An unvented cylinder can be installed in an airing cupboard, a loft, under the stairs, a utility room or a garage. The room itself is rarely the deciding factor. What decides it is the safety discharge pipe, frost protection, service access and the mains supply, as set out in Approved Document G for England. Choose the airing cupboard or loft if the cylinder already stands there, the space is insulated against frost and an engineer can still reach the controls. Choose under the stairs if a short, falling discharge pipe can reach an outside wall at ground level and nobody wants to climb into the roof to check the tundish.
Airing cupboard or loft, or under the stairs: what differs for an unvented cylinder
Both positions are allowed for an unvented cylinder in England. The differences show up in the discharge pipe run, the frost risk and how easily an engineer reaches the controls.
| Criterion | Airing cupboard or loft | Under the stairs |
|---|---|---|
| Discharge pipe route | Roof or upstairs level usually means a long pipe run. Approved Document G limits pipe resistance, so longer runs need a larger size. | A ground floor cupboard is often near an outside wall, so the discharge pipe stays short and keeps its fall easily. |
| Frost risk | Lofts, garages and outbuildings are unheated. Manufacturers ask for frost protection and insulation on all exposed pipework. | Inside the heated part of the house, so frost is normally a question only for the pipe once it leaves the wall. |
| Checking the tundish | The tundish must be visible and in the same room as the cylinder, so looking at it means going up into the loft. | The tundish sits behind a door at ground level and is in view as soon as the cupboard is opened. |
| Service access | The hatch and boarding have to let an engineer reach the controls and withdraw the immersion heater. | Cupboard depth decides it. Stair cupboards narrow towards the back, which can block access to the controls. |
| Benefit from height | None for an unvented cylinder. It runs at mains pressure, so height above the taps adds nothing. | None needed. Mains pressure feeds the taps, so a ground floor position loses no performance. |
A survey settles which of the two works in a particular house. Floor loading, the route for the discharge pipe, the mains supply and the position of existing pipework are all checked on site.
When an airing cupboard or loft suits an unvented cylinder
An airing cupboard or loft suits an unvented cylinder when the old cylinder already stood there and the hot pipework runs from that point. Keeping the position means shorter pipe alterations and less disturbance to finished rooms. It also keeps the cylinder near the upstairs bathrooms, where most hot water is drawn.
A loft adds two conditions. Manufacturers ask for frost protection and name lofts, garages and outbuildings as places needing particular care, with all exposed pipework insulated. The hatch and boarding have to let an engineer reach the controls and withdraw the immersion heater for inspection or replacement. Draining a cylinder mounted at high level has to follow the manufacturer's method, which is work for a qualified engineer rather than the householder. Putting a boiler in the loft raises the same access and frost questions.
A boiler can share an airing cupboard with the cylinder. Where the cupboard is also used for drying, the drying area must be separated from the appliance and the flue so that nothing combustible can touch them. A non-combustible partition does that, perforated if needed with holes no larger than 13 mm, in line with BS 6798 and BS 5440-2. Anything touching the flue or the boiler casing is work for a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 as amended in England.
When under the stairs suits an unvented cylinder
Under the stairs suits an unvented cylinder when a short discharge pipe can fall to an outside wall at ground level. Terraced and semi-detached houses often have a stair cupboard backing onto a side or rear wall, which keeps that pipe simple. The position also makes sense when the loft has become a bedroom, when there is no safe loft access, or when the family wants the airing cupboard back for storage.
Three points decide whether the cupboard is big enough. Where controls or the immersion heater face a wall, manufacturers ask for at least 250 mm of clearance so they can be removed and serviced. The tundish, the drain valve and any motorised valves have to be mounted clear of electrical components, which matters in a stair cupboard holding the consumer unit or the meter. The cupboard also narrows towards the back, so usable depth is measured at the cylinder position, not at the door.
The discharge pipe, not the cupboard, decides where an unvented cylinder can go
The discharge pipe from the safety valves rules out more positions for an unvented cylinder than cupboard size ever does. Approved Document G for England requires water released by the safety devices to be carried away visibly, without endangering anyone in or around the building. These are the dimensions that shape the route.
