Heat only boilers, sometimes known as open vent boilers, are the oldest design of boiler used today. Traditionally fitted on an open vent primary system to provide heating and hot water.

The primary pipework from the heat only boiler is routed around the hot water cylinder and or the heating circuit. This is typically achieved using a combination of a room thermostat and hot water cylinder thermostat in conjunction with a programmer/timer and a control valve. The control valve historically was usually a three port valve (“Y Plan”).

The primary water is moved around the primary pipe work with a circulating pump external to the boiler.

The open vent name comes from the fact that traditional heating systems have a feed and expansion tank, usually in the loft. The feed and expansion tank is fed from the cold main and has a pipe from it to fill the primary circuit. It also has a vent pipe from the primary pipe work over the top of the feed and expansion tank to allow water to vent into the tank in the event of excess expansion due to temperature.

An open vent hot water cylinder, used in conjunction with the heat only boiler also has a cold water storage cistern to feed it and also has a vent pipe.

Some heat only boilers can nowadays also be used on pressurized heating & hot water primary circuits where the feed and expansion tank is dispensed with. They can also be used on hybrid systems where a combination of pressurized and open vent systems are used on the hot water and heating circuits.

Heat only boilers available today are condensing boilers.