Unvented hot water cylinders 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. The other major benefit is that you don’t need to maintain a cold water tank in the loft (which vented systems require). This is good news since not only does it free up space, it also removes the potential freezing issue during our long cold winter periods. 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. Other advantages of installing an unvented system include reduced noise in the system (since there is no cold water filling of the water storage cistern), and since there is no water storage cistern and the system is essentially sealed, the cold water is not at risk from contamination. 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. There are two methods of allowing this expansion to take place safely. The first is the bubble top unit, which uses an internal air bubble that is produced and trapped at the top of the cylinder when it is installed. The other type is the external expansion unit that utilises an expansion vessel to contain the expanded hot water. 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 operate at higher pressure than 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.
