There are two types of hot water cylinders found in homes today. The newer pressurised unvented hot water tanks and the older style vented hot water tanks.
Unvented Hot water Cylinders
Unvented hot water cylinders were only made legal in the UK in the mid 1980s, 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.
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 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 such as GAS SAFE. This means they tend to be far more expensive to install than traditional vented hot water systems.
Vented Hot water Cylinders
Vented hot water tanks are still the most common type of hot water system found in the UK. Unlike newer unvented tanks, these copper tanks are fed by cold water from a header tank (normally located in the loft) and they use gravity to drive the hot water around the home. 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 is still an issue, but in this case the expansion simply takes place via the vent pipe and 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. This means that although on the ground floor of the home the pressure might be excellent, in rooms on upper floors the pressure will be lower. As a result many showers in homes with vented hot water tanks use electric pumps to drive the hot water to the shower at increased pressure.
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. This makes them a far cheaper option when compared to the unvented system.