Central Heating Boiler types and definitions
When boilers are assessed for boiler efficiency there are a set of strict definitions used so that the efficiency data is uniform and can be relied upon. Whilst there are many boiler types the main definitions are below.
Boiler – A gas or liquid fuelled appliance designed to provide hot water for space heating. It may be designed to provide domestic hot water as well.
Condensing boiler – A boiler designed to make use of the latent heat released by the condensation of water vapour in the combustion flue products. The boiler must allow the condensate to leave the heat exchanger in liquid form by way of a condensate drain. Boilers not so designed, or without the means to remove the condensate in liquid form, are called ‘non-condensing’.
Low temperature boiler – A non-condensing boiler designed as a low
temperature boiler and tested as a low temperature boiler as prescribed
by the Boiler Efficiency Directive (ie; the part load test was carried
out at average boiler temperature of 40°C).
Regular boiler – A boiler which does not have the capability to provide domestic hot water directly (ie not a combination boiler). It may nevertheless provide domestic hot water indirectly via a separate hot water storage cylinder.
Modulating regular boiler – a regular boiler with the capability to vary the fuel burning rate whilst maintaining continuous burner firing.
Combination boiler – A boiler with the capability to provide domestic hot water directly, in some cases containing an internal hot water store.
Keep-hot facility – A facility within an instantaneous combination
boiler whereby water within the boiler may be kept hot while there is
no demand. The water is kept hot either (i) solely by burning fuel, or
(ii) by electricity, or (iii) both by burning fuel and by electricity,
though not necessarily simultaneously.
Instantaneous combination boiler – A combination boiler without an internal hot water store, or with an internal hot water store of capacity less than 15 litres.
On/off instantaneous combination boiler – An instantaneous combination boiler that only has a single fuel burning rate for space heating. This includes appliances with alternative burning rates set once only at time of installation, referred to as range rating.
Storage combination boiler
A combination boiler with an internal hot water store of capacity at least 15 litres but less than 70 litres,OR a combination boiler with an internal hot water store of capacity at least 70 litres, in which the feed to the space heating circuit is not taken directly from the store.
On/off storage combination boiler
A storage combination boiler that only has a single fuel burning rate for space heating. This includes appliances with alternative burning rates set once only at time of installation, referred to as range rating.
Modulating combined primary storage unit (CPSU) – A single appliance designed to provide both space heating and the production of domestic hot water, in which there is a burner that heats a thermal store which contains mainly primary water which is in common with the space heating circuit. The store must have a capacity of at least 70 litres and the feed to the space heating circuit must be taken directly from the store. The appliance has the capability to vary the fuel burning rate whilst maintaining continuous burner firing.