Converting a vented system to an unvented cylinder

What actually changes in the house

A vented cylinder is fed by gravity from a cold water cistern, usually in the loft, and has an open vent pipe rising above it. Pressure at the taps depends on how far the tank sits above them, which is why the shower on the top floor is often the weakest outlet in the house. An unvented cylinder is sealed and fed at mains pressure, so the same pressure reaches outlets on every floor.

Because the cylinder is sealed, the expansion of the water as it heats has to go somewhere — water expands by up to about 4% — so an unvented cylinder uses either an external expansion vessel or an internal air bubble. Losing the loft tank frees up roof space and removes a stored volume that can freeze or become contaminated.

• Out: cold water storage cistern, open vent pipe, and the pipework that fed them
• In: sealed cylinder, expansion provision, temperature and pressure relief valves, tundish and discharge pipework
• Changed: the primary circuit, if the old system heated the cylinder by gravity rather than a pump

Do you need Building Regulations approval?

Yes. In England, hot water supply and systems are covered by Requirement G3 and the guidance in Approved Document G. G3 requires precautions that prevent the stored water temperature exceeding 100°C and that discharge any hot water released from safety devices safely. An unvented cylinder counts as a controlled service under the Building Regulations and may only be fitted by a suitably qualified person.

There are two routes. The work can be notified to your local building control body, or an installer registered with a competent person scheme can certify the work themselves, in which case it does not have to be notified to building control in advance. Ask which route is being used before the work starts, and keep the certificate with your house paperwork.

Is your mains pressure and flow good enough?

Mains pressure and flow decide whether a vented-to-unvented conversion works, because an unvented cylinder only performs as well as the cold supply feeding it. Manufacturers publish minimum figures: for a Megaflo Eco Plus 250–300, the installation manual states 0.15 MPa (1.5 bar) working pressure and 50 litres per minute; the smaller Megaflo Eco SystemReady range asks for 1.5 bar and 20 litres per minute. A supply below those figures can reduce performance, so the numbers need checking before a cylinder is chosen rather than after it is fitted.

Static pressure and flow rate are measured at the incoming main, not guessed from how the taps feel. Older properties on a small or shared supply pipe often read well below what a large cylinder needs, and a system that felt adequate on a gravity-fed vented tank can disappoint once it depends on the mains. FM Power carries out a free home survey before an installation, and the measured figures feed into the written fixed price and the cylinder size proposed.

Where does the discharge water go?

A converted system needs a discharge route that did not exist before, and it is often the part that decides where the cylinder can stand. The Water Supply (Water Fittings) Regulations 1999 require an unvented hot water storage vessel to have a temperature relief valve or a combined temperature and pressure relief valve fitted directly to the vessel, with enough discharge capacity to stop the stored water exceeding 100°C. Discharges from these valves and from the expansion valve must be made in a safe and conspicuous manner.

• The tundish sits vertically, in the same room as the cylinder, as close under the valve as possible
• No more than 600 mm of pipe between the valve outlet and the tundish
• At least 300 mm of straight vertical fall below the tundish before any change of direction
• The discharge pipe runs continuously downhill, stays frost-free, is never capped or used for anything else, and stays open to the atmosphere
• The pipe diameter follows the sizing table in Approved Document G, where every bend reduces the permitted length

That table is why an airing cupboard in the middle of a house can be harder to work with than one on an outside wall. The survey is the point at which the route is worked out, not the installation day.

Will your existing boiler still work with it?

An indirect cylinder is heated by a coil fed from the boiler; a direct cylinder is heated electrically. For a gas-heated home, indirect is the usual choice, paired with a system or regular boiler. If the old cylinder was heated by gravity, the primary circuit is converted to a fully pumped, fully controlled arrangement as part of the work. Manufacturers design their cylinder ranges around their own system and regular boilers, so the pairing matters.

If you are weighing an unvented cylinder against dropping the cylinder altogether, our guides on combi, system or regular boilers and on a system to combi conversion set out the trade-offs for a house with more than one bathroom. Any work on gas fittings in a home must be carried out by a Gas Safe registered engineer qualified for it — that is a requirement of the Gas Safety (Installation and Use) Regulations 1998 as amended.

Water temperature, scalding and landlords

HSE guidance on hot and cold water systems is to store hot water at 60°C or above and to distribute it so that 50°C is reached at the outlets within one minute. Approved Document G also sets a scalding limit: hot water delivered to a fixed bath must not exceed 48°C. That bath requirement applies to new dwellings, including those created by a change of use — it does not apply to baths in existing dwellings, although a thermostatic mixing valve can still be specified.

Removing redundant pipework is a recognised legionella control measure, so stripping out the old cistern, vent pipe and dead legs is worth doing properly rather than leaving capped stubs in the loft. Landlords have duties to assess the risk from legionella; in most residential settings the risk is considered low because water is drawn off regularly. If you let property, the same visit is often a sensible time to sort your landlord gas safety certificate.

What servicing looks like afterwards

An unvented cylinder has safety devices that need checking. Routine servicing includes confirming the safety valves are clear, and the pressure relief device is operated periodically to clear scale and confirm it is not blocked. Water discharging intermittently from the expansion valve can mean the air volume inside the cylinder has been lost and needs re-establishing — that is an engineer's job, not a tap to be tightened.

What you can safely keep an eye on yourself is straightforward: the boiler pressure gauge, the thermostat and programmer settings, the reset button if the boiler locks out, and bleeding radiators that stay cold at the top. If the heating runs but the taps stay cold, our guide on no hot water but the heating works covers what that usually points to.

To find out whether your mains supply and layout suit the change, you can book a free home survey and get a written fixed price for a Megaflo or unvented system.

Sources

• Approved Document G (with 2024 amendments) — sanitation, hot water safety and water efficiency
• Approved Document G: Frequently Asked Questions (March 2016)
• Building regulations approval: use a competent person scheme — GOV.UK
• The Water Supply (Water Fittings) Regulations 1999, Schedule 2 — hot water services
• The Gas Safety (Installation and Use) Regulations 1998, regulation 3
• HSE: Hot and cold water systems — legionella control
• Megaflo Eco Plus 250–300 installation manual (Heatrae Sadia)
• What is a tundish for a hot water cylinder? — Viessmann UK