Basic Sequence of Events.
Assuming that there is water pressure in the system and that there is a 240v supply and a fuel supply then there is a sequence of Events which can be applied to 95% of boilers working in the field today. By following this sequence you can normally determine in which area the boiler or system fails.
1) Initial Demand. - normally via a Timer / Programmer or the Turning on of a Tap (ie a flow switch)
2) Secondary Demand - normally via a Thermostat.
3) Motorised valve Opens – if fitted – if this Fails check for power from items in 1&2
4) Pumps starts to run – if this fails to operate check the motorised valve operation.
5) Boiler Flue Exhaust Fan starts to run – if this fails check the Pump ( it may be seized) also the boiler flow switch and power from the PCB
6) The ignition starts to spark – electronic ignition only – if this fails check the Fan/Air Pressure Switch/ Flueways
7) The primary solenoid on the gas valve opens and the pilot lights – electronic ignition only – if this fails check that there is a spark present as the leads or probe may be dirty and the primary gas solenoid is creating a magnetic field (normally you can hear a click and feel it grab with a small screwdriver placed against it). In the event of a permanent pilot the thermocouple may be faulty.
8) The main gas solenoid opens and the main burner lights.- should this fail check that the sparking has stopped as the leads or probe may be broken or dirty – Check the main gas solenoid for operation. Check that there is a FULL gas supply.
This is meant as a Very Basic Guide only and repairs should be ALWAYS undertaken by qualified Engineers