How does my boiler know when to fire?

[asktoby]

I have an Ideal Classic FF boiler. When I request hot water for taps or radiators it should fire. I'm trying to work out how it knows to do this.

Referring to the documentation and inspecting the boiler, it looks like it has just three external connections:

Mains power in.
Water in.
Water out.

The mains is provided by an IEC (kettle) lead that goes to a fused spur in the wall.

I think I also expected to find some kind of low voltage control connection to give it signals from the ST6450 digital programmer or thermostat so now I am confused.

My next guess was that the power supply to the boiler was somehow slaved off the ST6450 digital programmer or Honeywell T6360B thermostat so as an experiment I tried unplugging the boiler's IEC (kettle) lead from the fused spur and instead powering the boiler from a kettle lead into a wall socket, to provide permanent power, thinking that perhaps then it would permanently fire, but that doesn't seem to be the case.

So now I'm stumped. My question is: How does the boiler know when to start and stop firing?

 

[ianmcd]

your programmer and thermostats open either a 3 port motorised valve or 2x 2 port motorised valves depending on your sytem, these valves tell the boiler and pump when to come on

 

[oldbuffer]

1. That boiler has only three electrical connections, live, neutral and earth.
2. Neutral and earth are permanently connected.
3. The boiler has no pump overrun function, so doesn't need a switched live and a permanent live. Once 230 V is applied to the live connection the boiler should fire provided it is working.
4. Don't know why the boiler doesn't start with 230V to the live terminal. Are you sure your temporary arrangement is:
4a. Supplying 230V to live. AND
4b. Has a good neutral connection. AND
4c. Has a good earth connection.
5. There are no low voltage controls on a boiler of this age.
6. For central heating, the programmer switches 230V to the room thermostat. If heat is required, the thermostat switches 230V to the central heating motorised valve. The motorised valve opens, and once open switches 230V to the boiler.
7. For hot water, the process is the same using the cylinder thermostat and the hot water motorised valve.
8. If you have a three port motorised valve rather than 2 x 2 port motorised valves its a bit more complicated but broadly the same.
9. It is normal for all the connections to thermostats, motorised valves and sometimes the pump to be made in a special connection box called a wiring centre.

 

[fixitflav]

When I request hot water for taps or radiators it should fire. I'm trying to work out how it knows to do this.

You say it should fire. Do you mean it doesn't fire, so you have a problem, or just want to find how the system works?

 

[asktoby]

your programmer and thermostats open either a 3 port motorised valve or 2x 2 port motorised valves depending on your sytem, these valves tell the boiler and pump when to come on

Thanks. How do they "tell" the boiler to come on though? Via what means?

 

[fixitflav]

Thanks. How do they "tell" the boiler to come on though? Via what means?

oldbuffer explained it pretty well. 230 volt is applied to the boiler live terminal, then it fires, if working properly.

 

[ianmcd]

Thanks. How do they "tell" the boiler to come on though? Via what means?

well what system do you have is it a 3 port (Y plan) or 2 x 2ports (S plan) post pics if you are not sure, both use micro switches and the valves have little motors in them , when the motor opens the valve it makes a micro switch, this switch sends power to the boiler to fire, is your boiler not working ?