by Tal Potishman
Since their successful introduction to the UK market back in the 80s, pressurized water heating systems, alternatively known as unvented cylinders, have proven to be very popular. They are winning hearts both with the general public and the plumbing and heating industry. As an additional confirmation of their success, in 2007 unvented systems accounted for the first time for 50% of the hot water systems installed 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).
However, pressurized systems, as the name suggests, are effectively passing the incoming pressure of the mains cold water system into the hot water system. A typical residential area in the UK would have a mains cold water pressure of around 2-3 bar. By keeping the system sealed, unvented systems can sustain this pressure within the hot water loop, providing sufficient pressure in the shower, kitchen or wherever else it is need in the house.
The sealed nature of the system led to some technological developments that were needed to address issues such as water expansion. As water heats up it expands and increase the volume it occupies. Traditionally this was easily addressed by the feeder tank absorbing any such expansion of the water. For unvented systems, however, a set of safety solutions has been designed to ensure safety is maintained at the highest level while still allowing for reliable and efficient operation.
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.
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.
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 - normally use an electrical heating unit to heat the water. The electrical unit (often referred to as an immersion heater) is immersed within the water inside the cylinder and has a typical heating capacity of 2.5 to 3.6 kW to heat up the water electrically.
* In-direct cylinders - these cylinders use a heat exchanger format to transfer the heat from another heat source. Typically a gas boiler heats up water which is then run through the heat exchanger inside the cylinder. Heat from the boiler-heated water is then used to heat the water inside the cylinder in-directly through the heat exchanger.
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.
About the Author:
Tal Potishman, editor of Heating Central, writes articles on
home heating, plumbers,
Hove boilers, underfloor heating and solar thermal. He specializes in helping save money by advising on efficient heating.