Unvented or vented hot water cylinder
Updated: 17.10.2026
An unvented or vented hot water cylinder is a choice about how cold water reaches the store and how pressure is controlled. A vented cylinder uses a feed cistern and an open vent. An unvented cylinder takes a sealed mains supply and needs a safe discharge. Our [hot water cylinder comparison](/hub/hot-water/hot-water-cylinder) includes listings described as unvented. A title does not prove the installation or the box contents. Measure the supply and decide who will install it before you pick a capacity. Peter F. checked the listings, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
An unvented or vented hot water cylinder is a choice about how cold water reaches the store and how pressure is controlled. A vented cylinder uses a feed cistern and an open vent.
An unvented or vented hot water cylinder is a choice about how cold water reaches the store and how pressure is controlled. A vented cylinder uses a feed cistern and an open vent. An unvented cylinder takes a sealed mains supply and needs a safe discharge. Our hot water cylinder comparison includes listings described as unvented. A title does not prove the installation or the box contents. Measure the supply and decide who will install it before you pick a capacity. Peter F. checked the listings, with no lab test. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Pressure claim | Relief valve | Stated pressure | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  TESY TESY BiLight 30 litre | Item type says unvented | Relief stated, no pipes | n/a | 8.8 | about £139.99 | View offer |
| Alternative |  TESY TESY BiLight 100 litre horizontal | Title says unvented | Required, not in box | 6 bar | 7.2 | about £259.99 | View offer |
| Preis-Tipp |  TESY TESY BiLight 50 litre horizontal | Not called unvented | n/a | 6 bar | 6.5 | about £159.99 | View offer |
On this page
Two ways the cold water arrives
A vented cylinder receives cold water from a cistern, usually in the loft. The open vent is the escape path, and the height from cistern to outlet helps set the pressure. You keep a cold reserve if the mains stops, and you give up loft space. The cistern still needs protection from contamination and freezing.
An unvented cylinder is fed from the mains and stores water under pressure. It removes the feed cistern and can balance several outlets when the incoming main is suitable. It does not manufacture pressure. Measure static pressure and flow under demand, not a single open tap.
Pressure is separate from the heat source. Either type may be direct or indirect. Use the direct or indirect cylinder guide rather than assuming unvented means electric.
When the loft tank is a reason to stay vented
Stay vented when the cistern, open vent and gravity arrangement are sound and the incoming main would not support the flow you want. Like-for-like replacement also avoids redesigning older taps and pipework around a higher pressure. A pressure change can expose weaknesses that gravity hid.
Capping the open vent is not a conversion. A vented cylinder cannot become sealed by removing its safety path. Test the main while another outlet runs. A good static reading can collapse when a kitchen tap and a shower open together. Decide pressure first, then use what size hot water cylinder you need for storage.
What changes when the store is sealed
An unvented store needs controls for normal expansion and independent protection against overheating and excessive pressure. Approved Document G in England requires at least two independent safety devices in addition to the thermostat, and stored water must not exceed 100°C. If a relief valve operates, the discharge must be visible and carried safely without endangering people or damaging the building. The tundish and discharge route are part of the design, not accessories hidden after the cylinder arrives.
CNC Building Control states that an unvented store above 15 litres needs a competent person. A scheme member may self-certify suitable work. Otherwise agree Building Control before installation. Routes differ across England, Wales, Scotland and Northern Ireland, so do not copy the England point as a UK-wide form.
Keep the commissioning record. A new dwelling in England must limit the bath outlet to 48°C. That rule does not apply automatically because a new bath is fitted in an existing home. HSE guidance on legionella commonly uses 60°C storage and at least 50°C at the outlets, with mixing where scalding is a risk.
What an Amazon title does not settle
The TESY 80 litre, 120 litre and 100 litre horizontal titles say unvented, and the 30 litre item-type name uses the word. The 50 litre horizontal file does not. The GEESEN title does not settle vented status. Do not guess from the shape.
Unvented does not confirm the kit. The 100 litre horizontal TESY is cylinder only and requires a 3 bar relief valve that is not included. The 80 litre file does not clearly state the valves. The GEESEN includes a safety valve, which is not a complete installation.
Use the hot water cylinder buying guide for box contents and supply. Stay vented when gravity and a weak main favour it. Plan a competent unvented installation only when flow, pressure and a safe discharge route support the change.
Common questions
What is the main difference between vented and unvented?
A vented cylinder is supplied by a cistern and has an open vent, with pressure influenced by the cistern height. An unvented cylinder is sealed and mains-fed, using expansion and relief controls. This is a pressure-system choice, not a direct-versus-indirect heat choice.
Do I still need a tank in the loft?
You generally need a feed cistern for a conventional vented cylinder. An unvented cylinder removes that cistern because it takes mains water directly. Before converting, confirm that the incoming pressure and flow can support your outlets and that a safe discharge route is possible.
Is an unvented cylinder always higher pressure?
No. It can deliver mains pressure, but its performance is limited by the incoming main and property pipework. A poor supply remains poor. Test pressure and flow under realistic simultaneous demand before choosing an unvented system for stronger showers.
Can I convert a vented cylinder by capping its vent?
No. Closing the open vent would remove a fundamental safety path from a vessel not intended to operate as a sealed system. Conversion requires a suitable unvented cylinder, controls, expansion provision, safe discharge and competent installation.
Who can install an unvented cylinder in England?
For a store above 15 litres, use someone competent in unvented hot-water storage. A relevant competent-person scheme member may self-certify qualifying work. Otherwise agree the Building Control route. Ask for evidence of current unvented competence and the commissioning record.
Does unvented mean the safety valves are included?
No. It describes the intended pressure arrangement, not the box inventory. One TESY listing in this set explicitly requires a 3 bar relief valve that is not included. Have the complete safety and discharge package specified for the exact installation.
Related pages
Sources
Public UK sources listed in the research digest, and the amazon.co.uk listings for these six cylinders. Prices in the digest were captured on 16 October 2026.
- Approved Document G (2024 amendments) (retrieved 16.10.2026)
- Part G FAQ (retrieved 16.10.2026)
- CNC Building Control: unvented hot water cylinder (retrieved 16.10.2026)
- Hot Water Association, unvented note (retrieved 16.10.2026)
- Hot Water Association, sizing a hot water cylinder (retrieved 16.10.2026)
- HSE: legionella and hot and cold water systems (retrieved 16.10.2026)
- Regulation (EU) No 812/2013 on legislation.gov.uk (retrieved 16.10.2026)
- TESY BiLight series (retrieved 16.10.2026)