Vaillant VUW 242 E that suffers from intermittent ignition

[vincenthes]

Hi all

I have a Vaillant VUW 242 E that suffers from intermittent ignition.

I have checked the following and unfortunately bought and fitted a new control pcb with no success. If anyone can help then I would be very grateful.

I have checked the operation of the air flow switch. Also measured the volt drop at the circuit board when the boiler failed to light.

Cleaned the fan and the flue, also the air flow switch tubes back to the switch.

Checked gap and cleaned the ignition electrode. Ckecked the loom abck to the board.

The system fails to light and the following occurs.
The fan will run and the LED lights on the panel.

The valve LED lights up suggesting the valve opens, air flow sensor is triggered and the gas is flowing. But I do not get the sparking from the ignition.

But what I can tell from when the boiler works (fires up) is that the sparking does not start until what I believe is the boiler knows that gas is flowing to the burner. I now think that the sparking will not start until the boiler knows that the gas valve is open. Does the gas valve position feedback or is there some other feedback to the control showing gas flow that lets the control start the spark ignition??

 

[Agile]

We are not permitted to give detailed advice on gas work on an open forum.

Most of what you have done, apart from replacing a perfectly good PCB, is not so wrong.

However, you totally mis understand how the gas valve/sparking works.

The boiler starts the fan and the APS then operates and the sparking starts and only THEN is the gas valve opened.

Since the APS is outside the combustion chamber, I can just about ask you to check the "ON" resistance by gently blowing TOWARDS the positive air connection.

If the resistance is much above one ohm the boiler may not proceed with the start up sequence.

Tony

 

[vincenthes]

Thanks Tony

Yes i understand the/ and your position regarding the gas regs. As soon as have As soon as I have eliminated all possibilities and found this to be a gas problem I will call a registered engineer.

APS does this mean air pressure sensor/switch??? If so then I have measured the volt drop during a failed start of the system at the board. So for me, this means that the correct measurement at the switch when the system failed shows the switch not to be the problem.

The bit I have difficulty in understanding is the sequence for starting.
This is how I imagine it works.

If the control calls for heat/or DHW then,
The fan runs.
The APS is triggered (the fan LED lights on the panel)
The valve opens and lets gas to the burner for initial lighting. (the valve LED lights)
The board energises the sparking transformer and the sparks start at the burner.
The gas ignites
Heating the flame sensor (the flame indicator LED lights)

But what seems to happen is that

The fan runs.
The APS is triggered (the fan LED lights on the panel)
The valve opens and lets gas to the burner for initial lighting. (the valve LED lights)
(the missing bit) Some feedback to the control to say gas is flowing to the burner.
The board energises the sparking transformer and the sparks start at the burner.
The gas egnites
Heating the flame sensor (the flame indicator LED lights)


If the above is correct then gas valve or regulator could be the problem but until I understand the correct flow of events that start the boiler I cannot decide if the gas valve or regulator is the problem.

Any help with this would be gratefully received. And thanks again Tony for your input so far.

Regards, Vincent

 

[Agile]

I appreciate that you have quite a good understanding of electronics but on an open forum I cannot give you any advice that will encourage you do do any work on the gas train.

They all say they will call an RGI "when they have found what the fault is" !

What I will say is that there is no feedback to show what gas is doing. This is only a boiler not a power station gas turbine!

I would still like to know the resistance of the APS "on" contacts !

Tony

 

[vincenthes]

Ok, no gas discussion

With regard to the APS there is no resistenc value, it goes from O/C to C/C with the most gentle air blow down the P1 port.

From the sound the mechanism makes it seems quite a positive latch when you blow.

Thanks again for your input

Other than the APS and the call for heat (and not asking a gas question) what other inputs are there to the control for the process to complete??

 

[Agile]

Ok, no gas discussion

With regard to the APS there is no resistenc value, it goes from O/C to C/C with the most gentle air blow down the P1 port.

Is it a superconductor maintained at -263° ?

Can you measure the restance and tell me?

Tony

 

[vincenthes]

just to give some more background this problem first occured after i returned from holiday. Another thing was that the system was full of air (I would say the highest rad was completely full of air) However when the system lights the heating and hotwater run perfectly and the system pressure is maintained increasing to just below 2bar when fully hot

 

[vincenthes]

ok, the resistence is between 1.1 ohms and 2.8 when i blow very gently

 

[Agile]

In that case the implication is that its not the APS although sometimes they can be very intermittent with their "on" contact resistance. In a known intermittent situation I would repeat that test about 20 times!

I would not rule out the APS yet though.

The pressure is not monitored by the boiler. The hot pressure means nothing to me unless you advise the start ( cold ) pressure!

Tony

 

[vincenthes]

The cold pressure is by the book at 1.5bar

What aboyt the differential valve switch?? I assume this is the feedback to tell the system that the pump is working and that the water is flowing through the circuit. Is this the case??

 

[Agile]

Thats correct although in my experience on that model it very rarely gives any trouble ( apart from leaking sometimes ).

Tony