A short introduction to unvented hot water cylinders

What is an unvented hot water cylinder?

A hot water cylinder simply stores hot water for when it’s needed. Turn on the hot tap, and you’ll have hot water in seconds. You’ll usually find your cylinder in the airing cupboard.

What’s the difference between a vented and unvented cylinder?

Remember those big old header tanks in the loft? They were the traditional way for British homes to get their hot water between the 1940s and 1990s. The header tank was full of cold water that simply used gravity to feed into a vented cylinder, where the water was heated. Vented cylinders are still a good bet as a like-for-like replacement.

Nowadays, and definitely in newer homes, unvented cylinders are the way to go. They do away with the need for the water tank in the loft by using the mains cold water supply instead. They’re also known for more reliable hot water pressure compared to vented systems.

If you’re thinking about getting an unvented cylinder to replace a vented cylinder, get some advice first to make sure your heating system can handle the higher pressure!

What are direct and indirect unvented cylinders?

There’s a lot of terminology going on with these cylinders! Direct or indirect simply means how the water in the cylinder is heated. For example, an immersion heater in the cylinder is heating the water directly. Whereas a boiler, solar panels or a heat pump are heating it indirectly.

Indirect cylinders are more energy efficient, and you need to make sure you have the right type of cylinder for your heat source (there are solar models and more!)

How does an unvented hot water cylinder work?

The hot water in the cylinder is ready to be used whenever you need it. Because it’s not gravity fed (unlike those pesky vented systems!), the hot water pressure is good, which means it won’t take forever to fill your bath. And the shower shouldn’t dribble either.

Heated water expands and needs somewhere to go, so a cylinder will have an expansion vessel for this water to expand into. This might be in the cylinder itself, or a separate bladder type contraption known as an external expansion vessel.

Pros and cons of hot water cylinders

The alternative to a hot water cylinder is a combi boiler.

Combi boilers are cheap and don’t take up a lot of space. However, you know what happens when you’re in the shower and your mate/significant other decides to run a sink full of water! However, they can do the job in small homes such as flats.

Hot water cylinders may cost a bit more to install and maintain, but you’ll have hot water even during a power cut. There’s no more of those shower/sink pressure/temperature issues, and they work a whole load better with fancy drench showers than a combi boiler ever will.

Just make sure you select the right size cylinder for your property. As a (very rough) guide, more bathrooms = bigger cylinder.

I will always sing the praises of unvented hot water cylinders over combi boilers!

How to install an unvented hot water cylinder

It’s definitely a pro job to install a cylinder. In the UK you need a G3 certificate to install one, or to sign off someone else’s installation. You can watch my installation videos on how to install a Kinsgspan unvented hot water cylinder below: