Central Heating, Water & Boiler Types

The following information has been given as a basic guide as to how certain types of central heating systems work and what type of boilers you can get on the market. Please note, this information is about the most common types and you can combine nearly all of the systems with various boilers depending on your home situations, so if you think you might currently have a different system, you probably do!


Conventional open vented system

This system is where the boiler is fed water from a feed and expansion tank, situated at a high level (generally the loft). The tank is there to supply the system with water and accommodate any expansion in the system, and to vent the system of air.

This works alongside the rest of your heating system, i.e boiler.


Sealed Central Heating System

This is where there is no feed and expansion tank. The boiler is supplied water via a filling loop which is connected to the mains pipes and the return on the boiler.

Some boilers have integral filling loops which are built in.

The expansion is taken up by an expansion vessel, these can be remotely fitted or integral to the boiler.

These systems should have a pressure relief valve fitted, which will expel water to a suitable location if pressure in the system exceeds 3 bar in pressure.


Unvented System

An unvented system is where there is no cold water storage tank in the property and the system is fed from the mains supply coming into the property.

These systems combine a sealed central heating system and generally an unvented hot water storage cylinder or thermal store system.


Thermal Storage units

These units have direct mains supply going to the hot water which delivers good hot water flow rates.

These are filled via a cold water tank and heated by either a boiler or immersion heater, and are common in apartments built in the 1990’s to 2000’s.

Examples of these units are Gledhill, Pulsacoil & Boilermate.


Unvented Hot Water Storage cylinder

These systems operate at mains pressure as they are taken directly from the mains feed to deliver high output flow rates. These systems generally combine a series of safety devices which is purchased as a complete package when it is installed. These can be heated directly via immersion heaters or indirectly via a remote heat source, i.e a boiler. The most common and well known unvented cylinder’s are Baxi Megaflows, Tribune & Ariston, although there are several different manufacturer’s. 


Combination Boilers –also known as Combi Boilers

A combi boiler is fed from the mains and heats water on demand and when you use any hot water tap or shower.

The heating is heated and supplied to the radiators or underfloor heating (system) when the heating is turned on.

This is easily controlled to suit your needs by room thermostats and programmers.

A combi boiler has all the components required to heat your home in one box, ie pumps and diverter valve.


Heat Only Boiler

These boilers are also referred to as Conventional Boilers. They supply heat to the hot water cylinder (system)and this heats your radiators and hot water. 

These are fitted in conjunction with other components on the system i.e pumps, diverter valve, room thermostat and programmers.


System Boiler

This works in the same way as a Heat Only Boiler but has a pump and other components fitted inside it. This works in conjunction with a hot water tank to provide your hot water.

This system can be controlled to suit your needs with room thermostats and programmers.


Condensing Boiler

The main difference between a condensing and non condensing boiler is they have a much more efficient heat exchanger which used more of the heat from the flue gases before being expelled outside. They also have an extra pipe to convey the water which has condensed inside the boiler to a suitable point, ie soak away.

They are much cheaper to run and can save gas consumption by up to 40%


Back Boiler

These are installed in the builder’s space at the base of the chimney. The advantage of the boilers is they occupy no useful space in your home, and are generally behind gas fires. They generally work as a heat only boiler. These are generally open flued appliances which are not economical compared to modern SEDBUC A Boilers. There are condensing back boilers available but at present they work with electric fire fronts instead of gas.


Electric Boiler

An electronic boiler heats the water in the system via elements which work similarly to how a household kettle works.

These are an ideal solution for properties that currently have a gas heating system, which requires renewal, and due to the location of the boiler can make it difficult to fit a gas boiler flue.

These can be retrofitted to gas boiler systems and provide the same heat for hot water and heating.


Warm Air units

Warm Air Units work by warming air from the property and pushing it round the system which heats up rooms via air grills. These come in both room sealed and open flue models and are generally seated on a plenum, which is basically a square box which has the pipe feeds coming from it to feed the air grills and heats your home. 

Once the warm air is delivered into the room, it travels back to be re-heated via the return air duct on the unit. 

The most common warm air units are the ones made by Johnson & Starley.


WA Water Heater

These systems are fitted integrally to Warm Air Units and are a separate burner which heats the hot water cylinder to give hot water. These are basically a heat only boiler connected to the cylinder.


Heating & Interface Units:

These units are becoming increasingly popular in new developments. They have a communal boiler which has pipes going to several properties. These pipes are then put into a heating interface unit which consists of a pump and plate heat exchanger. This is similar to a combination boiler without a burner.