Choosing the right boiler requires more than just understanding what type to choose, or how to pick the right size. It can also be helpful to understand how a boiler works to make sure you understand what you are specifying.

Here, we explain what the components of a heating system are, how a boiler is installed and also look at how a boiler works.

Main components of a sealed heating system

 Sealed heating system - the boiler  Sealed heating systems - the cylinder
Boilers heats water on demand (combi) or tops up the cylinder (system). The cylinder stores a supply of hot water for instant use around the house. When it gets up to temperature, a thermostat switches off the supply.
 Sealed heating systems - the expansion vessel  Sealed heating system - the controls
An expansion vessel is required in sealed, pressurised systems to accommodate any excess water. A range of controls are required to operate the system. On a more advanced setup, controls can be weather-compensated.

Boiler Installation

Unless you happen to be a qualified, registered installer, you cannot fit your own boiler — so check with Gas Safe (gas) or Oftec (oil) that your installer is registered. However, you can keep an eye on things to ensure that they are doing a proper job (it’s worth contacting a few installers initially to see if their advice varies). The first thing you need to check is that they have correctly sized the boiler and system. As mentioned on p.144, you can get an idea yourself by using the calculator at, where you can also evaluate boiler ratings.

You must ask your installer to demonstrate how their advice complies with the recent changes to the Building Regulations Part L. You can view Part L at, along with Part J for guidance on the variety of flue options, siting and information on the condensate. A common problem is for the condensate pipe to freeze, so internally check it is a minimum of 22mm in diameter and externally at least 32mm. Check all pipework is insulated.

Ensure that the heating system is properly cleansed and flushed before the boiler is installed, and that inhibitor chemicals are added afterwards.

Inside a Boiler

Inside a boiler1. Fan

Assists the evacuation of combustion gases

2. Gas valve

3. Boiler thermostat Controls the temperature of the hot water flowing from pipes to the radiators

4. Flow connection

5. Drain point For draining the system

6. Return connection

7. Heat exchanger The hot gases transfer their heat to the water

8. Air/gas manifold

9. Syphonic trap Disposes of the condensate by discharging a given quantity of condensate fluid at a time, reducing the chance of freezing

Heating Controls for Boilers

A modern central heating system uses a range of controls to work at optimum efficiency. New (gas or oil) boilers should include the following:

Full programmer: Allows the water to come on at specific set times.

Room thermostat: The thermostat has boiler interlock, so when a set temperature is reached, an electrical contact is broken inside the thermostat to switch off the electrical supply to the heating circuit.

Cylinder thermostat (where applicable): Fixed to the cylinder, so that when the top reaches around 60°C, it switches off the electrical supply to the heat exchanger coil.

TRVs: Thermostatic radiator valves should be on all radiators, except in rooms containing a thermostat. It automatically closes off the water supply to the radiator when the desired temperature is reached.

Automatic bypass valve (if necessary): The pump runs for a few minutes after the boiler has been turned off; this stops static water in the heat exchanger boiling due to residual heat.

Dwellings should normally be divided into two space heating zones with independent temperature control (one in the living area). The heating circuit and hot water must be controlled separately.

Articles like this Comments
  • matchmade

    Most condensing boilers are over-specified and spend too little time in their efficient condensing mode. For example, an installer might specify a 24kW condensing boiler which only comes on for 10 minutes per hour, and spends most of that time reheating the pipework. A better approach would be to install a smaller boiler (12kW) that modulates down to 4kW and at least works for most of the time in condensing mode.

    They also very rarely run at maximum efficiency when supplying traditional small radiators with traditional thinner pipes, as they are designed to work at lower temperatures. They work best with underfloor heating at around 55 degrees C. As they are usually added to existing systems, they have to be set to run at a higher temperature than they are designed for, meaning they only work in condensing mode and maximum efficiency for a limited period at startup.

    They are vastly more expensive and have a higher carbon cost than old-style "inefficient" cast-iron boilers, because they are designed only to last 5-10 years, whereas cast-iron boilers easily last 25 years. Codnensing boilers therefore need replacing more frequently, involving the consumptuion of fresh carbon-generating resources. They also break down more often, which means expensive maintenance call-outs, carbon wasted in driving to the site, and so on.

    All in all, they are not an improvement over old-style boilers either in financial terms or real whole-life energy efficiency, once you factor in the maintenace cost and the need for regular replacement.

