Expansion tank

Expansion tank

An expansion tank is a small tank used in closed water heating systems and domestic hot water systems to absorb excess water pressure. Excess pressure can be caused by thermal expansion as water is heated, or by water hammer. The expansion tank usually contains a rubber diaphragm which divides the tank in two sections. One side contains a small volume of air, which is compressed so that it is equal and opposite to the water pressure. The other side of the tank contains water, which is openly connected to the plumbing system. Anytime the water pressure rises, it will push against the diaphragm, and gently compress the air. The compressibility of the air cushions the pressure shock, and relieves pressure in the system that could otherwise damage the plumbing system.

When expansion tanks are used in domestic hot water systems, the tank and the diaphragm must conform to drinking water regulations and must be capable of accommodating the required volume of water.

In the past, domestic plumbing often contained more air than they do currently. Thus, expansion tanks are being used more frequently than in the past.

In the UK, prior to the use of sealed expansion tanks, 'open' tanks were installed in the roof space to accommodate the water's expansion; these had the disadvantage of being exposed to the cold air in the roof space. This, with effective loft insulation could fall below freezing, and could cause the pipework supplying the tank to freeze. However with good pipe and tank insulation, this was in practice quite rare. Although such systems were remarkably trouble free, there are concerns about the potability of water from roof tanks due to the possibility of contamination. The other major disadvantage is that the water pressure from a roof tank is considerably lower than mains water pressure, making the use of mixer taps sometimes unpredictable.

An expansion tank is also used in the cooling system of most internal combustion engines, to allow the antifreeze and air in the system to expand with rising temperature and pressure.


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