How boilers work

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This is a reworking of part of Ed Sirett's Boiler Repair FAQ (which is a work in progress).

Ed's document contains extensive and important parts on Safety, Fault-finding Methodology, Tools and much else: read it before acting on anything in this article!

What class of boiler do I have?

There has been such major developments in the technology used in boilers over the last 30 years that really we have to treat the different 'generations' as completely different appliances.

I have chosen three models to describe as fully as I can. Together they span the bulk of the technologies used. I have chosen each model as the archetype of their technology level. Of course a feature in one level of technology may be found in the 'wrong' category on any specific model but in general these rules hold pretty true. See the article Boiler Evolution for a more detailed descritption

The low tech boiler

Typically has a cast iron heat exchanger. The pilot light operates whenever the boiler is on standby not just when the boiler is running. There are no electronics. The only electrical connection is a single mains input of line, neutral and earth and the boiler runs when power is applied. The flue will move the products of combustion by convection due the lower density of hot flue gases relative to cool air, this is known as a natural draught flue. The archetype is a Baxi Bermuda 50/4 back boiler unit. Since several million of these units were installed mostly in the 1970s and 80s. Baxi became a synonym for this type of boiler even though others made and sold similar units.

The middle tech boiler

Might have a cast iron or a light weight (low water content) heat exchanger. The pilot lights only operates when the boiler is running and is lit with a high voltage spark. There are electronics both for safety and ignition but no 'intelligence', no firmware, no digital displays. It does have both a mains supply which is normally on all the while (except for maintenance) and a 'demand' connection that causes the boiler to run. A fan will usually assist with the movement of the products of combustion. The circulation pump is generally under the control of the boiler. The boiler operates at a pre-set power rating. The archetype is the Potterton Profile 50e many units were made, many are still running, the makers built a solid reputation on this model which has a pretty good efficiency even by modern standards.

The high tech boiler

Might be a condensing model with a stainless steel or other light weight heat exchanger. The gas is ignited directly as needed without the use of a pilot light. There are electronics for everything, the PCB is connected directly to all components in the unit and reads or controls all of them directly. The burner is possibly of the forced premix type. The model may also be a combi boiler and can produce 'instant' and 'unlimited' hot water. The pump is internal and controlled by the electronics. There are mains connections, and a pair of 'demand' terminals which are connected together in order to run the heating. The boiler can alter its power output to suit conditions. There is a digital indicator and possibly the option to plug in a laptop, the control keeps track of problem 'events'. I have chosen the Vaillant Ecotec plus 831 combination boiler for the typical unit.

The low technology boiler

A Baxi Bermuda 50/4 inset back boiler unit.

General description

Back boiler units are no different from wall mounted and floor standing units of the same era. The slightly unusual aspect is that they are positioned in a fireplace and they are combined with a radiant gas fire with which they share a common flue. For the purposes of this document the fire front is simply an appliance which serves as the front cover for the back boiler unit (BBU), (and restricts access to the boiler). Apparently some 3 million BBUs were installed that's about 15% of the dwellings in the UK and given that they were generally not installed in flats that would indicate a still larger percentage of homes. My own house had one until 2003 (the model was a GlowWorm Galaxy) . They were popular because they made use of an existing fireplace and chimney for the boiler and flue location, almost all houses built before the 1970s have at least one fireplace. The fireplace space would have been otherwise unused so they were in some respects space saving, perhaps replacing a solid fuel boiler previously located in the kitchen.

Here is a link to a PDF file containing the entire service and installation manual. This manual mentions little about the fire front because this BBU could be combined with a selection of differing gas fires each of which has their own manual. The manual linked above is a scan of a modern version of this model, however it is almost identical in most respects to the original units except that it has a slightly modified pilot light assembly. This newer pilot light unit can detect the build up of carbon monoxide and/or the depletion of oxygen in the atmosphere and shut down the boiler down.

How it works

You probably know all this already but just in case there is someone who needs to know this.

When main electricity is sent to the boiler a solenoid operated gas valve is opened allowing gas to be burnt in the burner. The gas is lit by a pilot light which under normal condition burns continually. The hot products of combustion convect upwards through openings in a cast iron heat exchanger transferring heat to the water inside the heat exchange. The greatly cooled (but still around 100-150C) gases leaving the heat exchanger are diluted with more air to dry them and further cool them as they convect up a 125mm diameter (or bigger) flue pipe to the terminal at the top of the chimney. The water in the heat exchanger is moved by a circulation pump which is external to the boiler. There is a thermostat in series with the gas valve to control the temperature of the water.

Each part explained.

