Use Hot Water Pressurized Tanks for Proper Hot Water Pressure

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Use Hot Water Pressurized Tanks for Proper Hot Water Pressure

Monday, December 22nd, 2008    Subscribe To Our Feed

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.

Compare this with pressurized systems which use the pressure of the mains system to provide hot water at convenient pressure levels. As the name suggests, pressurized systems sustain the incoming pressure of the cold water mains (normally around 2-3 bar in most residential areas in the UK). By keeping the system sealed, the hot water is pumped out to the user at the same flow as the cold water that comes out of the cold water tap.

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 UK certification of unvented systems has strict requirements from manufacturers to ensure that proper safety measures are in place to handle the excess expansion in the system when the water heats up. Expansion vessels are attached to the loop (either externally or within the cylinder) to absorb volume expansion of up to ten bars. This level is normally enough to account for most cases of volume increase due to overheating of the water.

UK regulations for unvented cylinders set out a detailed spec of safety measures to ensure all unvented systems sold in the UK meet the minimum level of safety. One of the main measures to deal with water expansion within sealed systems is the introduction of an expansion vessel. Whether external or internal (within the cylinder) an expansion vessel can absorb the expanding volume of water due the increase in the water’s temperature. Expansion vessels vary but most can handle up to ten bars of pressure.

In addition, the legal safety requirements request a pressure and temperature relieve valve (PT valve) to be pre-installed in the cylinder. The valve is designed to open automatically whenever the pressure or the temperature reaches a point which is deemed to exceed the recommended levels of operation. In such case, excess water is discharged and is drained away from the cylinder, thus reducing the pressure and the temperature inside.

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 - this set up is based on a heat exchanger. Since the water inside the tank is heated by another medium using the heat exchanger, it is considered to be done indirectly. Typically a boiler is used to heat up water which is then pumped through the heat exchanger where the heat is transferred to the water inside the cylinder.

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, member 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.

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