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Also known as 'open vented boilers'. These boilers provide hot water and central heating similar to a system boiler where water is heated in the boiler and then pumped either through the radiator for central heating or through the heating coil in the hot water tank for hot water. The difference is that the water heated by the boiler is at atmospheric pressure which means that it has a fill and expansion tank located in an elevated position. The hot water is also at atmospheric pressure and requires a header tank in an elevated position as well. This type of heating system in only really found in the UK, and most people if they are looking to renew their heating systems will remove the fill and expansion tank and turn the open vented system into a sealed system.
These boilers are suited for larger homes with an already existing open vented boiler