The gas cock

This allows the gas supply to be isolated to perform maintenance. The only thing which is a little different on BBUs compared to wall hung and floor standing units is that it has three positions: Off, Boiler only supplied, Boiler and gas fire supplied. As part of removing the gas fire it will have already been moved to the “Boiler Only” position. The manuals shows the orientation of the little grooves on the gas cock and what they indicate. If the valve gets stiff of lets by it is best repaired by replacement, but that will entail work in the entire gas installation so you will need to be competent to do gas work at that level.

The gas union

Immediately to the left of the gas cock is a gas pipe union which is intended to be opened and rejoined as part of servicing the boiler. The union requires nothing except cleanliness to make a good gas tight seal. If it does not seal properly then it must be replaced.

The multifunction gas valve

This part is a complete set of gas safety related controls in one lump of metal. I will deal with the purpose of the various sub-components presently. This is a genuine “no user serviceable parts inside” piece of kit . Now let's be clear about this. There are no user serviceable parts inside. There are no professionally serviceable parts inside. There are no diy serviceable parts inside. In fact there are no serviceable parts inside even on Christmas eve. They cost less than £100, they do not need replacing very often if ever in the life of the boiler.

The box contains a thermionic valve (or pilot light valve), an electrically operated solenoid valve, a supply pressure test point, and burner pressure test point, a burner pressure regulator and a pilot light supply adjustment. Most of the valve is described in the manual. Here is some extra background info:

The thermionic or pilot valve: The pilot light heats up a rod of metal called the thermocouple, in fact it is two metals joined together and they are separately connected, electrically, to a coaxial copper wire that looks and feels much like a 2mm copper capillary tube. When (the rod part) is heated to several hundred Celsius (red hot is too much) a small voltage (say 25mV) is produced. This does not sound a lot but it is quite sufficient to drive enough current through a solenoid coil to hold the pilot valve open against a spring. There is not enough power to open the valve that has to be done manually by the user. Should the pilot light go out for any reason the valve will close under spring pressure and the boiler will not fire. If you extinguish a pilot light (say by briefly interrupting the gas supply) after a delay a 'clunk' will be heard from inside the multifunction valve as this valve springs shut. The maximum delay must be no more than 60s. You cannot relight a pilot light until this clunk has occurred. With experience you can sometimes briefly release a little of the pressure on the button as the pilot light is warming the thermocouple up. This is useful as it can tell you whether the thermocouple is good, finished or disconnected. If there is some 'stickiness' in the valve, after you have been holding the valve in for a good 30 seconds but not enough to 'stay in' then the thermocouple is either finished or the pilot flame is not doing its job right. If there is absolutely no stickiness whatsoever then a circuit break is more likely or just possibly a failed solenoid in the valve. The solenoid can be tested for electrical continuity to confirm.

The solenoid valve: This is not the same as the solenoid that's part of the thermionic valve above. It is mains operated (some other models include a transformer and use 24Vac). It simply switches the boiler on and off. If it's not working it is more likely that the problem is elsewhere in the supply chain to the solenoid than inside the unit.

Test points: No need to remove the screw just slacken it. Close it finger tight only and then test for leakage with leak-detection-fluid. The U-gauge tubing fits nicely on the tall 'cones' in which the screws sit. The screws can be very tight to undo if someone tightened the screws previously more than finger tight. If the screw is unmovable with the best sized screwdriver then that information will be unavailable.

Pilot supply and pilot adjusting screw: There is a tube of about 5mm diameter which supplies the pilot light, the amount of gas supplied to the pilot light can be adjusted by a screw near to this pipe.

The pressure regulator: tries to keep the outlet gas pressure at the required level even when the pressure of the gas supplied to the boiler varies. There is another feature of the regulator; it ramps the pressure up slowly to the required level over the period of a few seconds. This helps the main burner ignite more gently and without an alarming bang. The regulator is adjusted by removing a screwed on dust cap the real screw is then exposed. Only adjust the pressure if you are competent to do so, a lot more harm than good will come of blind fiddling. At the very least you need to have a U-gauge and know how to use it, know what the pressure should be, what the pressure is, and which way to turn the adjustment.

The burner

The burner consists of several parts all of which can be collectively referred to as the burner or simply the component out of which the flames emerge. The burner supply pipe comes out of the multi-function valve (usually just called “the gas valve” and goes to a critical component called the injector.

The injector in this model has six small holes rather than one slightly less small hole. The injector is a replaceable component. Over it's life the hole(s) erode to become marginally bigger. When that has happened the boiler will consume slightly too much gas than it was designed to. Combustion will be affected and there will be an increased level of carbon monoxide CO. The holes can also become blocked. If the boiler consumes less than 90% or more than 105% of the designed amount it has a problem.

The mixing tube: is where the gas which having emerged from the injector finds itself impelled towards, on its way it grabs hold of a lot more air to follow with it. The gas mixture then becomes mixed, there may even be a fine gauze inside the mixing tube to further assist this process. The gauze can of course become partially blocked, which is bad news.