- Tundish: visible to the user, in the same room as the cylinder, vertical, below the valve, with no more than 600 mm of pipe between the valve outlet and the tundish.
- Below the tundish: at least 300 mm of vertical pipe before the first bend, then a continuous fall of at least 1:200.
- Size: at least one pipe size larger than the valve outlet. From a G½ valve that is 22 mm up to 9 m equivalent length, 28 mm up to 18 m and 35 mm up to 27 m, counting 0.8 m for each bend.
- Material: plastic pipe is allowed where it is tested to at least 110 °C.
- Outlet: pointing downwards no more than 100 mm above the external surface, with a guard that still leaves it visible; or at high level into a metal hopper and metal downpipe with the pipe end in view.
- Route: continuously falling, insulated where exposed and kept in a frost-free environment.
No cupboard works without a discharge route. If the safety pipe cannot leave the chosen spot with the fall, the size and the visible outlet that Approved Document G requires, the unvented cylinder goes somewhere else, whatever the room looks like.
Mains pressure and flow decide whether an unvented cylinder works at all
Mains pressure and flow are tested before any cupboard is measured, because an unvented cylinder is fed straight from the incoming main. Megaflo Eco and Compact cylinders need at least 1.5 bar (0.15 MPa) working pressure and 20 litres per minute. The larger Eco Plus models ask for 1.5 bar and 50 litres per minute. Maximum supply pressure is typically 16 bar.
At those minimum figures, outlet flows can be weak when several taps or showers run together. High mains pressure on its own does not guarantee high flow, and manufacturers say so plainly. Both are measured at the stopcock during the survey. More on the cylinders themselves is on the Megaflo and unvented systems page.
Does an unvented cylinder need building control sign-off wherever it goes?
Installing or replacing an unvented cylinder with more than 15 litres of storage is notifiable building work in England and Wales. There are two routes. Your installer notifies the building control body before work starts. Or the installer is registered with a competent person scheme and self-certifies, and the scheme operator issues a Building Regulations Certificate of Compliance for the owner.
Requirement G3 of the Building Regulations 2010 (England) sets the safety side. The system must be installed by a competent person. It needs precautions that prevent the stored water exceeding 100 °C, and two independent safety devices in addition to the thermostat. Who may carry out that work is covered in the guide on who can install an unvented hot water cylinder.
The Water Supply (Water Fittings) Regulations 1999 (England and Wales) add a separate notice to the water supplier before certain work begins. Consent counts as given if the supplier does not reply within ten working days. Northern Ireland has its own water fittings regulations from 2009, and Scotland works to its own building standards. The paperwork you should end up with is listed under the unvented hot water cylinder certificate.
Which position to choose for your unvented cylinder, and when
Choose the airing cupboard or loft if the cylinder already stands there, the space is insulated against frost, and the hatch or door leaves room to service the controls and immersion heater. Choose under the stairs if the loft is converted or hard to reach safely, and a short falling discharge pipe can get to an outside wall at ground level. Where both positions work, the shorter and simpler discharge route is usually the better buy, because that pipe has to stay visible and frost free for the life of the cylinder.
A free home survey checks pressure, flow, the discharge route and the access in each candidate spot, and you get a written fixed price before any work starts. Where the cylinder is changing as part of a boiler replacement, the same survey covers a new boiler installation.
Sources
- Approved Document G: Sanitation, hot water safety and water efficiency (2024 amendments)
- Building regulations approval – GOV.UK
- Competent person scheme: current schemes and how schemes are authorised – GOV.UK
- The Water Supply (Water Fittings) Regulations 1999
- Megaflo Eco SystemFit unvented cylinders installation manual (Heatrae Sadia)
- Megaflo Compact unvented cylinders installation manual (Heatrae Sadia)
- PremierPlus unvented hot water cylinder installation and user manual (Heatrae Sadia)
- Megaflo Eco Plus 400–570 installation manual (Heatrae Sadia)
- High Efficiency Boiler Range Technical Guide (Glow-worm)