Home / Unvented cylinders
Unvented cylinders
Mains-pressure hot water to every tap and shower at once, which is what larger houses need and what a combi boiler cannot always deliver.
Why a bigger house often wants one
A combi heats water as it flows, so everything depends on the rate your incoming main can supply. That is fine for one bathroom. Run two showers at once and the flow splits, and both go from pleasant to disappointing.
An unvented cylinder stores hot water at mains pressure instead. Three people can shower at the same time without anybody noticing the others. For a four or five-bedroom house, that difference is usually the deciding factor.
The safety side, and why it is regulated
An unvented cylinder holds hot water under pressure, and that stored energy has to be controlled. The law requires anybody working on one to hold a G3 qualification, and it requires an annual safety check.
That check covers the expansion vessel, the pressure relief and temperature relief valves, the tundish and the discharge pipework. It is not optional and it is not a formality — a failed relief valve on a pressurised cylinder is genuinely dangerous. Our engineers hold the qualification and we will always tell you when yours is due.
Worth checking. If nobody has serviced your cylinder since it was fitted, it is overdue. It is also one of the first things a surveyor picks up when a house is sold.
What we do
Heating, cooling and plumbing, for homes and small businesses across Surrey and south-west London.
New boilers
From £2,211 fitted
A-rated boilers from Worcester Bosch, Vaillant, Baxi and Glow-worm, sized for your actual house. Guarantees up to twelve years.
New boilers →Boiler servicing
£89
A proper hour-long check, not a five-minute look. Keeps your guarantee valid and catches the faults that would otherwise pick January.
Boiler servicing →Boiler repair
£75 diagnosis, no call-out fee
We find the fault, tell you what the fix costs, and you decide. Most common faults are sorted on the first visit.
Boiler repair →Central heating
Designed room by room
New systems, upgrades and smart controls, sized on a proper heat loss calculation rather than a rule of thumb.
Central heating →Air conditioning
From £1,850 fitted
Cools in summer and heats efficiently in winter, because it is a heat pump running both ways. Fitted by F-Gas engineers.
Air conditioning →Plumbing
Hourly, told to you first
Leaks, taps, showers, toilets and pipework, plus genuine 24-hour cover when something bursts at midnight.
Plumbing →Common questions
How often should it be serviced?
Every twelve months. Manufacturers require it for the warranty, and the safety valves genuinely do need testing that regularly.
Is my house suitable?
Almost certainly, provided the incoming mains supply is decent. We check the flow rate and pressure at the survey and tell you honestly if it is marginal.
How much space does it need?
Roughly the same cupboard as a traditional cylinder, but there is no need for a tank in the loft, which usually frees up space overall.
What is the tundish for?
It is the small open gap in the discharge pipe that lets you see if a safety valve is releasing water. If yours is dripping, something has failed and it needs looking at rather than ignoring.
Where we work
We are based in Guildford and cover everything inside the orange line — thirty-one areas across Surrey and into south-west London. Tap any town to see what we do there.
Map data © OpenStreetMap contributors
Do we cover you?
Pop your postcode in and we will tell you straight away.
Most of our work is within half an hour of the door. That is deliberate — it is why we can usually reach a breakdown the same day.
Woking and north Surrey
Mid and east Surrey
South-west London
Accredited and checked by
Unvented cylinders: mains-pressure hot water, explained properly
Vented versus unvented
A traditional vented cylinder is fed from a cold tank in the loft. Pressure at the tap comes from the height of that tank above it, which is why hot water upstairs in an older house is often disappointing while downstairs is acceptable.
An unvented cylinder is fed directly from the mains. Pressure at every outlet is mains pressure, regardless of floor. Showers behave properly upstairs, two outlets can run at once without either collapsing, and the loft tanks disappear along with the risk of them leaking through a ceiling.
It is a pressure vessel, and that changes things
Because it is sealed and under mains pressure, an unvented cylinder contains stored energy. Installation is notifiable work under Building Regulations and must be carried out by someone holding the appropriate unvented hot water qualification. The safety devices are not optional extras.
Those devices are an expansion vessel to absorb the expansion of heated water, a pressure reducing valve to limit incoming pressure, an expansion relief valve, and a temperature and pressure relief valve as the final safeguard, with a discharge route to a safe visible position.
This is why annual servicing is a legal expectation rather than a suggestion, and why a cheap installation by somebody unqualified is a genuinely poor idea.
Sizing: the decision people get wrong
Too small and you run out mid-shower. Too large and you heat water nobody uses, every day, for the life of the cylinder.
A reasonable starting point is around 45 litres per person, adjusted for how your household actually behaves. A family that all shower between seven and eight needs more storage than the same number of people spread across the day. Add capacity for a bath, which uses considerably more than a shower, and for any high-flow shower or body jets, which can empty a cylinder alarmingly fast.
Recovery time matters as much as volume. A cylinder with a large coil and a well-matched boiler reheats far faster, which in practice can be worth more than extra litres.
What your incoming mains has to provide
An unvented cylinder cannot deliver more than the mains brings in. As a general guide you want at least 20 litres a minute at 1.5 bar or better to make the most of one. Below that the cylinder will still work, but it will not produce the transformation people expect.
We measure flow and pressure at survey. If your supply is genuinely weak, the honest answer may be an accumulator or a pumped system rather than a cylinder, and we will say so.
Annual servicing: what it covers
The expansion vessel loses its charge over time, which is the most common fault and causes the relief valve to discharge. The temperature and pressure relief valve is tested. The immersion heater and thermostat are checked. The discharge pipework is inspected. And where the water is hard, scale on the immersion and in the coil is assessed.
Skipping it risks both the guarantee and, in the worst case, the safety devices you rely on.
Hard water and cylinder life in Surrey
Much of Surrey sits on chalk and the water is hard. In a cylinder, scale forms on the hottest surfaces — the coil and any immersion heater — insulating them and slowing recovery. Running storage temperature sensibly, rather than as hot as possible, slows it considerably, and a scale reducer on the incoming main is worth fitting at the same time as the cylinder.
Storage temperature does need to be high enough to control legionella, which is why the control strategy matters and why this is a job for someone who understands both requirements rather than simply turning everything down.
Questions we get asked
What is the difference between vented and unvented?
A vented cylinder is fed from a loft tank and pressure comes from height. An unvented cylinder is fed from the mains, so every outlet runs at mains pressure regardless of floor.
Do unvented cylinders need servicing every year?
Yes. It is a pressure vessel with safety devices, and annual servicing is both a legal expectation and a guarantee requirement. The expansion vessel loses charge over time.
What size cylinder do I need?
Around 45 litres per person is a starting point, adjusted for how your household actually behaves. A family who all shower within an hour needs more storage than the same number spread across the day.
Can any plumber install an unvented cylinder?
No. It requires a specific unvented hot water qualification and the work is notifiable under Building Regulations.
Will an unvented cylinder fix my weak shower?
Only if your incoming mains can supply it. We measure flow and pressure first. Below about 20 litres a minute you will not see the transformation people expect, and we will tell you that before you spend.
Get a price without anyone knocking on your door
Answer a few questions and see real numbers in about ninety seconds. No sales visit needed to find out roughly what it costs.