Pressure and expansion vessels

The diaphragm vessel holds a reserve of water under pressure. The pump does not start every time a tap opens, starts less often, lasts longer and is protected from water hammer.

It depends on the pump's flow and how many starts per hour it allows. A bigger vessel means fewer starts. For a house with a standard pump, 50 to 100 litres is the usual choice, and a variable-speed pump can do with less.

About 0.2 bar below the pressure at which the pump switches on. Check it with a gauge on the valve when the vessel has no water in it. If it keeps dropping, the diaphragm or the valve may be damaged.

They work the same way. A vertical vessel takes less floor space, while a horizontal one fits under the pump or in a low room. Wates makes both types from 24 to 2000 litres.