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