Home Heating Boilers

Types of Home Heating Boilers

There are a number of different types of gas, oil and solid fuel boilers for home heating on the market and they can be free/floor standing or wall hung/mounted.

Note: the boilers/furnaces that are used in the USA are differnet to the boilers that are typically used in the UK and most of Europe. This page looks at the European type of home heating boilers.

Central heating boilers can be used for hot water provision, for space heating or both.

  • The new Free/Floor Standing boilers are made from pressed steel or cast aluminium/aluminum which makes them 'slim' (unlike the older iron and cast iron models) so easier for fitting between kitchen units.
  • They are available as Room Sealed, (oxygen is provided via the flue) or Conventional Flue versions where oxygen for the boiler is taken from the room.
  • Wall Hung/Mounted boilers make up the majority of home heating boilers these days. These central heating boilers are lighter in weight, have more compact and efficient heat exchangers, and are constructed from materials such as copper, aluminium/aluminum, stainless steel or even lightweight cast iron.

They are available as Room Sealed appliances only.

The sections below describe the following types of home heating boilers in more detail:

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Conventional Home Heating Boilers

Conventional home heating boilers can be either the condensing or non-condensing type. A condensing boiler reuses the heat from the flue gases to reheat the system making it more efficient, enabling it to be able to operate at lower temperatures saving energy and reducing costs.

A non-condensing boiler does not reuse the heat from the flus gases.

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System Boilers

System boilers are low water content boilers (a low water content boiler only holds a small amount of water in the heat exchanger) that incorporate within them some of the central heating system components that are installed independent of a conventional boiler in central heating system using that type of boiler.

The components which are in the system boiler are:

  • circulator pump
  • automatic by-pass valve
  • diverter valve
  • automatic air vent
  • expansion vessel
  • flow and return pipe temperature sensor
  • filling loop

System boilers can be either condensing or non-condensing type.

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What is A Condensing Boiler?

A condensing boiler is a boiler that reuses the heat that would normally escape from the boiler flue to reheat water in a secondary heat exchanger. When hydrocarbon fuel such as coal, gas and oil are burnt, as part of the combustion process they combine with the oxygen in the air to form water vapour.

The latent heat that is used to produce this vapour forms a significant part of the heat given up by the fuel. Because this vapour leaves the boiler, together with the combustion gases through the flue, this heat is lost. If this vapour can be condensed into liquid within the boiler it can then give up this heat which can be transfered to the heating system therefore raising the efficiency of the process.

With boilers made from materials such as steel and cast iron it was important not to cool the combustion gases down to the 'dew point' which is about 55°C, the point where gas condenses to liquid. This liquid (condensate) is a corrosive acid that can attack boilers and flues.

Condensing boilers are made from stainless steel or cast aluminium/aluminum and therefore resistant to the mild corrosive acid of the condensate. The flue gases leave a condensing boiler at about 5 - 10°C above the return water temperature and this is cool enough to allow the use of plastic pipes for the flue construction.

Condensing boilers are also less harmful to the environment than conventional boilers because their high efficiency enables them to produce more heat from a given amount of fuel and because less harmful elements in the combustion gases are released into the atmosphere (even in non-condensing mode condensing boilers are still markedly more efficient than conventional boilers).

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Combination (Instantaneous) Boilers

The combination (Combi) boiler is one of the most popular types of domestic home heating boiler. Because it is an instantaneous boiler it heats water on demand (direct acting), and so allows you to free up loft space (if you have a loft) by the removal of the cold water storage cistern/tank and associated pipework and fittings, saving you money on installation costs.

Combination boilers are practical in small flats/apartments where there is minimal or no space for either hot or cold water storage.

Combination boilers are also very easy to install and cheap to run. However, they can be expensive to repair and many are designed to be replaced after five years, so you may find that replacing your combination home heating boiler is cheaper than repairing it. With regular yearly services these appliances will last a lot longer.

Note: Combination boilers are 'mains fed' and must have enough water mains pressure to operate. Combination boilers can also be condensing improving efficiency.

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If you have a family with a number of hot taps/faucets and showers in use at the same time then a standard Combi boiler may not meet you requirements, but don't worry read on to find a solution.

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Combination (Storage) Boilers

The combi storage boiler has an added storage tank, therefore the problems of progressive flowrate reduction, associated with multiple users at once, are overcome as there is stored hot water to assist in hot water demands.

Now you can use a couple of taps simultaneously without an unacceptable drop in performance. The advantage of this system over a conventional boiler and storage cylinder/tank is that your hot water never runs out. Even after running a bath, a combi storage boiler doesn't need time to recover before you can use it again. So there's no need to plan your hot water requirements as you always have hot water on hand.

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Combined Primary Storage Unit (CPSU)

This type of combination boiler incorporates very large hot water storage, 80 litres or more, which creates a high hot water flow rate at your taps/faucets and a faster responding heating system.

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Combined Heat and Power (CHP) Systems

The CHP system is a boiler that provides both heat for hot water and heating and generates electricity for power. It is also called a 'Cogeneration System'. 

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Electric Boilers

Electric boilers can be used in place of fossil fuel type boilers in conventional home heating systems. They are very compact, light and can be run off cheap rate electricity (night rate). They are also completely silent.

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Solid Fuel Boilers

Solid fuel boilers are floor standing boilers that can be run on materials such as wood, coal, straw and carbon neutral pellets. However, unless using carbon neutral fuel they are not environmentally clean.

With solid fuel boilers at least one radiator must continually be on as a 'heat sink'. Solid fuel home heating boilers run constantly unlike automatic boilers that only start up when needed .

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Central Heating Combination Boilers: Fault Finding and Repair


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