Water expands when heated. In a heating system, its volume grows by about 3-4% from cold to operating temperature. If the extra water has nowhere to go, the pressure rises and the safety valve opens.<\/p>\n\n\n\n
A membrane expansion vessel takes in this water and returns it to the system as it cools. The pressure stays stable, and the boiler and radiators work without overload.<\/p>\n\n\n\n
Inside the steel body, a rubber membrane divides the vessel into two parts. On one side there is air or nitrogen under pressure. On the other is the heating water from the system. When the water expands, it compresses the air cushion.<\/p>\n\n\n\n
| Parameter<\/th> | For heating<\/th> | For water supply<\/th><\/tr><\/thead> |
|---|---|---|
| Body color<\/td> | Usually red<\/td> | Usually blue<\/td><\/tr> |
| Membrane<\/td> | Designed for hot heating water<\/td> | Food-grade, for drinking water<\/td><\/tr> |
| Where it is installed<\/td> | Heating system next to the boiler<\/td> | Pumping station, well, hot water cylinder<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n You must not use a water supply vessel in a heating system or vice versa: the membrane is not designed for those conditions.<\/p>\n\n\n\n How to calculate the volume<\/h2>\n\n\n\nA simple guideline is 10-12% of the water volume in the system<\/strong>.<\/p>\n\n\n\n The water volume in the system can be estimated from the boiler output: roughly 10-15 litres per 1 kW for a radiator system. With underfloor heating there is more water.<\/p>\n\n\n\n Example.<\/strong> A 20 kW boiler with radiators: 20 \u00d7 13 = about 260 l of water in the system. Vessel: 260 \u00d7 0.1 = 26 l. Choose the next larger standard size \u2014 35 l.<\/p>\n\n\n\n Wall-hung boilers already have a 6-10 l vessel. That is usually enough for an apartment. For a house with many radiators or underfloor heating, an external vessel is added.<\/p>\n\n\n\n The air pressure in the vessel is set before the system is filled:<\/p>\n\n\n\n Vessel pressure = system height above the vessel (m) \/ 10 + 0.2 bar.<\/strong><\/p>\n\n\n\n For a two-storey house about 7 m high above the boiler: 0.7 + 0.2 = 0.9 bar, rounded up to 1 bar. The pressure in the system itself is set 0.2-0.3 bar higher when filling.<\/p>\n\n\n\n |