  • john walsh

    The above comments are a load of nonsense! modern Condensing boilers are just as reliable as the old cast iron 75% efficient things Mr Atkins loves so much, I have had one for over 8 years with no trouble working in a system I designed myself which is open vented, with as many bends as possible and as few elbows as you can get away with to increase water velocity and aid flow , Most boilers whatever their max rated output made these days can be range rated electronically and virtually turned into whatever sized boiler you need for your heating requirements, while maintaining their max output for indirect cylinders ( which are usually around 15.5 Kw coils) on the DHW front.

    As for condensing mode and radiator sizes the with condensing boilers it is actually better to have slightly over-sized panels as you are looking for a dew point return temp of around 54oc as Mr Atkins states they are designed for low temp operation. A heat only condensing boiler coupled into a well-designed 22/28 mm main manifold system with as short as possible 15mm runs to the rads ( under a metre if possible) on a S plan system with modern digital controls and an outside weather sensor with correct balancing will run in condensing mode most of the time modulated on the burner down to what a layman would consider a pilot flame on an old cast iron jobby.

    Stainless steel heat exchanges (not ally, on this I agree with him) will last every bit as long as a cast iron unit in the past I have written off many old conventional boilers with cracked iron H/E, further modern modulating burners are so highly efficient, silent with excellent coils designs on the exchangers they use only a fraction of the gas that the old simple on max full fire/no fire operation of conventional units.
    I can virtually leave my boiler on all year round with outside weather sensing and use only 1/4 of the gas used by my former potterten flamingo 11kw and mine is a 24kw range rated to 10k watt on heating and heats hot water from cold in the DHW tank in less than 9 mins on high fire.
    I used top in bottom out with TRVs on all rads except the towel rail ( acting as a permanent bypass) and the hall, as for carbon nonsense, it

  • Bryn Jones

    Hi John Walsh. In the process of renovating a 3 bed detached house circa 1993 so quite modern. One job I wish to do is modify the HWHG System. Taking on board your valuable design comments, rather than use the current open system (I wish to remove roof storage tanks because of the wasted potential usable space) is there the possibility to create a pressurised system and have say storage of hot water (placed in remote corner of loft) so not using a combi (which I dislike for the sharing reasons you state). Do you have a schematic available for your set up ? Appreciate your or anybody else’s advice – Thanks Bryn

  • Mark Larr

    John Walsh, with your comment "the trees produce more CO2 every month than any human activity does in 10 years and that includes air travel", how can anyone reading this be sure you know what you are talking about? Your whole post sounds legitimate but I suggest you get your facts right before posting… trees actually absorb CO2 and release Oxygen, so much so one large tree can supply a day’s supply of oxygen for four people. So it seems what you say is also nonsense.

  • james bardsley

    I want a combi boiler without a fan that goes on and on and on after demand has finished. The 24i boiler I have has a 5minute factory set fan time. This is ridiculous. Can anyone tell me how to find a boiler where the fan runs for twenty seconds which is more than enough! Ideal run for 4 minutes. Too long!

  • Tony Cansfield

    Reading John Walsh’s article I would kindly appreciate it if he could give me some further assurance about such a system he installed.
    My existing system is a 35000btu/10.5 kw wall mounted gas boiler supplying an elson indirect feed hot water tank,gravity fed,living room stat,radiator load of 12kw,8mm micorbor connecting radiators. Boiler is original to the house built in 1980 and I have extended and added extra rads over the years.
    Our main problem is of course we soon run out of hot water if a bath and shower is run the same time. 2 year ago I fitted a timer to have the immersion element heat for 1.5hrs each morning.
    My system still just manages to get all the rads hot to the touch from cold in about 1.5 hours.
    Going by what your old system was sounds similar to mine and after making a comparison I seem to think I should upgrade to a similar system as you have now.
    One thing to ask you is what litre capacity cylinder do you have and do you find it still retains hot water after running a bath and shower off at the same time. And, if it does run cold it will start to recover hot water in 9 minutes. This seems quite an incredible re-heat time to what we have been used to. Also having an outside stat,is this worthwhile.
    Should I also consider to replace some of my rads for greater output as if I fit a bigger boiler its heat exchange to the rooms might be restricted, I won’t feel the benefit.
    If you could please pass on any other advice to consider it will help to give me more confidence in my end choice and discuss it with my installer.

    Thanks, Tony.

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