ferolli 77ff boiler fault

[Jimmyjaffa]

Hi i was wondering if anyone can help me?

I have a 77ff combi boiler which has a fault,when the boiler is switched either to ch on or hw on,it does not ignite the main boiler,i have found that i have no voltage feed from the pcb to the main gas valve,i have tested the valve by putting the thermostat wires on it just to test it and the boiler ignites ok,i got hold of a secondhand pcb which was supposed to have been tested but when fitted it there seemed to be even more problems.i still had no feed to the gas valve ,no feed at the thermostat wiring at the gas valve,also the fan is running constantly,i have now fitted the original pcb back in,is there some sort or sensor that tells the pcb to give the voltage to the gas valve,if anyone has got any ideas it would be much appreciated,jim.

 

[ArtfulBodger]

Usually it's the APS which is next in sequence after the fan. You shouldn't really be doing this yourself though .........

 

[ollski]

Test continuity across the overheat stats, should be closed circuit.

 

[Jimmyjaffa]

Where abouts is this over heat stat,i know loads about cars but boilers is another thing but when i know where things are im very competent to have ago.

 

[ollski]

hw overheat stat is in the middle beind the pcb, ch I think on the left hand side....alternatively if you have the instructions you could link across the terminals on the board just to test of course, almost certain it will be one of them.

 

[Jimmyjaffa]

Hi, if they're the things which are held in by spring clips, they are closed circuit as the meter beeps for both. Any other ideas pls? jim.

 

[ollski]

which board number have you got in at the moment vfr something I think, and did you test the air pressure switch is switching?

 

[Jimmyjaffa]

Hi the board is a vmf 5 pcb.

 

[ollski]

ok with hw demand check the resistance across pin 66 on the board and a mains neutral, if you get 24v its ok, if not the problem will lie with your air pressure switch / filter / venturi

 

[Jimmyjaffa]

Hi i put the mutimetre across pin 66 and 67 and when i turned the switch to hw i got a reading of 24v . jim

 

[ollski]

Oh can see this being a long thread but now I've started..........

not sure what 67 is but never mind, push the gas control knob in with the hot tap on wait 30 seconds or so for the purge delay and test across 55 and 56 for mains voltage..if none test them thermostats again (not he thermistors above)

 

[ChrisR]

i have tested the valve by putting the thermostat wires on it just to test it

Eh? Can you explain?

I don't have a manual for one of these - is there a primary flow switch?

 

[Jimmyjaffa]

Hi im so great full for al ya help, can not tes across 55 56 as 56 is blanked off but tryed across 55 57 but only got 6v,oh bet you wish ya never posted ah,ha ha ha.jim

 

[ollski]

Hi im so great full for al ya help, can not tes across 55 56 as 56 is blanked off but tryed across 55 57 but only got 6v,oh bet you wish ya never posted ah,ha ha ha.jim

there will be a blank pin between 55 and 56 but you are testing the brown and blue wire. Get those wires off the thermostats and tell us the exact resistance you are getting across each one, plus check you have refitted all the multitude of plugs back on the board....and please don't say you have been checking resistance with the alarm setting . Not a problem to give you a hand, but be very surprised if it isn't a faulty stat.

 

[Jimmyjaffa]

Hi right the left one gives a reading of 1.0645 k ohms,the middle one gives a reading of 1.0326 k ohms.jim.