Unvented vs vented hot water cylinder: which suits your home?
An unvented cylinder is fed directly from the mains, so it gives mains-pressure hot water to every tap and shower and needs no cold water tank in the loft. A vented cylinder is fed by gravity from a cold water tank above it, so pressure depends on how high that tank is and is often weak upstairs. Unvented cylinders need good incoming mains flow and pressure, a safe discharge route and installation and annual servicing by a qualified installer. Vented cylinders are simpler and work where the mains is poor or old pipework cannot take mains pressure.
Loyal Plumbers · Updated
Plan a cylinder installationHow each type works
A vented cylinder is open to the atmosphere through a vent pipe that rises above the cold water tank in the loft. The tank feeds cold water into the bottom of the cylinder, and hot water leaves from the top. If the water gets too hot, it can expand safely up the vent pipe. The pressure at your taps comes from the height difference between the tank and the outlet.
An unvented cylinder is sealed. Mains water enters through a pressure-reducing valve, so the cylinder runs at a controlled pressure close to mains. Because it is sealed, expansion is taken up by an expansion vessel or an internal air gap, and several safety devices stop it overheating or over-pressurising. If one of them operates, water discharges through a tundish and a pipe to a safe, visible outlet.
Unvented vs vented at a glance
Neither type is better in every home. The comparison below shows where each one has the advantage.
| Feature | Unvented cylinder | Vented cylinder |
|---|---|---|
| Water supply | Directly from the mains | From a cold water tank in the loft |
| Hot water pressure | Close to mains pressure, good on every floor | Depends on tank height, often weak upstairs without a pump |
| Loft tank needed | No | Yes |
| Mains requirements | Needs adequate flow and pressure | Works with poor mains |
| Safety devices | Temperature and pressure relief valves, expansion control, discharge pipe | Open vent to the tank |
| Installation rules | Qualified installer, notified under Building Regulations | Simpler, fewer controls |
| Servicing | Annual service usually required for the guarantee | Periodic checks of tank and cylinder |
| If the mains goes off | No hot water supply to outlets | Stored water in the tank keeps supplying for a while |
Advantages of an unvented cylinder
The main reason people choose an unvented cylinder is pressure. Showers upstairs run as strongly as the kitchen tap, and several outlets can run at once without the pressure collapsing, as long as the mains supply keeps up. There is no need for a shower pump.
Removing the loft tank frees space and gets rid of a tank that can be a source of leaks, freezing and contamination. Because the cylinder does not have to sit below a tank, it can often go in a cupboard on any floor, subject to space for the discharge pipe and service access. Megaflo is the best-known brand, and our guide to what a Megaflo is compares a Megaflo cylinder with a combi.
Advantages of a vented cylinder
A vented cylinder is simpler, with fewer safety components to maintain, and it is often the right choice in older properties. If the mains flow in the street is poor, an unvented cylinder cannot perform well however good it is. If the existing pipework, taps or shower valves are not rated for mains pressure, switching to unvented can lead to leaks.
Keeping a vented cylinder is also less disruptive when replacing like for like. The loft tank and pipe routes already exist, and there is no new discharge pipe to route to the outside. Our guide to vented cylinder replacement options explains when keeping the existing arrangement makes sense.
Safety rules for unvented cylinders
Unvented hot water storage systems over 15 litres are covered by Part G of the Building Regulations. They must be installed by a competent person and the installation notified to building control. In practice that means an installer who holds an unvented hot water qualification and can self-certify the work.
The discharge pipe matters as much as the cylinder. It has to run from the relief valves through a tundish to a safe, visible point of discharge, and its size and length are set by the manufacturer's instructions. Finding a route for it is one of the main things checked at survey, especially in flats.
If water is running from the tundish on an unvented cylinder, a safety device has operated. Switch off the heat source and immersion heater, and call a qualified engineer. Do not block the tundish or discharge pipe.
Can you change from vented to unvented?
Usually, yes. The installer checks the incoming mains flow and pressure, the size of the supply pipe and whether the existing pipework and fittings can take mains pressure. They also plan the discharge pipe route and the cylinder location. The loft tank and old vent pipe are then removed or made redundant.
If the mains flow is not enough, the options include upgrading the supply pipe, staying with a vented cylinder, or considering other arrangements. Testing before committing avoids fitting an expensive cylinder that disappoints at the shower.
- Mains flow and pressure measured at the property, ideally at a busy time of day
- Supply pipe size from the boundary checked
- Taps, shower valves and older joints checked for mains pressure
- Discharge pipe route agreed
- Cylinder size matched to bathrooms and household use
Talk to us about your cylinder
Loyal Plumbers installs, services and replaces vented and unvented cylinders across London and inside the M25. If you are unsure which suits your home, call us or send an enquiry with the size of the current cylinder, whether you have a loft tank, how many bathrooms you have and any pressure problems. We will check the mains and recommend the option that works in practice.
Common questions
- Is an unvented cylinder better than a vented one?
- It gives better pressure and frees the loft, but only if the mains supply is strong enough and the pipework can take mains pressure. In some older homes a vented cylinder is the better choice.
- How do I know if my cylinder is vented or unvented?
- A vented cylinder has a cold water tank in the loft and usually a copper cylinder with no expansion vessel. An unvented cylinder has a pressure-reducing valve, an expansion vessel or label, and a tundish on the pipework.
- Does an unvented cylinder need servicing?
- Yes. Manufacturers usually require an annual service by a qualified installer to keep the guarantee, covering the safety valves, expansion vessel and pressure settings.
- Can I fit an unvented cylinder myself?
- No. Unvented systems over 15 litres must be installed by a competent person and notified under Part G of the Building Regulations.
- Will an unvented cylinder work with low water pressure?
- Not well. It can only deliver what the mains supplies. Have the flow and pressure measured before choosing one.
Sources and further reading
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