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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Nick
3 September 2026
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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).
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Laura Peters
17 August 2026
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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!
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James Ross
8 August 2026
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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.
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Ian Chevalier
11 June 2026
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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.
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Karen Flackett
20 May 2026
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Excellent service. Matt came & fitted 2 taps & waste pipes he was polite & friendly & carried out the work efficiently & in a timely manner.
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Paul Norris
30 April 2026
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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!
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Julie Galvin
21 April 2026
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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.
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Chesca Haley
16 April 2026
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Lee and the team are amazing! So helpful, so knowledgeable and really look after their customers. Highly recommend.
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Sarah Rogers
6 April 2026
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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
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Alison Molan
25 March 2026
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Lovely engineers, Matt and Jack, were a pleasure to have in the house. Thorough, knowledgeable, tidy and polite. Would highly recommend.
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.