The burner: has many holes in it out of which the mixed gas/air mixture emerges and burns. The burner and mixing tube are a single assembly.

The lint arrestor: attempts to catch hold of lint before it can get into the burner and make trouble. Open flued appliances are susceptible to lint, especially back boilers in carpeted rooms or other gas appliances installed in laundry rooms. The heat exchanger

This is device which transfers the heat from the hot gasses produced by the burner into the central heating water. It has three water holes to allow for having a convection driven circuit to heat a hot water cylinder indirectly, known as a “gravity hot water”, and also a pumped radiator circuit. The common return is the bottom lowest hole. If the unit begins leaking it is probably due to severe corrosion in the primary water circuit. That would probably render the boiler beyond economic repair. The gaps for the hot gasses to pass through can becomes partially of even fully blocked, this is bad news. Once soot starts to form it will rapidly build up until the boiler is burning very badly indeed, the products of combustion contaminate the supply of air for combustion so producing even more soot and carbon monoxide. The toxic gas can escape from the fireplace and so put the users in danger. To avoid this newer models have an Atmospheric sensing device (ASD) which is part of the pilot light assembly. Should the flue gasses become depleted in oxygen below a certain point the pilot flame will become ineffective and the thermocouple driven pilot valve will then close.

The boiler hood

This serves a number functions. Firstly it gathers the flue gasses from the top of the heat exchanger and directs them to the flue pipe. Secondly it offers an opening to receive the flue gasses from the gas fire at the front and direct those gases to the flue. Thirdly at the back it has an opening which allows additional air to be drawn into the flue, this opening is known as the draught diverter. The purpose of allowing air to mix with the flue gases is to make them less humid so that upon cooling they are less likely to fall below the dew point and form condensation inside the flue. The flue pipe is made from light alloy and would be susceptible to corrosion from the condensate which due to the trace presence of oxides of nitrogen and sulphur will be mildly acidic.

The flue pipe

This is a flexible metal pipe made of light alloy, it is called a liner because it has been drawn down the chimney and so it lines the inside of the chimney. It is typically secured in the bottom of the chimney by rock wool and chicken wire, which also serve to prevent any debris from the chimney falling on the boiler.

The pilot light assembly

This is broadly similar to the more modern ASD unit shown in the manual. It consists of a pilot light, a thermocouple and an ignition electrode.

The thermocouple is a coaxial tube of two metals which are joined internally at the tip of the tube. The outer metal is chosen to be a material resistant to prolonged heating by a gas flame. Thermocouples can last from a few months to many years. The workings of the thermocouple are further explained in the section on the multifunction valve. The pilot light should heat the last 10mm of the thermocouple. It should not heat it so much that it glows red; that will waste gas and shorten the life of the thermocouple.

The pilot flame usually has two or more flames, one ought to be heating the thermocouple the other should be pointing toward the main burner to light that when the main gas comes through. There may be a small additional flame between the other two to make sure that if one is alight the other is also lit. This pilot flame has a very small injector just like a tiny version of the main injector, this too can become blocked. Likewise the air supply to the pilot burner can become blocked this will cause the pilot flame to be more like a candle flame. This has two effects; firstly it can be a source of soot. Secondly the flame may be less effective at heating the thermocouple, in the case of the modern ASD unit this is intentional. If the gas supply pressure falls too much when the boiler is running, due to inadequately sized supply pipes or a partial blockage, the pilot flame may stop being good enough and the boiler will stop.

The ignition electrode, this is placed so that a high voltage electrical pulse will form a spark and so light the gas of the pilot light.

The piezo-electric igniter

This is a mechanical device which when pushed causes a piezo-electric crystal to be given a sharp knock and so generate a high voltage pulse. The pulse, if you are lucky, will go all the way to the ignition electrode of the pilot light assembly. Usually the lead has become disconnected, or the insulator on the pilot light electrode has cracked and is now shorting out. Try to avoid making yourself part of the spark circuit as it hurts. Even if the pulse, by some miracle, finds its way to the end of the electrode and into a spark its troubles are not over. it also has to spark to the right place. Some people find the only way they can get the pilot to light is with a flame.


The boiler's bimetal thermostat is a temperature controlled switch. It opens when the circulating water output has reached its set temperature. This closes the gas supply valve, shutting off the main burner. It has a different function to the room stat, which controls room temp.

The thermostat is adjustable; a higher primary circuit temp makes radiators hotter, giving more space heating power, a lower primary circuit temp gives better fuel efficiency, but less max heating ability. The stat is best set low in autumn & spring, and turned up when necessary in winter to give enough heating.

RF interference suppressor

Rating Plate

Information on gas type & pressure, power etc