Boiler Types

What are the different types of boiler ?

When you look into upgrading or installing a new boiler, do you know what the difference is between the different types ? e.g. condensing system boiler, or open vented heat only boiler.

Hopefully the information below will be helpful.


Condensing Boilers

For the last few years all boilers fitted must be a condensing boiler (some extreme circumstances let you fit a non-condensing unit, but these are very rare).

Without going into too much detail, these boilers will operate in condensing mode that makes them very efficient (over 90% efficient compared to a back boiler that may be 65% efficient). The boiler will produce condensation inside the unit, this runs to the bottom of the boiler and has to be discharged to a drain, soil pipe, waste pipe or purpose made underground soakaway.

The condense pipework can make new boiler positioning more awkward as many existing boilers are situated in the centre of the property away from outside walls and waste pipes. There are a few ways of overcoming this problem, one of which is a condense pump that can pump the water upwards, possibly into a loft, and through the loft to a suitable outside wall near a gully or soil pipe.

Flue position is now more important with a condensing boiler. The boiler will produce 'Plume', it will look like steam coming out of the flue ( but is actually only slightly warm), it can be pushed up to one meter from the flue by the boilers' fan and is something that you would not want pointing towards a neighbour or near any cars. There are ways of avoiding this in awkward situations with the use of plume kits that can direct the flow of plume where required.


Boiler Types

There 3 main types of boilers ( gas and oil ), all of these are condensing.

  • Heat Only
  • Combination
  • System

Heat Only Boilers

This is the standard type of boiler. If you have an hot water tank in the airing cupboard, and storage tanks in the loft or at high level then you have a heat only boiler.

These can be in the form of a wall hung, floor standing or back boiler/fire unit.

Heat Only boilers produce hot water that is sent to radiators and usually a storage cylinder. The domestic hot water comes from the storage cylinder which in turn is kept full by the cold water storage tank in the loft.

The radiators and boiler are kept full of water from a header tank usually installed in the loft.

This type of system is called an open vented system, or low pressure open vented.

As the name suggests, there are open ends on the system. these are at the tank area on the vent and cold feed pipework. this is to help fill the system and for expansion of the system water when it is heated. Heat only boilers can be fitted to a sealed system (with the addition of an external pressure vessel) but this normally only seen if the heating system has had problems.

This type of system is usually controlled by a timeclock, room thermostat, and cylinder stat ( although older systems may have very little control ), and may also have motorised valves controlling the flow of water to heating or hot water (or both). A common setup would be an 'S-Plan' with 2 motorised valves or a 'Y-Plan' with one 3-way motorised valve. All of the valves, pump, thermostats would normally be fitted in the airing cupboard.


Combination Boilers

This type of boiler supplies heating and hot water from the same unit. It has 2 heat exchangers, one for heating and one for domestic hot water built into the boiler casing.

Combination boilers are generally all wall mounted (some floor standing oil versions, and even external oil versions are available), and require a bit of thought when positioning because they require 7 pipes to be connected to or from the boiler. All of the main components of a heating system a built into the boiler body including the pump, valves, thermostats etc. This means that all you will see is a boiler and associated pipework, everything else is internal.

The heating system is filled via the water main to a higher pressure than a Heat Only boiler and will require a sound radiator and pipework installation to be place.

This is called a pressurised system.

Combination boilers do not require storage tanks in the loft or hot water cylinders which makes them ideal for saving space and obtaining a 'dry loft'.

The boiler has a hot water priority system that will turn off the heating when domestic hot water is in use, and then continue with the heating when the domestic hot water stops flowing.

Standard controls usually consist of a built in timer on the boiler and a room thermostat, although there are a vast amount of programmable controls available for combination boilers. Combination boilers do not require a timer for the domestic water because the boiler will heat the water as soon as a tap is used or hot water starts to flow.


System Boilers

System boilers are a hybrid of a Heat Only boiler and a Combination boiler. They produce hot water that is sent to radiators and a hot water cylinder usually via a 'S-Plan' or 'Y-Plan' setup positioned in an airing cupboard or next to the hot water cylinder.

The system boiler is filled from the water main just like a combi boiler and is considered a pressurised system. This means that a cold feed tank is not required in the loft. The expansion of hot water is taken up by a built in pressure vessel in the boiler. The boiler also has a build in pump within the boiler casing. If the boiler is connected to a standard hot water cylinder, a cold water tank is still required to supply the cylinder.

This type of system is usually controlled by a timeclock, room thermostat, and cylinder stat, and will also have motorised valves controlling the flow of water to heating or hot water (or both). A common setup would be an 'S-Plan' with 2 motorised valves or a 'Y-Plan' with one 3-way motorised valve.

These boilers are often fitted in properties that are too big for a combination boiler and are usually fitted with an unvented cylinder such as a 'Megaflo Unvented Cylinder' ( pressurised domestic hot water ) that does not require a cold water storage tank in the loft. Many boiler manufactures now have their own range of these types of cylinders.