By Matthew Marks 26/3/1998 with subsequent updates
Many gas boilers have a pilot light. The pilot light performs one function or two. On boilers without automatic ignition, it will be lit permanently, and provides the means to light the main burner when required. On all boilers with one, the pilot light provides a safe way of lighting the main burner quickly, preventing a build up of gas which might detonate explosively if the only means of ignition were a spark. (Boilers with a modulating burner can be lit safely without using a pilot light, because the main burner can be started at a low level.)
A permanently lit pilot light wastes gas, and needs user intervention to re-light it if it blows out or if the gas fails. However, it can be quite useful in a combination boiler, as it keeps the water warmish, thus reducing the delay in the production of hot water when it is required after a long period of standing.
The gas valve has a means of shutting off the pilot light, and the main burner, if the pilot should fail. The presence of the pilot light is sensed by a simple system: a thermocouple. A thermocouple is any junction of dissimilar metals. A small voltage is generated at the junction, proportional to its temperature. If the dissimilar-metal leads from a thermocouple are joined together, current will flow, but only if the two junctions are at different temperatures, because otherwise the voltages generated at each will cancel out.
In a gas boiler, the "hot" junction is in the pilot flame, and the "cold" junction is in the gas valve. Thus, if the pilot is lit, a current will flow round the circuit. This passes through an electromagnet, which holds the gas valve open. The current is far too small to pull in an electromagnet, so the clever bit is that the user does this by pressing in the button on the gas valve. There will also be some sort of interlock to prevent the main burner activating while the button is pressed in.
If the pilot is then lit (either with a match, a manual piezoelectric spark or an electronic spark generator), after a few seconds the hot junction will be hot enough for the electromagnet to hold the valve open against a spring. If the pilot goes out for any reason, after a few seconds the hot junction will cool and the electromagnet will release, closing the valve until it is manually opened again.
If there is an ignition system and it does not produce sparks, it may be faulty, its high tension connection to the electrode may be breaking down (the return path is always through the body of the boiler), or the electrodes may be too far apart (due to being interfered with or corroding away). If there are sparks and the pilot will not light, the electrode positioning or the pilot adjustment may be wrong, or the pilot jet may be blocked, resulting in no gas getting near the sparks. No gas can also result from a faulty gas valve, or air in the pipes. (With my old combi, the only way to re-light it was to fool it into thinking that the pilot was lit, and activate the main burner for a couple of seconds to purge air.)
The thermocouple is rather confusing in that it looks like a capillary, as used in thermostats. The hot junction is a probe which sits in the flame, and a thin flexible copper tube, acting as one conductor and containing the other, leads back to a fitting on the gas valve secured with a small back nut. Thermocouples do fail, and are cheap to replace at about a fiver, but before this is done it is worth making sure that the probe is correctly positioned in the flame, and the flame is the right size (as specified in the boiler manual). Pilot jets can get blocked, and there will be a pilot adjustment on the gas valve. Sometimes there is a deflector to direct part of the pilot flame towards the thermocouple, and part towards the main burner.
If the pilot flame is correctly adjusted but the gas valve will not stay open at all, the thermocouple (or the gas valve, but unlikely) is open or short circuited. Sometimes thermocouples may fail intermittently, leading to the confusing symptom of the boiler working for a while and then giving up, as the heat of the main burner causes the thermocouple to expire temporarily. However, this may also be caused by strong air currents due to heating by the main burner blowing out the pilot. The pilot needs to be watched, to see if it goes out before or just after the characteristic click of the gas valve releasing.
These use an electronic spark generator (a capacitor, charged repeatedly, and then discharged rapidly into a step-up transformer) to light the pilot whenever the main burner is required. Once the pilot is lit, the main burner is activated. As part of the electronics required to achieve this, the presence of the pilot is detected not with a thermocouple, but by electrodes in the flame - a flame looks like a resistive diode. (It is unsafe merely to check for conductivity, as many faults could cause this.) On some boilers these electrodes are the same ones from which the spark jumps to light the pilot in the first place: between sparks, they are monitored.
These boilers have a gas valve containing two mains or low voltage solenoid valves, for pilot and main burner. The ignition sequence of the Potterton Netaheat is given, as an example of how the system functions:
1) Thermostat calls for heat, supplying mains power to the electronics
2) The fan starts - this boiler has a fan-assisted flue;
3) A pressure switch detects that air is flowing through the boiler. (In some boilers a air flow is detected by the change in resistance of a thermistor, which is heated by current flowing through it, caused by the moving air cooling it down.)
4) Ignition is delayed for a "purge period", to remove any unburnt gas in the boiler
5) The pilot valve is opened (first irritating loud click), and the spark generator is activated
6) Pilot ignites and is detected by ignition electrodes
7) Spark generator is de-activated, the main valve is opened (second irritating loud click) and the main burner is ignited by the pilot flame.
The pilot flame and the air flow are continuously monitored.
Failure in this type of system can be harder to diagnose, but can be localised by seeing at which point the ignition sequence sticks. If the relevant input or output device is functioning correctly, the electronics is at fault. Pilot adjustment and electrode positioning (as opposed to thermocouple positioning) are just as important for this type of boiler.
It should go without saying that the safety devices incorporated in gas boilers must not be overridden, or there is a possibility of boiling water, fire, gas explosion or carbon monoxide poisoning.
Suppressors can be fitted to boilers to prevent radio interference when they switch on and off. These usually consist of an encapsulated series connected resistor and capacitor. These will perform their function if connected across the gas valve or the thermostat, but they should *not* be connected across the thermostat because if they were to fail short circuit, they would activate the valve permanently. A shorted suppressor across the gas valve will merely blow the fuse in the supply to the boiler, closing it down.