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Gas Boilers & How They Work

Gas Boilers & How They Work

What does the boiler actually do?

Your boiler is the most crucial part of your central heating system. It’s like a large fire that has a continuous supply of natural gas flowing into it from a pipe that goes out to the gas main in your street. When you decide to heat your home, you switch it on using an electric switch. The valve opens and the gas flows into a sealed combustion chamber through lots of tiny jets then an electric ignition system sets light to them. The jets point onto the boilers heat exchanger which is connected to a pipe that is full of cold water. The heat exchanger then takes the heat from the jets and starts heating the water to around 60°C (140°F).

The water pipe is actually one tiny part of a large circulating circuit of pipes that travel completely round your house. It goes through each radiator in turn and then it returns to the boiler once again. As the water runs through the radiators it gives off a part of its heat into your rooms. By the time it gets back to the boiler it has cooled down by a fair bit. This is why the gas boiler has to keep firing in order to keep the water at the correct temperature to heat your house. The electric pump inside the boiler keeps the water flowing around the circle of pipework and radiators.

What is Inside Your Combi-Boiler?

Have you had a combi-boiler installed and maybe thought about the parts are inside your combi-boiler? Here is an image which shows the main parts inside the big white box.

Internal parts of a combi-boiler

  1. The primary heat exchanger
  2. The air intake system
  3. The water pressure sensor
  4. The gas valve
  5. The secondary heat exchanger
  6. The bypass valve
  7. The hot water impeller sensor
  8. The flue connection ring
  9. The thermal module/gas burner
  10. The expansion tank
  11. The gas ignition electrode
  12. The fan
  13. The air vent
  14. The electronics and controls box (under the cover)
  15. The diverter valve with bypass
  16. The pump

How do combination boilers make hot water?

Most gas boilers are also hot-water heaters. Some open vented boilers heat water that is stored in a tank while others such as combi-boilers will heat water on demand.

How do combi-boilers work?

Commonly, combi-boilers have two autonomous heat exchangers. The first one takes a pipe through to the radiators, while the second one takes a similar pipe through to the hot water supply. When you turn on a hot water tap, you open a valve that lets water get out. The water flows through a system of pipes that lead back to the gas boiler. When the boiler recognises that you have opened the tap, it fires up the boiler and heats the water. If it is a central heating boiler, it usually has to pause when heating the central heating water while it is heating the hot water, because it cannot supply enough heat to do both of these things at the same time. That’s why you can sometimes hear boilers switching off and on when you turn on the taps, even if they are already lit up to power the central heating.

How does central heating work?

You can think of a central heating system as a continuous circle moving hot water coming out from the gas boiler, through the radiators, and then back again to absorb more heat. In practice, this circuit is usually more complicated than this. Instead of a series arrangement, most new systems are likely to have parallel trunks and branches with many of the radiators being fed from a common pipe. The water is permanently sealed inside this system and the same water goes around around your house every single day. Here’s how this works:

Gas combi boiler central heating diagram

Gas enters your home from a pipe in the street. The boiler burns the gas to make the hot jets that fire on to a heat exchanger, this is basically a copper pipe holding water that bends back and forth through the gas jets so it gathers the optimum amount of heat. The heat from the gas is conveyed to the water. Now a pump pushes the heated water through the entire system.

The the water runs around a closed loop inside each of the radiators, entering at one side of the radiator and leaving at the other side. Because each of the radiators are releasing heat, the water is much cooler when it leaves a radiator than it is when it enters it. After the water has passed through all the radiators, the water has cooled down a lot and it has to return to the gas boiler to absorb more heat. You can see the this water is really just a heat-carrying system that picks up heat from the gas in the boiler and moves some of it off at each radiator.

The pump is strong enough to push water upstairs in your house through the radiators there too.

There is a thermostat situated in one room monitors the temperature and switches off the gas boiler when it’s hot enough, switching the boiler back on again when the room gets cools down too much.

The waste gases from the boiler leave through a flue and dissipates into the air.

If you would like some advice on fitting a new combi-boiler just get in touch here.