Your hot water cylinder is vented if it is fed by gravity from a cold water storage cistern, usually in the loft, and an open vent pipe rises from the top of the cylinder and turns over that cistern. It is unvented if it is fed straight from the mains cold supply, has no vent pipe, and discharges through a tundish — an open funnel fitted in the safety pipework so any leak is visible. A few minutes looking in the loft and at the cylinder itself normally settles the question, and this guide also covers which type suits which home.
Vented or unvented hot water cylinder: what actually tells them apart
A vented cylinder is open to the atmosphere and fed by gravity from a cistern above it. An unvented cylinder is sealed and fed at mains pressure, so it carries safety valves and a visible discharge.
| Criterion | Vented (cistern-fed) | Unvented (mains-pressure) |
|---|---|---|
| Water supply | Fed by gravity from a cold water storage cistern, usually in the loft | Fed directly from the mains cold water supply |
| Vent pipe | An open vent pipe rises from the cylinder and turns over the cistern | No vent pipe; expansion is taken up by a vessel or an air bubble inside the cylinder |
| Pressure at the taps | Comes from the height of the cistern, so upstairs taps and showers feel weaker | Mains pressure at every outlet, upstairs and downstairs alike |
| Safety fittings | Open to atmosphere, so no pressure reducing valve group on the cold inlet | Pressure reducing valve, expansion valve, strainer and a tundish in the discharge pipe |
| Look of the cylinder | Usually copper, often under a sprayed foam jacket | Usually stainless steel, with a data plate and a cluster of valves at the cold inlet |
| Installation rules in England | No sealed storage, so the Part G3 rules for storage without a vent pipe do not apply | Part G3 of the Building Regulations 2010 requires a competent person, and the work is notifiable |
Both types store hot water, and both can be heated by a boiler coil, by an immersion heater, or by both. Direct and indirect describe that heat source, not the vent: direct means the water is heated only by an immersion heater, indirect means a boiler heats it through a coil. A cylinder can be indirect and vented, or indirect and unvented.
Where to look in your home to tell a vented cylinder from an unvented one
Start in the loft. A vented hot water cylinder needs a cold water storage cistern above it, and an open-vent boiler adds a second, smaller feed and expansion cistern for the heating water. If the loft holds nothing but pipework and insulation, the hot water below is almost certainly at mains pressure.
- A large plastic or galvanised cistern in the loft with a pipe running down towards the cylinder — a sign of a vented system.
- A pipe that climbs from the top of the cylinder and bends over that cistern — only vented systems have this open vent pipe.
- A tundish, meaning an open funnel with a visible air gap, fitted upright in the pipework near the cylinder — a sign of an unvented system.
- A small pipe discharging through an outside wall where it can be seen — the discharge from an unvented cylinder's safety valves.
- A group of valves on the cold feed into the cylinder, often with a round expansion vessel beside it — unvented.
- A copper tank under a foam jacket, as against a stainless steel tank with a data plate on the side.
A visible tundish is the clearest single sign of an unvented cylinder. Approved Document G, which supports Part G3 of the Building Regulations 2010 in England, expects the tundish to sit vertically in the same room as the cylinder, within 600 mm of the safety device, so that any discharge is seen. Look only — the safety valves, the expansion vessel and anything inside the casing are work for a qualified engineer.
A system boiler does not prove your cylinder is unvented
Judge the cylinder by its own pipework, not by the boiler. A system boiler is pressurised and filled from the mains, so the heating side needs no loft tanks. That tells you nothing about the hot water cylinder it feeds, because a pressurised heating circuit can still serve a vented, cistern-fed cylinder.
A heat-only or regular boiler works the other way round. It needs a small feed and expansion cistern in the loft for the heating water. That cistern is for the radiators, so it does not prove the hot water cylinder is vented either. If there is no cylinder anywhere in the house, you have a combi boiler heating water as you draw it, and the vented question does not arise — our guide on combi, system or regular boilers explains the three types.
