ComGas installs, services and replaces vented and unvented hot water cylinders for homes across Farnham, Guildford and the surrounding area. Cylinders are sized to the household's actual hot water demand, unvented systems are installed and maintained to G3, and cylinders destined for a heat pump are specified with the right coil for it.
The right cylinder depends on your household's demand, not just the size of the airing cupboard.
A hot water cylinder that's too small runs out at exactly the times a household needs it most — mornings, or when several people want to bathe close together. One that's too large wastes energy keeping water hot that never gets used. Getting the sizing right depends on genuinely understanding how the household uses hot water, not simply matching the cylinder that's already there.
The choice between a vented and an unvented cylinder, and the safety requirements that come with an unvented system, are just as important as the size. We work through both before recommending a specification, and treat unvented cylinder safety as a genuine annual requirement rather than an optional extra.
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How we approach hot water cylinders
Vented, unvented, sized correctly and serviced properly.
Cylinder type, sizing and safety requirements are worked out from the household's actual demand and the wider heating system, including any plans for a future heat pump.
Vented vs Unvented Cylinders
The two cylinder types deliver hot water in genuinely different ways, and the right choice depends on the house's existing pipework, roof space and pressure needs.
Vented cylinders are fed from a cold water storage tank, usually in the loft, and operate at relatively low pressure
Unvented cylinders connect directly to the mains, delivering strong, even pressure to every outlet without a tank or pump
Removing a loft tank when moving to unvented frees up storage space, but needs mains flow and pressure to be adequate first
Vented systems remain the practical choice in some older houses or where mains pressure genuinely can't support an unvented system
Because an unvented cylinder is pressurised, its installation and ongoing servicing are governed by Building Regulations G3, with specific safety devices required by law, not just recommended.
Installation and servicing must comply with G3 and be carried out by someone competent to work on unvented hot water systems
Two independent temperature and pressure safety devices are fitted, so a single failure can't lead to an uncontrolled pressure or temperature rise
A discharge pipe routes any relief safely to a visible, suitable outdoor termination point, sized and positioned to current guidance
An expansion vessel or internal air gap accommodates the water's expansion as it heats, and is checked at every service
Annual servicing is a genuine safety requirement for unvented cylinders, because the safety devices need testing and, where necessary, replacing
Cylinder capacity is decided by the household's peak simultaneous hot water demand, not simply matched to the outgoing cylinder without question.
Capacity is sized to peak simultaneous demand — number of occupants, bathrooms and how they're typically used at the same time
An undersized cylinder runs out of hot water at genuinely busy times of day
An oversized cylinder wastes energy keeping unused water hot for no benefit
Recovery time — how quickly the cylinder reheats after use — depends on coil size and the boiler or heat source's output, not on cylinder capacity alone
Immersion Heaters, Replacement and Heat-Pump-Ready Cylinders
Immersion heaters, cylinder replacement and heat pump compatibility all come down to matching the cylinder's specification to how it will actually be used and heated.
Immersion heaters provide electric backup or standalone heating, and are serviced or replaced as part of routine cylinder work
A failed or corroded cylinder is generally replaced rather than repaired, since the cylinder itself has a finite working life
Cylinders intended to run from a heat pump need a larger heat exchanger coil surface area, because a heat pump delivers heat at a lower temperature over a longer period than a boiler
Specifying an undersized coil for a future heat pump is a common reason a heat pump installation underperforms on hot water recovery
Sizing for the heat source, not just the household
A cylinder's coil size decides how fast it recovers — and that depends on what's heating it.
Recovery time — how quickly a cylinder reheats a full tank of hot water after use — is governed mainly by the size of the internal heat exchanger coil and the temperature and flow rate of the water passing through it, not by the cylinder's overall capacity. A boiler typically supplies water to the coil at a higher temperature than a heat pump does, so a coil sized for a boiler will recover more slowly, or not fully, if the same cylinder is later connected to a heat pump instead.
This is why a cylinder specified for a future heat pump needs a larger coil surface area from the outset, to compensate for the heat pump's lower flow temperature with more heat exchange area rather than higher temperature. Fitting a boiler-specification cylinder now and expecting it to work equally well with a heat pump later is one of the more common reasons a heat pump conversion disappoints on hot water performance.
Recovery time depends on coil size and the heat source's flow temperature, not cylinder capacity alone
Boilers typically supply higher flow temperatures to the coil than heat pumps do
A heat-pump-ready cylinder needs a larger coil surface area to compensate for the lower flow temperature
Specifying the wrong coil is a common reason a heat pump conversion underperforms on hot water
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
Hot water cylinder questions
Frequently asked questions.
If you're replacing a failed cylinder, planning a bathroom addition, or thinking ahead to a heat pump, get in touch and we'll confirm the right type and size.
What's the difference between a vented and unvented cylinder?
A vented cylinder is fed from a cold water storage tank, usually in the loft, and runs at relatively low pressure. An unvented cylinder connects directly to the mains and delivers strong, even pressure to every outlet without needing a tank or pump.
Do unvented cylinders need to be serviced every year?
Yes — annual servicing is a genuine safety requirement, not just good practice, because the temperature and pressure relief devices fitted to an unvented cylinder need to be tested and, where necessary, replaced.
What is G3 and why does it matter for my cylinder?
G3 is the Building Regulations requirement covering the safe installation of unvented hot water systems, including the safety devices, discharge arrangements and competency of whoever installs and services it. It exists because an unvented cylinder is pressurised and needs specific safeguards against overheating or overpressure.
Can I fit an unvented cylinder if my mains pressure is low?
It depends on how low — unvented cylinders need adequate mains pressure and flow rate to perform properly. We check your incoming supply before recommending an unvented system, and would advise against it if the mains genuinely can't support it.
How do I know what size cylinder I need?
Sizing is based on the number of occupants, bathrooms and how hot water is typically used at the same time in your household, not simply matched to whatever cylinder is currently fitted. We work through this before recommending a capacity.
Can I replace a failed cylinder with a bigger one?
Often yes, provided there's adequate space and the boiler or heat source can recover the larger volume in reasonable time. We check both before confirming a larger cylinder is the right choice.
Do I need a different cylinder if I'm planning to get a heat pump?
Very likely, yes. A cylinder intended to run efficiently from a heat pump generally needs a larger coil surface area than one sized for a boiler, to compensate for the heat pump's lower flow temperature.
Where does the discharge pipe from an unvented cylinder go?
It routes any relief from the safety devices to a visible, suitable outdoor termination point, sized and positioned according to current guidance — not simply run to the nearest gulley or drain.
