Heat Only Boiler
This is a traditional or conventional open vented boiler, if you have tanks in the loft and a hot water cylinder in an airing cupboard then you have this type of boiler.
The boiler heats water, which is diverted between either the hot water cylinder and or your radiators depending on the type of controls fitted and demand. The heated water travels around the cylinder inside a coil, this then transfers heat to the water in the cylinder and gives you hot water for washing etc.
In the airing cupboard you will have a pump, diverter valves, bypass valve & cylinder thermostat. Any expansion within the heating system “vents” into the feed and expansion tank in the loft. In older systems the boiler may heat the hot water cylinder by gravitation rather than being pumped.
Because hot water is heated indirectly and stored these boilers are suitable for larger more demanding households.
Heat only boilers are the most compact, easily fitting inside a kitchen wall unit if required. Some good examples of heat only boilers are Glow worm Flexicom HX, Ideal Icos, Worcester Greenstar and Baxi Solo.