Condensing Boilers

Condensing Boilers

A condensing boiler is a modern, high-energy-efficient boiler that incorporates an extra heat exchanger so that the hot exhaust gases lose much of their energy to pre-heat the water in the boiler system.

Since 1st April 2007, it has been a legal requirement that all gas boilers installed in England and Wales must be condensing boilers

When condensing boilers are working at peak efficiency, the water vapour produced in the combustion process condenses back into liquid form releasing the latent heat of vaporisation. A side effect is that this water, known as condensate, which is slightly acidic, has to be piped away to a drain or soak away.

This photo shows a cutaway combination condensing boiler. It is mounted on a wall and the exhaust gases will rise through the plastic flue in the top left corner. Hot water is provided by a small storage tank on the right. The tank (which is covered by insulating foam) has been cut open to show the tightly wound quick refresh coil inside it. At the bottom of the photo are a number of pipes going into the boiler. One carries the gas for the burner and there are two (in and out) for the central heating system. The plastic pipe on the right carries the condensed water vapour produced by burning the gas. This water contains dissolved oxides of sulphur and nitrogen, making it slightly acidic.

Do condensing boilers work?

Ian Byrne, our Deputy Chief Executive, writes: "YES! Although condensing boilers are relatively new in Britain, they have been commonplace in the Netherlands for a number of years. The National Energy Foundation carried out extensive monitoring work on new homes built in Milton Keynes in the late 1980s, and it was found that operating efficiencies of 90% or better could be obtained in normal use. I have had a condensing boiler in my own home for over 20 years; they have worked well and my heating bills are lower than my neighbours in similar homes with old-fashioned boilers (although most have installed condensing boilers themselves since this page was first written in 1996)."

What size of condensing boiler do I need?

Old boilers may have their output measured in British Thermal Units per hour (Btu/h), but all current boilers are sold in the metric equivalent of kilowatts (kW). Use this calculator to convert Btu/h to kW. You should bear in mind that, in the past, central heating engineers often installed boilers with a higher output than was strictly necessary. Although this meant that there was no possibility of the boiler failing to meet the demand for heat, even in the most arctic of conditions, it also meant that they were mainly operating at a part load, and so running below their maximum efficiency. If you have installed additional loft or cavity wall insulation since the last boiler was fitted, it is highly likely that you will need a smaller boiler than before.

We recommend seeking professional advice from a Gas Safe or OFTEC registered installer before choosing an appropriate replacement boiler. Please note that it is illegal for most types of gas boiler to installed on a DIY basis.

Where can I find an installer for a condensing boiler?

All Gas Safe registered gas engineers or Oftec registered oil installers in the UK should be able to fit a condensing boiler. The Gas Safe scheme replaced the CORGI scheme on 1st April 2009. There are no longer nationally available grants or cashbacks for high efficiency boilers but some local schemes do exist.

What controls do I need with a high-efficiency condensing boiler?

Good heating controls require a minimum of four things:

  • Electronic timer or programmer that allows separate switching of heating and hot water.
  • Room thermostat.
  • Thermostatic radiator control valves (TRVs).
  • Separate thermostatic control on the hot water system.

Additional controls that may be worth considering to get the very best from a central heating system include intelligent heating controls, a weather compensator, a boiler energy manager and full zone control. These are all explained on our page about Central Heating Controls.

Can condensing boilers work with warm air systems?

In general, yes, condensing boilers can be used with warm air central heating. These systems were commonly fitted into homes in the late 1960s and 1970s, and many are now in need of a major upgrade or replacement. There are limited number of UK firms specialising in this upgrading work, including Johnson & Stanley who maintain a library with the technical specifications of almost all warm air system types ever installed in the UK. New or upgraded warm air systems must comply with the British Standard BS 5864:2010 "Installation and maintenance of gas-fired ducted air heaters of rated heat input not exceeding 70 kW net (2nd and 3rd family gases) - Specification", which specifies the selection, installation and maintenance of flued, gas-fired, ducted-air heaters of rated input not exceeding 70 kW (based on net calorific value) used to heat one or more rooms in either domestic or commercial premises. The British Standard is applicable to warm air heaters that incorporate a fan to circulate the warm air. It is also applicable to combined air heater/circulator installations and to the servicing and maintenance of heaters that distribute warm air by natural convection.

Are there alternatives to condensing boilers?

If you are off the gas main, and are concerned about price fluctuations of oil and liquid propane gas (LPG) there are three main alternatives to condensing boilers. All three can work with traditional radiator-based central heating systems, but all of them are likely to cost more to install than a normal gas boiler system:

  • Biomass boilers burn wood or, more commonly, processed fuel derived from wood, such as woodchip or pellets. Pellets are the easiest to handle and can be supplied in bags or by a tanker. They can be used with fully automatic heating systems, require very little de-ashing and take up least storage space. However, they tend to be a fairly expensive fuel, and bulk deliveries by tanker are not available everywhere. Woodchip is a useful compromise, especially for larger boiler systems. All biomass heating systems have the added advantage that they are broadly carbon neutral when using wood grown from carefully managed sources.
  • The second most popular alternative is to use a heat pump. This most commonly uses a ground source collector loop or boreholes to capture heat stored in the ground. Heat is extracted from the external collector loop in a heat pump that produces warm water for use in the central heating system. Ground Source Heat Pumps (GSHPs) often work best with relatively low temperature central heating circuits, utilising underfloor heating (or oversized radiators), so are best selected when undertaking a new build or major refurbishment, although some systems are designed to feed existing high temperature heating circuits. Air to water heat pumps can also be used in some situations. More information on GSHPs can be found on the Ground Source Heat Pump Association website.
  • The final, and least common, alternative to a condensing boiler is a type of electric storage heating. This uses cheap off-peak electricity to store heat in a large insulated container of water or bricks. The central heating circuit then pumps water through the store to the radiators to release the stored heat into the home, when needed.

As with all central heating systems, running costs should not be a major issue for homes that are well-insulated. As these systems tend to be more expensive to install, it makes even more sense to make sure that a home is well-insulated and relatively draught-free before sizing a new heating system.