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Central Heating System Explained

Everyone has a heating system in their home, but not everyone knows what a heating system does or the type of system they have.

Knowing about your heating system is always helpful, especially in times of crisis, as it means you can explain with confidence to a heating engineer or plumber the problems you are experiencing with your system.

Basic Overview of how your central heating works;

Your central heating system moves in a constant circle which pushes hot water from your boiler, to the heating elements and back again to accumulate more heat. The same water is locked within your system, and loops around again and again, unless you flush your heating system.

The basic steps;

  1. Fuel enters your home from an external source or storage unit.
  2. Your boiler then burns the fuel, which creates flaming jets that heat a copper pipe containing water. This pipe is run back and forth over and around the jets making this copper pipe a heat exchange. As the jets heat the copper pipe, the heat created is transferred from the pipe to the water within.
  3. You have an electric pump that pushes the hot water through the system, this includes upper floor levels.
  4. Water flows around the closed circuit delivering hot water to each radiator connected to the system via pipework. The hot water enters the radiator from one side and leaves from the other side, cooler than when it started its journey. Once passed through all radiators the cooled down water returns to the boiler to be heated again.
  5. Boiler activity is monitored by a thermostat placed in a communal area of the home. This switches on the boiler when the temperature has dropped below the setting and the demand for hot water is requested. This also works the opposite way, once the temperature hits the too high thermostat setting, the boiler is switched off.
  6. Waste gasses from the boiler are expelled from the boiler via a flue.

Boiler Types

Combi Boiler – Is the most widely used boiler in UK homes to date. The reason for this is that it provides a heating system for your hot water and central heating demands instantly.

System Boiler (Also known as ‘Sealed System Boiler’) - With this system you do not need a large water storage tank to keep your heated water stored like a conventional boiler. This frees up a large amount of space in a home, which is beneficial when every square foot counts. This system needs a water cylinder, which is much smaller than the storage tank and can be placed neatly in your property.

Conventional Boiler (aka ‘Open Vent System’ or ‘Regular Boiler’) - These boilers have both a cylinder and a tank. Not many homes install new Conventional boilers. This style of boilers is good for homes that need hot water in several different places at one time because with any other system the demand for hot water in several areas of the home reduces pressure and causes the hot water to fluctuate, often burning the unsuspecting person in the shower.

All of the below make up a basic central heating system.

The Boiler

The boiler is the heart of your central heating system; it delivers different amounts of heat energy depending on the size of the boiler and fuel type used.

Boilers can come in many different sizes, which will be determined by how many kilowatts (kw) the boiler requires to heat your home. To achieve the correct sized boiler many calculations are carried out by a professional heating engineer. Such calculations will be determined by the size of your property, the construction of your building, materials used to build your property and finally how you will be using the system, as everyone uses it differently.

Type of Fuel

Everyone thinks boilers only use gas to create the heat for your home but this isn’t the case. There are several different fuels that are available and can be used.

Compatible fuel types are listed below:

  • Natural Gas – A gas that comes from the mains, the majority of homes have this supply
  • LPG – Liquid petroleum gas
  • Oil type C2 – Kerosene same as jet fuel
  • Oil type D – Gas Oil, mainly used for oil Aga’s
  • Solid Fuels – Coal
  • Biomass Fuel – Logs, wooden pellets or chippings
  • Electric

Heating Emitters

What is a ‘heating emitter’? These are the components that actually expel heat into your desired rooms from your boiler. These can consist of:

  • Radiators – The most common way to heat a home
  • Underfloor Heating – This type of heat emitter is getting more common on extensions, renovations and new builds. Underfloor heating works by running a series of pipes under a solid concreate slab. This works by heating the concreate slab which pushes hot air up and cold air down
  • Fan Heaters – These aren’t as popular in homes. This style of heating uses heated water passing through tubes. Hot air is then blown over the tubes


Pipes are what feed the central heating system from the boiler to the heat emitters. Pipe work materials can be either copper of plastic. Plastic pipe work is becoming more and more popular as it’s easier for the heating engineers and plumbers to fit. Plastic is a push-fitting joint system, whereas copper pipes need soldering, which can be risky because if they aren’t soldered correctly it will lead to leaks. The sizing of pipes will depend on the boilers and what fittings the pipes are joining, the common sizes for pipe work are 28mm, 22mm and 15mm but they can range anywhere from 8mm to 35mm.

Expansion Vessel

The expansion tank is often talked about but if you asked people what it actually is and what it does, not many people would be able to tell you.

An Expansion tank is used to relieve pressure within the pipe work. When water is heated within the pipes it causes the hot water to expand by 4%. This expansion of the water needs to go somewhere to relieve the pressure in the heating system or the system will explode. The job of the expansion tank is to relieve this pressure.

Expansion Relief Valve

Yes you have guessed it; this is a failsafe against the failure of the expansion tank. If the tank fails to do its job it releases pressure through its valve.

Header Tank

Aka water tank. This is normally located in a loft or a high place and is only used on an open heating system. The water tank has two jobs;

  1. Feeding water to the central heating system when water is low.
  2. Providing a place for the hot water to go when your system has overheated.


A pumps job is to push hot water around your heating system. Without the pump the water will just sit still in the pipes. The location of a pump is usually placed inside your boiler or in your airing cupboard.

Motorised Valve

This valve chooses which direction to push the hot water. The valve has a motor within which controls the flow of heated water to either the central heating system or the hot water system.

Central Heating Controls

Controls are the brain of the system, they control all the sensors and valves. There are several controls attached to a heating system see below;

  • Boiler Thermostat – This controls the temperature pumped around the system in your home. You will find this as a dial with degrees shown.
  • Programmer/Timer – If fitted to your system, this will control the on off function to your heating, as well as the flow of hot water to your radiators to heat the individual rooms in your home.
  • Individual Room Thermostat – These are placed in individual rooms to measure the air temperature in each room. You set these to the desired temp in your rooms. If the temperature drops below the set temperature it triggers the heating system to turn on. These controls aren’t as popular in modern heating systems.
  • Thermostatic Radiator Valves (TVR) – You will find these on many heating systems as they are a popular choice to control your radiators. The TVR regulates the flow of water around the radiator as well as controlling the on/off setting on a radiator, depending on stings. These do not control the boiler directly.

As you can see installing a new heating system in your home can be quite a daunting prospect, which will need professional guidance to make the correct decision for the type of home you live in.

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