I recently went through the fault analysis in the manual as i was
having problems with my combi and through all the tests i did came to
the conclusion that the pcb was Knackered so i went out and bought a
new one and installed it.It still wouldn't fire up and so i checked
various other things which seemed to be wrong and sorted them
out,evereything seems to be fine now apart from the gas valve will not
open. The fan fires up and is working fine, the pump is working
fine,the pilot is working lovely,the room thermostat is fine, but the
gas valve just will not open. The modulater is getting the correct
voltage and seems to be working fine, the gas valve though does not
seem to have any power going to it. If i take 24v ac and connect it to
the gas valve it does work, but this is not right as i shouldn't need
to re-wire on a new board, also when the heating goes of with this
arrangement it doesn't cut off completely and the gas still comes
through although the flames are really tiny, and the pump and fan are
still working fine. I wanted to know if anyone out there knows if the
fault lies with my new pcb or if i need to check something else as
something is preventing the relay on the board from swithching the 24v
ac onto the gas valve. Please help as i am desperate.

In message . com,
frank1 writes
I recently went through the fault analysis in the manual as i was
having problems with my combi and through all the tests i did came to
the conclusion that the pcb was Knackered so i went out and bought a
new one and installed it.It still wouldn't fire up and so i checked
various other things which seemed to be wrong and sorted them
out,evereything seems to be fine now apart from the gas valve will not
open. The fan fires up and is working fine, the pump is working
fine,the pilot is working lovely,the room thermostat is fine, but the
gas valve just will not open. The modulater is getting the correct
voltage and seems to be working fine, the gas valve though does not
seem to have any power going to it. If i take 24v ac and connect it to
the gas valve it does work, but this is not right as i shouldn't need
to re-wire on a new board, also when the heating goes of with this
arrangement it doesn't cut off completely and the gas still comes
through although the flames are really tiny, and the pump and fan are
still working fine. I wanted to know if anyone out there knows if the
fault lies with my new pcb or if i need to check something else as
something is preventing the relay on the board from swithching the 24v
ac onto the gas valve. Please help as i am desperate.

Do you have a broken wire in the loom between pcb and gas valve ?
Do you have 24 volts coming off the pcb ?

--
geoff

geoff
many thanks for replying to my plea.
the wire from pcb to gas valve is OK.
The problem i believe is that there isn't 24 volts coming from the pcb,
I would think that a relay on the board would switch 24 volts to the
wire if everything is OK, but something must not be OK or the new PCB
was duff. Maybe a thermistor isn't working properly, but how do check
it.
many thanks

Frank1