Vented and Unvented Hot Water Cylinders

When it comes to installing a hot water cylinder the biggest decision a buyer will have to make is whether to buy a cylinder that is unvented or vented.

Deciding on one or the other depends on having a ‘direct’ or ‘indirect’ system. The terms direct and indirect refer to how the mains water is fed into the system – either directly into the pressure vessel contained within a cylinder or indirectly into the cold water storage tank – or how the water is heated within the hot water cylinder – either directly using an electric immersion heater or indirectly using a heat exchanger supplied from a boiler.

Many homes use a combination of the two with indirect for normal heating and an immersion heater as a back-up or storage for extra hot water at short notice.

Before 1989, only vented cylinders were allowed to be used in domestic heating systems. A vented cylinder has a vent pipe which allows it to be subject to atmospheric pressure. The mains water is fed to and stored in the cold water storage tank (usually in the loft), and from there it goes to the appliances in the home. The appliances requiring hot water are fed indirectly via the cold water storage tank to the water cylinder and heated using a heat exchanger. Because the water supply is gravity-fed and therefore low pressure, sometimes a pump needs to be added to assure a strong flow of water both for the domestic hot water and the heating.

An unvented water cylinder works directly from the mains water. The principles of heating are the same as vented by using an emersion heater rather than a heat exchanger, but because the pressure comes direct from the mains water the flow is much better. The systems have many safety devices inbuilt to help it cope with the high pressure and expansion of water. You may find you have a small tank in the loft for venting and feeding your central heating, there is no need for a cold water storage tank.

What are the advantages and disadvantages of a vented hot water cylinder?

The main advantages of installing vented hot water cylinders – like a Heatrae Sadia Megalife vented water cylinder – over an unvented alternative is that they are much less complicated – making them easier to install – and cost less to buy and maintain.

The biggest disadvantage is the fact you need to have a cold water storage tank so the system will ultimately take up more space than an unvented hot water cylinder.

In an indirect heating system the water will be heated indirectly by a primary heat exchanger within the unit being supplied by the boiler. The heat exchanger is a much more economical option – the water is fed into the hot water cylinder which has a series of coils that heat the water. But if something goes wrong with your boiler, you’re left without any hot water at all until it’s repaired.

What are the advantages and disadvantages of an unvented hot water cylinder?

Because unvented cylinders – like the Baxi Megaflo HE – provide hot water constantly at mains pressure, there is no requirement for a cold water storage tank and the extra pipework to link it up. And because you’re relying on the pressure from the mains rather than just gravity, you can situate the hot water cylinder almost any location.

The biggest domestic issue is that you cannot use an unvented hot water cylinder with some power showers and some mixers. Another disadvantage is that they need specialist installation, making them pricier than traditional vented hot water cylinders.

Using an immersion element to heat the water means you’re relying on electricity which means less reliance on your boiler but it will be more expensive to run it as electricity is far more expensive than a heat exchanger. However, immersion elements are easy to get hold of and simple to install.

What else do I need to know about hot water cylinders?

Hot water cylinder capacity varies between 25 to 50 gallons for normal domestic supply with the larger being enough to supply a family home for a day. Most cylinders are made from thin copper and you should make sure your cylinder is well insulated to prevent heat loss. Insulated cylinders are available, which are coated with foamed polyurethane.