How a boiler works – boiler efficiency

Most houses are supplied with heat by a boiler, and in Ireland heating your house is the most energy intensive thing you do. So having your boiler operate efficiently is really worthwhile. This post will give a brief outline on how a boiler works, particularly looking at the main things that effect efficiency.

A boiler operates by burning natural gas, oil, LPG or wood pellet to heat water that circulates through hot water cylinders, radiators, or under-floor systems. Boiler efficiency refers to the ability of the boiler to transfer energy from the fuel supplied to the boiler to the water or thermal oil of the distribution system. Standard boilers are generally quite efficient, though this is largely dependent the boiler being well maintained and set up correctly.

The factors that affect the efficiency of the boiler are dependent on

1. The design of the boiler
2. The burner air/fuel ratio being set up correctly (this is usually something that can be changed)
3. The heat transfer surfaces of the boiler being clean (this means servicing the boiler)
4. The insulation in a boiler being intact, clean and well sealed.

How boilers work

  • The first stage in the operation of a conventional boiler is the fuel supply to the burner (in most domestic boilers this is inside the main casing, but the principle is the same as the diagram I have there). In the case of natural gas this is from the mains gas pipe network. For oil or LPG this is though a pressurised tank or vessel located outside. In the case of a wood pellet boiler the pellet is blown or screwed to the burner


  • The fuel is mixed with air and ignited. The correct amount of air is key to ensuring complete combustion. Too much air means that that a lot of oxygen will remain un-combusted in the flue gases. This un-combusted oxygen will reduce the efficiency of the boiler by carrying much of the heat up the chimney with it. Too little air means incomplete combustion releasing carbon monoxide (and soot!). Carbon monoxide is poisonous and so too little air is very serious situation. Also incomplete combustion means that not all the fuel is burned, which is wastful

Air/fuel ratio

(this graph shows the amounts of carbon monoxide (CO), carbon dioxide(CO2), and oxygen (O2) in the flue gases (smoke) of a boiler. If there is too little oxygen, there is CO in the flue gases. If there is too ,much oxygen going in, there will be a lot of oxygen in the flue gases)

So air/fuel mix is the first reason to have your boiler serviced. The service man should adjust the mix to optimise the efficiency of the boiler.

  • The flame from the burner, and the resultant combustion gases escape into the boiler, transferring their heat to the water circulating through the heat exchanger within the boiler. If the heat exchanger within the boiler is dirty, due to rust or soot, it reduces the heat transfer (soot is a good insulator). That is another reason why you should have your boiler serviced regularly. The service man should clean the heat exchanger as part of the job.
  • Boilers distribute heat from around the house using a wet system. In most cases water flows through the heat exchanger in the boiler, absorbing heat from the flam and the combustion gases. This hot water is then circulated around the house to radiators, underfloor heaters and hot water cylinders.

Another efficiency factor at this stage is the temperature you are running your system at. The hotter you run the boiler, the hotter it is trying to make the water that is to circulate around the house. Hotter water improves the response time of the system (the speed it can heat the radiators up at), but reduces the efficiency of the system.

  • The gases of combustion escape from the boiler through the chimney or vent. If the air/fuel ratio is set up correctly, this should be relatively clean. If the vent gas smells this can be a sign of incomplete combustion and the boiler should be serviced immediately. It is also essential that vents and chimneys are not blocked. It an obstacle is placed in the vent, this can lead to back pressure on the boiler, and reduce the efficiency
  • Age of a boiler – following the points made above it should be noted that older boilers are going to be less efficient due to dirt and soot on the heat exchanger (no one can clean them that well) and the air fuel mixer will become looser and so get out of “tune” more regularly.

Boiler controls – this is worthy of a post in itself, and will get one, but in short there are two main points

  • Running your boiler for less time is a good thing. If you don’t have a timer on your boiler, get one. They are very convenient, and they allow you to run your boiler for short periods when you need the heat.
  • Don’t have your boiler switching on and off too often. Like most machines, boilers are less efficient, and not really decided to operate, when they are being switched on and off constantly. If you are going to switch it off, leave it off for at least an hour.

Other topics I plan on covering in this boiler section

If there is one you want me to fast track, let me know

77 Responses to How a boiler works – boiler efficiency

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    i would like to know how a boiler work in detail from comtrol to temperature setting

  7. Tomer says:

    I just got one of these energy efficient boilers and although I didn’t see much difference at first, now that I’ve had it for a few months I can really tell the difference (especially on the bill :P).

  8. M S RAGHURAM says:

    You have explained the working of a boiler in very simple and concise terms so that any one can understand. I only feel if you can introduce the technical terms, it will be very useful.

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    who can tell me clear to operate boiler ?

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    Great information! What is the optimum temperature and pressure for a gas bioler?

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    Of course you can increase the efficiency of your boiler with a tigerloop dearator in the system.

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    hows the ratio of co2 and o2 from the wood fuel boiler? co2 greater than or less than o2?

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