An unvented hot water cylinder receives cold water directly from the mains water supply and supplies all of the hot taps, hot water at mains pressure.

The cold water comes into the unvented hot water cylinder, is then heated by either an immersion heater or gas from a boiler before being sent to the hot water tap requesting hot water.

Unvented hot water cylinders can be both direct or indirect;

• direct unvented cylinder uses the electricity to heat water using immersion heaters
• indirect unvented cylinder uses gas from a boiler to heat the water 

In theory, as the water is fed off the mains supply, the pressure sent to showers and other taps should be a better stronger pressure than a gravity fed system of a vented hot water cylinder. Of course, this is as long as the mains on your road is a decent strong water pressure.

Why install an unvented cylinder?

If space is a premium (like in so many UK homes) then an unvented system is the way to go. An unvented ‘sealed’ system does away with the need for a gravity fed water tank in the loft. Unvented cylinders come in slimline options taking up even less space, a well-known brand to look out for is Gledhill's Stainless Lite Plus range which can be installed almost anywhere. Having water come off the mains should always give a better flow rate to the hot water taps in the property.

Another great unvented cylinder option is Megaflo manufactured by Heatrae Sadia (often misspelt as Megaflow system or Megaflow boiler) is a popular choice for a stainless steel unvented cylinder that stores hot water as part of a central heating system.

Reasons that make the Megaflo popular is;

• water pressure in the system is consistent even if more than one outlet is being used at the same time, so it is suitable for families
• Megaflo cylinders are easy to install, and parts are easier to replace and maintain
• A Megaflo system is compatible with solar PV
• The system is fitted with several different safety features, including a thermostat to shut the boiler down when it has reached the correct temperature. If it becomes too hot, the Temperature/Pressure Relief Valve (TPRV) would dump water out of the cylinder into a drain to reduce pressure

Can I replace my vented cylinder with an unvented cylinder?

In short, yes. A vented system has been plumbed in with the knowledge that water will only be fed by gravity, therefore upgrading a system and replacing a vented with an unvented system sometimes requires the heating engineer to replace pipework to ensure that the pipes and joints can handle water flowing through at a higher pressure. This is especially necessary if it is older pipework.

A lot of UK homes prefer to upgrade the system to the unvented hot water cylinder to make use of the space that has been housing a water feed tank. Another idea is if you have a combi boiler, you can also install an unvented hot water cylinder to the system. This will help with the demand of a home with multiple bathrooms and where the demand for hot water can be simultaneously from different hot taps.

How much will it cost to replace an unvented hot water cylinder?

Replacing an unvented hot water cylinder is a pricey job. The cylinder alone can cost from £600 and then on top of this cost is the price of labour, which might include your heating engineer and a helper needing a full day to install and fit the unvented cylinder. 

If the heating engineer is upgrading the system and removing a vented cylinder to fit an unvented hot water cylinder, this will need the extra time of ensuring pipework can handle the possibly higher pressure and means labour costs will run into an extra day of work.

Cost to replace an unvented hot water system	Per	Average total labour cost
Replacing an unvented hot water cylinder	Per day	£450
Installing a Megaflo system	Per day	£450
Upgrading a vented to an unvented hot water cylinder	Per day	£650
Maintaining an unvented hot water cylinder	Per hour	£90
Diagnosing a fault on an unvented hot water cylinder	Per hour	£90

Is it time to replace my unvented hot water cylinder?

An unvented hot water cylinder has a longer lifespan than a vented cylinder, and as long as they are maintained, they should last for 30 plus years.

 If you notice damp patches around the cylinder or any signs of leaks, the cylinder may have a pin-hole or a crack and may be time to replace the cylinder before damage is caused to your home.

Another sign of a cylinder needing replacing is if it is filled with limescale and no longer heating water effectively or becomes noisy during the heating of the water.

When it comes time to replace your cylinder, if replacing with a new unvented hot water cylinder, the choices available aside from direct or indirect, include;

• standard unvented cylinder
• pre-plumbed unvented cylinder
• horizontal unvented cylinder
• slimline unvented cylinder
• solar unvented cylinder

Which cylinder should be installed, vented or unvented?

Both vented and unvented cylinders come with their own set of advantages and disadvantages. Older UK properties tend to already be fitted with a vented system, and therefore at the time of replacement it is cheaper to just install a modern version of the same type of system.

Newer properties are fitted with either combi systems or unvented "sealed" systems. Unvented is a better choice between vented and unvented for the space saving as well as for the consistent higher pressure, but if the mains water pressure coming into your home is not a decent pressure, then a vented cylinder system would be the better choice for your home. Eliminating the need for the water tank in the loft also avoids the damage of another potential area for a water leak and pipes freezing in winter. There is also no concern of the threat of contaminated water in case any debris gets into the water tank in the loft.

The downside to an unvented system is the higher cost of installing the hot water cylinder and the on-going maintenance needs.

Estimates for common heating engineer jobs;

Rates for other trades