Types of Boiler and heating systems


There are many different types of heating systems out there. Please click on a heading below so that you can attain a better understanding of each system and which one may suit your needs.

+Condensing Boilers

A condensing boiler uses heat from exhaust gases which in turn to use this heat the water vapour from the exhaust gas is turned into condense. With a condensing boiler it extracts more heat and less heat is loss through the flue.

With modern systems, only about one litre per hour of condensate is produced, which has a pH in the range of 3.5 to 5 – about the same acidity as tomato juice. As previously discussed, this is carried to a normal drain by means of a simple plastic overflow pipe.

Condensing boilers are suitable for replacing most existing boilers. As with any replacement boiler, the effectiveness of the control system and type of hot water cylinder should be assessed when conducting a site survey. Consideration should also be given to cleaning and flushing the system before fitting a new boiler.

A competent installer will have no problem in deciding on a suitable location for the drain and fitting a normal, plastic overflow pipe. Flue locations will also need consideration as condensing boilers create pluming.

Because the flue gases leaving a condensing boiler are cool, they tend to produce a noticeable mist or plume of water vapour around the flue terminal itself (as they condense upon contact with the atmosphere), especially under cold conditions. This is not a problem and in fact indicates that the boiler is working as intended. However, consideration should be given to boiler and flue location prior to installation so that the plume will not be too close to neighbouring properties or windows, doors and paths regularly used in the winter.

+Combination Boilers

A ‘Combi’ boiler is called this because it heats your house and heats water for your taps on demand, there is no need to let the water heat up. This means that when you run the hot tap your boiler fires up and heats that water that will help you wash up or clean your hands.
At the same time it could be heating your home’s radiators so doing 2 things in ‘combi’nation.

The benefits are:

  1. Cheaper to install
  2. Can be cheaper to run as there is no hot water store to heat
  3. Hot water when you want it
  4. Rules introduced in 2005 say that all new boilers have to be Condensing Boilers. This is sometimes misunderstood as meaning they have to be ‘Combi’.
  5. This is not true. Your boiler can either be ‘Combi’ or have a Hot water cylinder to heat water. The important thing is that the boiler is Condensing.
+Fully pumped Systems

The term for any boiler that uses a pump to move all the heat from the boiler to every part of the system is “Fully Pumped”. As a rule, one pump is used to circulate the water through the valves which divert the flow to the relevant part of the system. A fully pumped system, in conjunction the appropriate peripheral controls, can provide the user with a lot of control over how the heat is distributed, as well as at what time and temperature the heat is supplied.

With all fully pumped systems that incorporate a remote hot water cylinder, you can have an electric Immersion Heater as a backup for the hot water in case of a fault occurring in the gas central heating system.

+Sealed Systems

Central heating systems currently being installed in the UK are increasingly of the sealed primary circuit type. This means the water heated by the boiler, which circulates around the radiators is held in a closed circuit under pressure. Such an arrangement has a number of advantages over ‘open-, vented’ or ‘conventional’ heating systems. Sealed systems are low maintenance, but are not maintenance free. Combination boilers invariably have sealed circuits.

Some of the advantages of sealed systems are:

  1. They are simpler to install since there is no header tank in the roof space, and consequently none of the associated problems (i.e. overflowing or freezing).
  2. They are easy to fill since air is expelled under pressure when bleeding the radiators
  3. As the system is sealed, the problem of air being drawn into the system through open vents is eliminated, therefore the likelihood of airlocks, corrosion and sludge build up is significantly reduced
  4. Increased pressure raises the boiling point of the water within the boiler, promoting quieter operation
  5. There is a fixed volume of water within the system (i.e. the system will not replenish any losses automatically), so in the unfortunate event of a leak the escaping water is limited
  6. They save space in locations such as flats, where there are usually no convenient locations for the header tank
  7. The system can be flushed out under pressure using the mains water supply
+Wall boilers

The above systems can be used with any type of boiler although the most popular is a wall boiler.

As the name suggests this type of boiler is wall mounted and can either a “combi” or a regular heat only boiler. The main advantage of this boiler is flexibility of location and amount of space taken up by the appliance.

+Floor Boilers

As above a floor boiler will provide the necessary requirements for all central heating applications though due the size and location issues they tend not to be very popular.

+Back Boilers (BBUs)

This type of boiler is situated behind a gas fire (the fire and boiler are sold as one unit) usually in the lounge. These systems are fairly outdated and very inefficient and therefore should only be considered as a replacement of an existing back boiler unit.


The flue on a gas appliance takes the products of combustion from the appliance to the outside air. An open flued appliance takes the air required from within the room the appliance is located and requires a purpose provided air vent depending on the amount of gas the appliance burns

The only gas appliances still for sale with an open flue are gas fires or back boiler units. Most modern appliances have a room sealed fan assisted flue. This has the advantages of taking air directly from outside and therefore requires no purpose provided ventilation and also greatly reduces the risk of carbon monoxide poisoning.

As they products of combustion are expelled with the assistance of a fan there is also much greater flexibility with siting the appliance.


Boilers account for around 60% of the carbon dioxide emissions in a gas heated home. By fitting a new high efficiency condensing boiler and improving your heating controls, you will significantly cut your home’s carbon dioxide emissions and could save as much as £275 a year.

As the current lifespan of a boiler is around 15 years, choosing a heating system with a high efficiency condensing boiler with the correct heating controls can make a huge difference to your heating bills over time.

A high efficiency condensing boiler works on the principle of recovering as much as possible of the waste heat which is normally wasted from the flue of a conventional (non-condensing) boiler. The best high efficiency condensing boilers convert more than 90% of their fuel into heat, compared to 78% for the best conventional types.

If you’re shopping for a new boiler always look for the Energy Saving Recommended logo. It’s your guarantee that these products have met the strict criteria on energy efficiency, will cost less to run and help reduce carbon emissions.

+Unvented Cylinders

The traditional British domestic hot water supply system, in which water is heated and stored for use, is based on cold water feed from a storage cistern to a heated cylinder that is vented to the atmosphere. The flow rate and pressure of the hot water discharged from the hot water outlets is then dependent on the height of the storage cistern above the outlets.

However increasing demand to remove cisterns from the roof space means greater difficulties in meeting consumer demands for high performance hot water systems, from traditional open vented systems. This is particularly the case with the growing market for high performance showers.

Unvented domestic hot water systems can, however, meet these expectations by utilising cold water supplied directly from the mains to the heated storage cylinder. Included with the cylinder is a package of safety devices and controls to regulate temperature and pressures within the system. This type of system can provide flow rates in excess of 25 litres/min thus meeting BS6700 specification for design flow rates of 18 litres/min whilst retaining the facility for simultaneous draw-off from other hot water outlets.

Means of Water Heating:

The means of heating the water in the storage cylinder may be either a) direct, which entails one or more immersion heaters in the cylinder, or b) indirect, in which case the contents of the storage cylinder are heated by a primary heater of the coil heat exchanger type.

In both cases the heat source is subject to thermostatic control. Should the thermostat fail the energy input to the storage cylinder, either from an immersion heater or a coil heat exchanger, will be interrupted by an energy cut-out

Advantages of the Unvented System:

The advantages of the unvented system are:

  1. Balanced hot and cold water pressures.
  2. Improved shower performance.
  3. Less pipework and no storage cistern.
  4. Reduced risk of frost damage to system.
  5. Saving on installation time and costs.
  6. Excellent heat transfer capability.
  7. Allows the use of a wider range of modern tap fittings and showers.
  8. Flexibility of vessel siting and hence system design

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