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Stored Hot Water For Domestic Use

Unvented Cylinders

Unvented hot water cylinders have only been available since 1986, but have since rapidly grown in popularity. In an unvented system there is no cold water tank, instead the sealed hot water cylinder is fed directly by the water mains. Since they are operating at mains pressure, they generally offer a much better flow rate.

The major benefit is that you don’t need to maintain a cold-water tank in the loft (which vented systems require). This is good since not only does it free up more space but it also removes the potential freezing issue during cold winters.

In addition, since you aren’t relying on gravity to move hot water around the your property, the unvented cylinder can be located anywhere inside.

Another advantage is the reduced noise in the system since there is no cold water filling of the water storage cistern, and with the system sealed, the cold water is not at risk from any contaminates.

Vented systems

Vented hot water tanks are the most common type system found around the United Kingdom. Unlike the newer unvented tanks, these copper tanks are fed by cold water from a header tank (normally located in the attic) and they use gravity to drive the hot water around your property. A vent pipe links the vented hot water cylinder and the cold water in the header tank.

As with the unvented system, expansion of warm water can still be an issue, but in this case the expansion simply takes place via the vent pipe in the header tank.

The hot water pressure tends to be governed by the height of the water tank above the tap or shower feed, which means the ground floor of the home the pressure might be excellent, in rooms on upper floors may be lower. As a result many showers in homes with vented hot water tanks use pumps to drive the hot water to the shower.

Vented hot water cylinders are far less complicated than the pressurised vented systems and for this reason they are much simpler to maintain and install – making them a far cheaper option when compared to the unvented system.

Indirect Systems

Most hot water cylinders are heated via an external heat source such as a gas boiler or solar thermal. In this case the hot water is heated and then travels through a copper coil in the hot water tank. The heat is then transferred from the from the external heat source to the water inside the hot water tank.

Indirect cylinders tend to be fitted with a direct backup (such as a immersion heater) – so even if the boiler is broken you can still produce hot water as and when you need it.

You can get both vented and unvented indirect systems.


Mild-steel tanks may corrode over time; maintenance will help prevent this. They usually have five to ten year warranties.

Stainless steel tanks are more expensive, but last longer and don't require as much maintenance as the mild-steel tanks. They usually carry a 10-year warranty, but still require occasional maintenance, such as the replacement of valves and seals.

Local water quality may dictate which type is best for you; we can help you here.

Tanks are insulated, but there is always a little heat loss over time, so it's good to install them in a sunny spot or in a well insulated space. 

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