Unvented Cylinder vs Vented: What the Difference Is and Which Your Home Needs
An unvented cylinder stores hot water at mains pressure with no tank in the loft, so every hot tap in the house runs at the same strong flow as the cold. A vented cylinder is fed by gravity from a cold water tank above it, which is why the upstairs shower in an older house is often a trickle. That one difference decides most of what follows: the pressure you get, the safety kit the cylinder needs, who is allowed to fit it, and what it costs.
Here is how our engineers explain the choice on visits across Deal, Dover, Sandwich, Canterbury and Thanet, and how to work out which one your home actually needs.
What is an unvented cylinder?
An unvented cylinder is a sealed, insulated steel tank connected straight to the incoming cold main. Water enters through a pressure reducing valve, is heated inside the cylinder, and leaves for the taps at mains pressure. Megaflo is the brand most people know, but Gledhill, Joule, Telford and Vaillant all make the same thing.
Because the water is stored under pressure and expands as it heats, the cylinder carries a set of safety devices:
- A pressure reducing valve on the cold inlet, capping the incoming pressure at roughly 3 bar.
- An expansion vessel (or an internal air gap on some models) that absorbs the extra volume as the water warms.
- A temperature and pressure relief valve and an expansion relief valve, which open and dump water if either climbs too high.
- A tundish below those valves - the small open funnel in the pipework - so you can see if a valve is discharging.
- A discharge pipe that carries any released water safely outside.
That kit is why unvented cylinders fall under Building Regulations Part G3. They can only be installed, and should only be serviced, by an engineer holding the G3 unvented qualification, and the installation has to be notified to the local authority. Our engineers hold it, and you can read what an annual check involves on our unvented cylinder service page.
What is a vented cylinder?
A vented cylinder is the traditional copper tank in the airing cupboard, fed from a cold water storage tank in the loft. Gravity provides the pressure, so the flow at any tap depends on how far below the loft tank it sits. An open vent pipe runs from the top of the cylinder back over the loft tank, so it can never build pressure. It is simple and forgiving, with no safety valves to check, but the weaknesses are the ones you notice every morning: weak showers, a loft tank that can freeze or pick up debris, and hot water that runs out quickly.
Unvented cylinder vs vented: the differences that matter
| Unvented | Vented | |
|---|---|---|
| Pressure at the taps | Mains pressure, same upstairs and down | Gravity - depends on loft tank height |
| Loft tank needed | No | Yes |
| Annual safety check | Yes, G3-qualified engineer | No pressure valves to test |
| Two showers at once | Yes, with a good mains supply | Struggles |
| Depends on incoming mains | Yes - needs decent flow and pressure | No |
The last line is the one our engineers check first. An unvented cylinder can only give you what the main delivers. If the incoming supply is old lead or narrow galvanised pipe, or the pressure at the stopcock is poor, an unvented cylinder will disappoint. We measure flow and pressure at the stopcock before recommending one - five minutes that saves an expensive mistake.
Direct or indirect unvented cylinder?
- Indirect cylinders have a coil inside. Hot water from your boiler runs through the coil and heats the stored water, so it works with a system or regular boiler. Most homes with a gas boiler want this type. It usually has an immersion heater as a backup, which is useful in the week a boiler part is on order.
- Direct cylinders have no coil and are heated entirely by immersion elements, so they suit homes with no gas boiler or all-electric flats, usually paired with an off-peak tariff.
If you are on a combi now and want a cylinder because the combi cannot keep up with a second bathroom, that is a boiler change as well as a cylinder job. Our instant quote gives you a price for the boiler side in a few minutes, and we can add the cylinder to the same visit.
What size unvented cylinder do I need?
Size is decided by how many people use hot water and how many bathrooms can run at once, not by the size of the house. The rule of thumb our engineers use:
- One or two people, one bathroom: 120 to 150 litres.
- Three or four people, a bathroom plus an en-suite: 180 to 210 litres.
- Four or more people, two bathrooms or a large bath: 250 to 300 litres.
Go bigger if anyone takes long showers or you have a deep bath. An indirect cylinder on a modern system boiler recovers a full tank in well under an hour; a direct cylinder on a single 3 kW immersion takes considerably longer, so size a direct one generously.
The East Kent problem: hard water and immersion elements
Deal, Dover, Sandwich and the whole of Thanet sit on chalk, and the water is among the hardest in the country. That matters more for a cylinder than for almost any other part of the system, because the cylinder is where water sits hot for hours, and that is exactly when scale drops out of it. On cylinders that have never been treated our engineers find immersion elements furred to twice their thickness, scale on the indirect coil that slows the reheat, and a layer in the bottom of the tank that takes litres off the usable capacity. A scale reducer on the cold feed is cheap compared with a replacement element, and it is one of the first things we suggest. Our guide to hard water and sea air on the East Kent coast covers the wider effect on boilers.
How much does an unvented cylinder cost?
A straightforward swap of an existing unvented cylinder for a new one of similar size, supplied and fitted, is typically £1,800 to £3,000, depending on the make, the capacity and how much of the valve set and pipework needs renewing. Converting a vented system to unvented is more work - the loft tank comes out, the cold main is upgraded if it needs it, and the discharge pipe has to be routed correctly - so that is typically £2,500 to £4,000, depending on the property and whether the boiler is changed at the same time. Those are guide ranges, not quotes; the real figure needs a look at the airing cupboard and the incoming main.
Repairs to an existing cylinder - a dripping tundish, a flat expansion vessel, a failed immersion - are charged at our published hourly rates, £90+VAT for the first hour and £45+VAT per half hour after that, plus parts. The full rate card is on our pricing page.
Why the annual check is not optional
An unvented cylinder that has never been serviced is the fault we least like finding. The expansion vessel loses its air charge over time, the relief valve starts to lift every time the cylinder heats, and water drips into the tundish and out through the discharge pipe. Left alone, the valve seat wears, the drip becomes a run, and you are paying to heat water that goes straight outside. If the boiler side of the system starts showing a low-pressure fault as a result, our boiler fault code guides will help you read it, but the cause is usually in the cylinder cupboard.
The annual check recharges the vessel, tests the relief valves, inspects the immersion and thermostats and looks at the general state of the cylinder. Book it alongside your annual boiler service so both are done on one visit. If your cylinder is the vented type and simply past its best, our plumbing team can replace it like for like or talk you through the change to unvented.
Not sure which you have?
Look in the loft. If there is a cold water tank up there, your cylinder is vented. If there is no tank and the hot taps run as hard as the cold, it is unvented, and it is due a check if it has not had one in the last twelve months.
Call 01304 381999 and a real person will answer, tell you whether your cylinder is the type that needs a G3 check, and book an engineer to come out from our base in Deal. Whether it is a service, a repair or a full change to mains-pressure hot water, we will give you a straight answer on what your home needs and what it will cost.