When a vented hot water cylinder is the better system to keep
Keep a vented cylinder when the incoming mains cannot support a pressurised system. An unvented cylinder only delivers mains pressure if the mains supply itself gives enough flow and pressure, and in some older streets and upper-floor flats it does not. A vented system sidesteps that, because the stored water in the loft cistern feeds the taps by gravity.
- The mains flow into the property is weak, so mains-pressure hot water would disappoint at the taps.
- The loft cistern and vent pipe are in good order and the household is happy with the hot water it gets.
- There is no sensible route for a discharge pipe from the cylinder to a safe, visible point outside.
- A short interruption to the mains still leaves stored water in the cistern for the taps.
- A shower pump is already fitted and working to boost the upstairs shower.
Changing from one to the other is a bigger job than swapping a cylinder for a like-for-like one. Our guide on converting a vented system to an unvented cylinder sets out what the work involves.
When an unvented cylinder is the better choice for your home
An unvented cylinder suits a home that needs the same pressure upstairs as down. Because it is fed straight from the mains, a second-floor shower runs at the same pressure as the kitchen tap, without a pump. That matters most where several outlets are used at once.
- Two or more bathrooms, where a gravity-fed shower upstairs is too weak.
- A loft conversion is planned, and the cistern and vent pipe are in the way of the new room.
- A flat or top-floor home with no roof void above the ceiling for a cistern.
- The existing copper cylinder and loft tanks are tired, and the pipework is being reworked anyway.
- The incoming mains has been checked and gives good flow and pressure at the stop tap.
An unvented cylinder does bring duties with it. Manufacturers such as Heatrae Sadia, which makes the Megaflo range, recommend an annual service by a competent person, covering the strainer, the pressure reducing valve cartridge and the expansion valve. Our guide on servicing unvented cylinders explains what that check covers.
Who may install an unvented hot water cylinder in England
In England, an unvented hot water storage system may only be installed by a person competent to do that work, under Part G3 of Schedule 1 to the Building Regulations 2010. The installation is notifiable building work. Either the building control body is told before the work starts, or an installer registered with a competent person scheme self-certifies it afterwards.
Approved Document G sets out what the system must have. There must be at least two independent safety devices on top of the thermostat, and water temperature must be prevented from exceeding 100 degrees Celsius. Systems up to 500 litres and under 45 kW are expected to be fitted as a proprietary unit or package to the relevant standard, with discharge arranged so it is visible and safe.
In England and Wales, the Water Supply (Water Fittings) Regulations 1999 add rules for the expansion valve. It must sit close to the cylinder with no valve able to isolate it, and it must not discharge until the pressure is at least 0.5 bar above the normal operating pressure. Our guide on who can install an unvented cylinder goes through the qualifications in more detail.
Vented or unvented: which system your home should have
A vented hot water cylinder is the right system to keep if the mains flow into your home is weak, the loft cistern and vent pipe are sound, and the hot water at the taps already suits the household. An unvented cylinder is the right change if you want mains pressure at every outlet, the loft space is wanted for something else, and the incoming mains has been measured and found strong enough.
Neither answer can be settled from a photograph. The mains flow rate, the cylinder position and the route for a discharge pipe all have to be checked at the property. FM Power carries out a free home survey and gives a written fixed price before any work starts, and you can read what is involved on our page for Megaflo and unvented systems.
Sources
- Approved Document G: sanitation, hot water safety and water efficiency (2024 amendments)
- The Building Regulations 2010, Schedule 1 (Part G3, hot water supply and systems)
- The Water Supply (Water Fittings) Regulations 1999, Schedule 2 — hot water services
- Viessmann — A guide to vented vs. unvented cylinders
- Baxi — The different types of hot water cylinders explained: vented, unvented, direct, indirect
- Heatrae Sadia — Importance of servicing your cylinder
- Ideal Heating — The difference between combi, heat only and system boilers