Vaillant combicompact stays on for one second

Vaillant combicompact stays on for one second. The precise model is VCW GB 242. The problem is the same in heating and hotwater moded. The fan starts, the air pressure switch moves, the pump starts, there is 20 volt on the valve. The fan creates 2mbar underpressure in the combustion chamber, which can be measured at the burner pressure test point. About three seconds after the sparking starts there is a one second change on the test nipple from 2mbar negative to about 3 or 4 mbar positive. The burner comes on for that one second and dies out. 10 seconds later the boiler locks out.
I have taken the differential valve off and the pin goes in and out smoothly under thumb pressure. I have checked the pressure switch, the tube to the ring and the ring in the flue itself and all is clean. The diverter-valve moves with hot water demand and operates the micro-switches.
Is it the valve or one of the pcb’s?

CHeck APS pressure as it ignites. Needs a T.

Thanks for the tip chris. Must admit that APS does not ring any bells but then again I was a bit puzzled the other day when my supplier called to inform me my aav had arrived. The only abbreviations I use regularly are pcb and trv; other than that I just call thing by their name. Not that it makes any difference; they never jump out of their box and walk over to me no matter what I call them.
By the way, how much should the aps be?

air pressure switch

It dawned on me as I pressed the enter key. I did not measure the pressure on the switch but I did get a change on i and j on the board. It went nicely form 240 to 0. Or was it the other way around? Don’t remember. Whichever way around it was, am I wrong to conclude from finding a voltage on the valve and seeing the sparking start that the switch must be operated? I always thought that the sequence would not move to sparking unless the switch was operated with all boilers?

10 second timer is running with lockout at the end as no flame has appeared to disconnect the spark (flame rectification)

Negative pressure is due to negative pressure in the combustion chamber and positive as the gas valve opens to indicate gas flow.

I would be looking for continuity of spark, presence of gas at the burners and air pump running and providing air for the gas valve.

What lights do you see.

Not sure what you mean by looking for continuity of sparking. The sparking continues until lockout occurs. The 1.5 mbar gas-pressure starts after about 2 secs of sparking and lasts for one second, hence the dying flame. I was told that the pump, presumably you mean the diaphragm pump, only effects the modulation. I have removed both pipes to the pump one at a time and blocked them off. No change. Also tried gently blowing into them with same result. As I keep the 20 volts at the valve, I have the feeling that is where the problem is. At a price tag of over 200 quid, I would rather be sure before forking that out.

When the burner lights does the ignition carry on if so it might be flame sense, pcb( ignition) or flame sense lead may be u/s have had the flame sense lead break down were it is clamped behind the ali box that holds the pcb,s, if you look closely you can see black tracking.

before it locks out when alight it sould go from amber to green +amber light,showing flame sense

the ignition gas can be adjusted on the gas valve i think its about 4mb, left hand screw(with red paint) have had these go out of calibration

have had one other fault similar to that gas meter valve only just turned on, (out side meter )and while servicing working boiler kids turned meter nearly to the off position, good fun finding that one

What I was alooodin to was the possibility that the APS might be affected by the woomph when the main burners come on. Sometimes causes momentary drop-out which CAN cause lock out but I don't know if it would on this boiler.

While you may be able to depress the pin under the servo valve on the gas valve, by hand, have you actually eyeballed the pin going in by itself? If the pin is not pushed in and stays in (function of positive and negative pressure created by the pump), you can put 240 on the gas valve and still no gas will appear at the burners. 20 volts on the coil only sends the gas to the next stage. Pump pressure then opens the gate for gas to appear at the burners. Air pump malfunction will result in low flame only.

Are you a heating engineer or an end user? If heating engineer I will be able to provide you with comprehensive instructions. If end user suggest you get a heating engineer in as you are dabbling in area that is tricky to fault.

Thanks for all the replies guys. In order of appearance: the flame appears for such a short time that I can not really see the light sequence but it is definitely not a sensing problem as the flame stops way before the sparking stops.
It is fairly unlikely that the pressure switch is effected by the whoomp of the oncoming burner as the voltage on I and j on the pcb does not change after the sparking has started. Apart from that, the ignition pressure is only 1.5 so very small flame to start with.
To answer DP: I have not seen the pin go in by itself but have removed the diff. and manually depressed the pin myself whilst starting the boiler. Needless to say I got the same one second flame. I am a heating engineer and to make things worse, I specialise in boilers. It really mifs me off that I can’t get this one to work. First time in at least two years that a boiler seems to be winning. Grrrrrr. Comprehensive instructions would be very welcome. I have called just about anyone I could think of including Vaillant technical and the bugger is still refusing to work properly. I have the original service and installation manual as well and that has not helped me a lot either. The last tip I got tonight was to remove the operator and check the resistance of the coil. Apparently that should be between 200 and 240 ohms.
Any other suggestions, tips, advice no matter how unlikely are very welcome as this has now become a matter of principle.

Email me at [email protected]. REMOVE 999 for email to work


MOD

dp put your e mail in your profile please

I was wrong. You were all wrong. Vaillant technical were wrong. So was the ************* ********** ******* ************** * ************* * ****** ********* ************* ecv!! Enough gas for the hob but not for the boiler.