Boiler Systems in the United Kingdom Case Study



We discuss heat sources for the systems in the United Kingdom in a series of interesting facts about boilers.

In its most fundamental form, this is simply a metal box that is surrounded by fire. In fact, the first heating systems were just this, a metal box referred to as a back boiler, found within the fireplace of the lounge. Surprisingly, there are a few still out there in some older properties.

A boiler is an appliance used to heat water for the purpose of supplying a central heating system and hot water to taps. The term ‘boiler’ is not ideal, however, because the water never actually boils inside the appliance – if it did, there would be something seriously wrong. Boilers today are fully automatic devices that turn up the heat as necessary and, with the exception of solid fuel systems,
completely turn off when not required. The water is just heated until the required temperature is achieved, as set by its built-in thermostat, and then the heat source turns off. The fuels that could be used for the boiler include:

  • CC solid fuel, including coal, wood, and straw
  • CC electricity
  • CC gas
  • CC oil

Electric boilers are quite rare and so they fall beyond the scope of this book. The remaining fuel types, however, have been used in boilers for many years, and the design of the boiler has developed
into a very efficient appliance, unlike those of yesteryear. Solid fuel has limitations in its design, and because these boilers tend to be more labor-intensive – i.e. you need to load the fuel and empty the ash – they are not very popular and account for around only 0.5 percent of all installations. Around 92 percent of installations use gas and the rest use oil. Due to developments over the years, there are many different boiler designs from many different manufacturers, with a neverending list of models applicable to the particular designs as well as varied price options for home heating and cooling. But fundamentally they all fall into one of four basic types:

  • CC natural draught open-flued
  • CC forced draught open-flued (fan-assisted)
  • CC natural draught room-sealed
  • CC forced draught room-sealed (fan-assisted)


Essentially, these names relate to the method by which air is supplied to the boiler:

  • CC Natural draught or forced draught indicates whether or not the appliance has a fan incorporated to assist in the removal of the combustion products to the outside
  • CC Open-flued boilers take their air from within the room where the boiler is located
  • CC Room-sealed means that the air is taken into the boiler from outside the building

The boiler in your home will be one of these. For example:

  • If you have a back boiler situated behind a gas fire in the
    living room, you have a natural draught open-flued boiler.
  • If you have a large free-standing boiler in your kitchen, with
    a flue pipe coming from the top, traveling into a chimney or
    passing through a pipe to discharge up above the roof, again
    this is likely to be a natural draught open-flued boiler.
  • Both of these types take their air from the room in which they
    are installed, and this air is replaced via an air vent from the
  • If your boiler has a terminal fitting flush with the wall, it is most
    likely to be a room-sealed appliance.
  • If this terminal is quite large, it will be of the natural draught
  • If it is smaller, say about 100 mm in diameter, it will be a fan
  • These boilers do not take the air required for the combustion
    the process from the room, but directly from outside.

There are many variations of boiler design, where the location
of the fan or the route of the flue pipe – which may be vertically
through a roof or horizontally out through the wall – may vary,
but they all fall within one of the four basic types listed above.

In addition to the basic boiler designs, boilers are further classified into four generic types:

  • CC non-condensing regular boiler
  • CC non-condensing combination boiler
  • CC condensing regular boiler
  • CC condensing combination boiler

The differences between regular boilers and combination boilers will be discussed in the next post, but a new term is used here: ‘condensing’.