Unvented hot water cylinders were only made legal in the UK in 1986, but have since grown rapidly in popularity. In an unvented system there is no cold water tank – instead the sealed hot water cylinder is fed directly by the cold water mains. Since they are operating at mains pressure, they offer much better flow rates, meaning your shower and bath performance should be higher.
In addition, since you aren’t relying on gravity to move the hot water around the home, the unvented cylinder can be located pretty much anywhere in your property.
Unvented water cylinders and the water expansion issue
Since water increases in volume as it gets warm, unvented cylinders need to include a mechanism that allows the expansion to take place thereby keeping the cylinders operating at a safe pressure.
The major issue with unvented hot water cylinders is that since hot water flow depends on the cold water main pressure, if for any reason the mains water is turned off, your home will be without access to any hot water.
Since unvented hot water tanks are operating at higher pressure than the vented systems and have additional safety features installed, these cylinders need to be installed by boiler specialists who hold a qualification that complies with G3 of building regulations. This means they tend to be far more expensive to install than traditional vented hot water systems.