If your showers are weak and you have a cylinder in the airing cupboard with tanks in the loft, you have a vented system, and the pressure at your shower head is dictated by how far below the loft tank it sits. An unvented cylinder replaces that arrangement with stored hot water at mains pressure — which usually transforms the showers.
It is not automatically the right upgrade for every property. This guide compares the two honestly, including the maintenance obligations that come with an unvented cylinder.
How each system works
A vented cylinder is fed by gravity from a cold water storage tank in the loft. It is open to atmosphere through a vent pipe, so the pressure at any outlet is simply the height of the water above it. Reliable, forgiving, cheap to maintain — and weak at the top of the house where the head is smallest.
An unvented cylinder is a sealed pressure vessel connected directly to the mains. Hot water leaves it at close to mains pressure, so a shower on the top floor performs as well as one downstairs. Because it is a sealed vessel containing hot water under pressure, it carries safety devices and legal requirements that a vented tank does not.
Direct comparison
| Unvented cylinder | Vented cylinder | |
|---|---|---|
| Hot water pressure | Mains pressure at every outlet | Depends on height below loft tank |
| Loft tanks required | No | Yes, cold storage plus feed and expansion |
| Depends on mains flow rate | Yes, heavily | No |
| Installed cost | £2,000 – £3,800 | £1,200 – £2,200 |
| Annual service requirement | Yes, by a G3 qualified operative | Minimal |
| Typical service cost | £90 – £160 | Usually covered in a boiler service |
| Space required | Cylinder cupboard only | Cupboard plus loft tanks |
| Suits a heat pump | Yes, with a large enough coil | Yes, but pressure remains weak |
| Risk if neglected | Pressure vessel — safety devices must work | Overflow at worst |
Mains flow rate decides whether it will work
An unvented cylinder cannot create pressure; it passes on what the mains gives it. If your incoming supply has a poor flow rate — common on long private supplies, shared spurs and older rural connections — an unvented cylinder will deliver disappointing showers despite the expense.
Any installer proposing one should measure static pressure and flow rate at the stopcock first. Where flow is marginal, the alternatives are an accumulator to store mains-pressure water, or keeping a vented system with a shower pump. Our low mains water pressure guide covers what can and cannot be improved.
The legal and safety side of unvented cylinders
- Installation must be carried out by an operative holding the relevant unvented hot water qualification, commonly referred to as G3. We are qualified for this work.
- The installation is notifiable under building standards.
- Safety devices — expansion vessel, pressure relief valve, temperature and pressure relief valve, tundish and discharge pipe — must all be present and correctly installed.
- The discharge pipe must terminate safely, visibly and where scalding water cannot injure anyone.
- Annual servicing is required to check the expansion vessel charge and test the relief valves.
- A neglected unvented cylinder with a failed expansion vessel and a seized relief valve is genuinely hazardous, which is why the servicing requirement is not a sales device.
Sizing matters as much as type
An undersized cylinder produces the same complaint as a weak one: running out of hot water. Sizing follows the number of bathrooms, the peak simultaneous demand and the recovery rate available from the heat source.
It is also worth thinking ahead. A cylinder sized and specified for a boiler today may be inadequate for a heat pump tomorrow, because heat pumps heat water more slowly and need a much larger internal coil surface area. If a heat pump is a realistic prospect within the cylinder's life, specify a heat pump ready cylinder now — the extra cost is modest and it saves replacing a serviceable cylinder later.
When to keep a vented system
- Where mains flow rate is genuinely poor and cannot be improved.
- Where the property has a long-standing gravity system that works acceptably and the budget is better spent elsewhere.
- Where the loft tanks are in good order and freezing has never been an issue.
- Where there is no practical route for a safe unvented discharge pipe.
- In a listed property where the pipework alterations would be intrusive.
The immersion heater backup
Whichever type you have, a working immersion heater is a genuinely useful backup. If the boiler or heat pump fails on a Friday evening in December, an immersion gives you hot water while the repair is arranged, which a combi cannot.
It is worth testing it once a year — many households discover the immersion has been dead for a decade at precisely the moment they need it.
Frequently asked questions
- Will an unvented cylinder improve my shower?
- Usually dramatically, provided the incoming mains flow rate is good. If the mains supply is weak, it will not, which is why the flow rate must be measured before quoting.
- Do unvented cylinders really need annual servicing?
- Yes. It is a pressure vessel with safety devices that must be tested — the expansion vessel charge and the relief valves in particular. It is a legal and safety requirement, not an upsell.
- Can I fit one myself or use a general plumber?
- No. The work requires an operative with the relevant unvented hot water qualification and is notifiable under building standards.
- Can I keep my cylinder if I get a heat pump?
- Only if it has a large enough internal coil for low temperature operation. Many existing cylinders do not, so a heat pump ready cylinder is usually part of the installation.
Weak showers? Start with a flow rate test
We measure your mains supply, then recommend an unvented cylinder, an accumulator or a vented solution based on what your property can actually deliver.