Pressurized hot water storage tanks, also known as unvented cylinders, have seen a remarkable increase in demand since their introduction to the country in the 1980s. They have proven to be highly popular both with the wider population and the plumbing and heating industry in equal measures. Latest results show 2007 was the first year where unvented systems have accounted for 50% of the hot water systems installations in the UK.
Unvented systems provide good pressure of hot water at home. They differ substantially from traditional systems in that they do not rely on a header tank. Traditional systems build the hot water pressure by having a feeder tank in the loft which builds the water head and creates the pressure for the system. However, there is typically not enough head room for the water to build up sufficient pressure resulting in, for example, low pressure in the shower (especially if it is on the top floor).
Unvented systems are sealed and thus have an internal pressure which is likely to increase as the water within the tank heats up. In traditional systems, expansion of water as it heats up is handled by the feeder tank, which easily accepts any such expansion. For unvented systems, the solutions come in the form of specially designed checks and safety equipment to ensure safe and reliable operations even as water heats up and expands.
It is important to point out that the system is sealed and hence pressurized. As such it requires special training for correct design and installation. Since water expands upon heating it creates a few additional issues to account for. Unlike traditional systems where the header tank can take any expansion of the hot water, in sealed systems the technology has been designed to address this using a few checks and safety equipment along the hot water loop.
An additional safety mechanism is the pressure and temperature relief valve. The PT valve is a mechanical instrument that is designed to open whenever the temperature or the pressure exceeds a preset level, thus creating a potential hazard. When the temperature or pressure becomes too high, the PT valve opens up, discharging excess water into the drain (normally through a tundish), and in doing so reduces the temperature and pressure.
Another safety measure is the temperature and pressure relief valve. The PT valve is a simple yet smart mechanical valve which senses whether the temperature or the pressure within the cylinder is too high (or is breaching a preset level). At that point the valve opens up to let the extra steam out into the drain, thus immediately reducing the temperature, the pressure and the volume of water in the system.
The safety checks and valves ensure a safe and reliable hot water system. The Cold water enters the system at mains pressure, keeping the system (which is sealed) at a constant pressure throughout. The water heats up within the cylinder and is then delivered to the user (still at mains pressure). As the hot water is drawn to the user (normally from the top of the cylinder), fresh cold water enters the cylinder (normally from the bottom) to replenish the levels and sustain the pressure.
There are two main types of unvented cylinders – direct and indirect:
* Direct cylinders – direct systems heat the water through a direct contact between the heating unit and the water that is ultimately drawn by the user. The heating is done by an electrical element with a typical capacity of 2.5 to 3.6kW which is normally enough for a domestic installation.
* In-direct cylinders – using a heat exchanger, the water in the cylinder acquires the heat from the heating medium. Typically a boiler is used to heat up water which is then pumped through the heat exchanger where it loses its heat to the water inside the cylinder. This system is widely used in conjunction with solar thermal systems.
Since their introduction, unvented cylinders have grown in their market share and their popularity. With competition in the market improving availability and reducing prices, it is fair to assume that unvented tanks have a bright future.