specified where a hot radiator surface is a safety concern
Designer and vertical radiator options considered where appearance matters, sized to the room's heat loss rather than chosen on looks alone
Radiators intended to eventually run from a heat pump are sized larger, since heat pumps typically supply water at 45–55°C rather than the 70°C+ a boiler can reach
Sizing generously now, even while still on a boiler, avoids replacing radiators a second time if the heat source changes later
Sizing radiators for a lower flow temperature protects against replacing them twice.
A boiler can comfortably push water round a heating system at 70°C or more, which is why many existing radiators are sized smaller than they would need to be at a lower temperature. A heat pump doesn't work that way — it's most efficient supplying water at somewhere around 45–55°C, and pushing it much higher costs efficiency. A radiator sized for a 70°C system will usually be undersized if the same house later moves to a heat pump running at 45°C, because the radiator simply can't shed as much heat at the lower temperature.
This doesn't mean every radiator needs replacing before a heat pump goes in — some rooms will already have enough radiator surface area, and a proper room-by-room survey at the time will confirm which do and which don't. What it does mean is that if you're replacing radiators now for any other reason, and a heat pump is a realistic possibility in the next several years, it's worth sizing generously rather than like-for-like, since it costs little extra now and avoids doing the job twice.
Boilers typically run radiators at 70°C+; heat pumps are most efficient nearer 45–55°C
A radiator sized for a boiler's flow temperature is often undersized once a heat pump is fitted
Sizing generously now avoids a second radiator replacement if the heat source changes later
Heat pump sizing and running costs are covered separately on our renewable energy page
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
Radiator questions
Frequently asked questions.
If you're replacing radiators, adding to an existing system, or thinking ahead to a heat pump, get in touch and we'll size the work to your rooms.
How do you decide what size radiator a room needs?
We calculate the room's heat loss from its construction, glazing and orientation, then match a radiator output in kW to that figure at the system's actual flow temperature. It isn't based on the size of the radiator currently fitted or a fixed rule per square metre.
Can I keep my existing radiators if I get a new boiler?
Usually yes, provided they were correctly sized in the first place and the new boiler runs at a similar flow temperature. We check this as part of any boiler replacement rather than assuming the existing radiators are adequate.
What's the difference between a TRV and a lockshield valve?
A TRV lets you set and control the temperature of an individual room by responding to the air temperature around it. The lockshield valve, on the other radiator connection, is set once during commissioning to balance how much flow that radiator receives relative to the rest of the system.
Can radiators be moved without redoing the whole heating system?
Yes, in most cases a radiator can be relocated by extending or rerouting the pipework feeding it, without altering the rest of the system. The practical scope depends on the pipe route available and how far the new position is from the old one.
Are low surface temperature radiators only for care settings?
No — LST radiators are also fitted in family homes wherever a hot exposed radiator surface is a concern, such as a young child's bedroom or a small bathroom. They're sized the same way as any other radiator, just with a protective casing added.
Should I size my radiators differently if I might get a heat pump later?
If a heat pump is a realistic possibility in the next several years, it's worth sizing new or replacement radiators generously now, since heat pumps run at a lower flow temperature and need more radiator surface area to deliver the same heat output.
Why is one room in my house always colder than the rest?
It's usually one of a small number of causes: an undersized radiator for that room, a system that isn't properly balanced, or a TRV that has failed or is fitted incorrectly. We check all three rather than assuming it's simply a bigger radiator that's needed.
Do designer radiators heat a room as well as standard panel radiators?
They can, provided the output is genuinely matched to the room's heat loss — vertical and designer radiators often have a different surface area to a panel radiator of similar dimensions, so the specification needs checking rather than assumed from appearance alone.
