Central Heating Systems – How do they Work?

This is a question that is often asked.
The average modern central heating system typically consists of the boiler, hot water cylinder and pump, radiators, thermostat, programmer and timer.

The main central heating component is the boiler, as this is responsible for heating the water which in turn heats the house. The boiler is controlled by the programmer which contains the timer that switches the central heating system off and on at pre set times during the day and night.
The temperatures are controlled by a thermostat of which there are two, the room thermostat and the hot water cylinder thermostat. Both of these can turn the main central heating boiler on and off when heat is and is not required.

There is also a diverter valve, which is designed to control the flow of hot water from the central heating boiler to the radiators or hot water cylinder tank, this is carried out electronically on a selective priority basis, meaning that it will supply hot water to either the hot water cylinder or the radiators depending on which thermostat is asking for the heat.

However, if both the radiator thermostat and hot water cylinder thermostat need heat at the same time as each other. The diverter valve will automatically switch itself to the mid position and supply both simultaneously.

So let’s have a look at the different types of central heating systems.

These are the three main types of central heating systems around today.

Gravity fed.
Combination or “combi”
Fully pumped.

Gravity Fed Systems

The majority of homes in the U.K have this type of system. It typically consists of a cold water tank in the loft, and a hot water cylinder tank in an airing cupboard on a lower floor.
In this type of traditional central heating system the water is heated by the boiler.
The water for the central heating is supplied via a separate pump directly to the radiators. Water is also gravity fed to the hot water cylinder where it is heated and stored.


A combination boiler system is very popular, as it saves a lot of space because there is no need for a main header tank or hot water cylinder tank.
Water is taken directly from the mains to a second heating circuit in the boiler where it is heated and sent directly to the radiators. With this type of boiler hot water is unlimited and instantaneous when the boiler is on. Temperature can also be controlled with a room thermostat.
This type of central heating system is ideal for flats and homes where space is an issue.

Fully pumped system.

Over the last 20 years or so the fully pumped, open vented central heating system has now become the first choice of installation, regardless if it’s for a new build or a replacement for an older system.
Both the central heating and hot water are controlled by a single pumped circuit from the boiler.
This system heats radiators faster and is more efficient.