Heat only boilers
Heat only boilers are also known as 'regular' or 'conventional' boilers. If you are replacing an old boiler and have a small expansion tank and a larger cold water storage cistern in your loft space, you probably already have a heat only boiler.
Heat only boilers are usually installed on an open vented system. The system will usually comprise two tanks in the loft, a hot water storage cylinder in the airing cupboard and a separate pump.