Vaillant TurboMax Plus 837 intermittent flue fan fault

[daveb0001]

My Vaillant TurboMax Plus 837 combi boiler has developed an intermittent fault whereby the flue fan fails to start, so the boiler's controller eventually ends up in the S33 "Check air/flue system" state. Unlike other posts on this subject whereby the fault was with the air flue switch, the problem here is that the flue fan does not start. Having said that, the boiler has so far started working within 12 hours of originally finding that it had failed to work.

I've had the fan unit out and determined that the motor spins freely. I've also remade all of the connectors (2 on the fan, 2 on the controller, and 1 intermediate one). I've measured the 230V AC power fan being switched on by the controller. And I can see via register D33 that a fan speed of 210 is being requested, with an actual fan speed of zero being displayed in register D34.

My current conclusion is that there is bad connection somewhere and that this is sensitive to either temperature, humidity, or both.

Any suggestions on how to narrow this down? I've found that a replacement fan assembly is £175, but as the fault could be in on the main board, or in the cable loom, it would be good to be able to absolutely sure that the fault is with the fan's electronics. Having a better idea of how the main controller board controls the fan might help.

Many thanks in advance for your help,
Dave B.

 

[mickyg]

Your talking about buying a £175 fan, not being totally sure its the problem. For another £75 you could get vaillant out for a guaranteed fix.

 

[daveb0001]

I have considered this. They have quoted £275 inc VAT for parts included one time repair (except for the heat exchanger and expansion vessel) or £312 for the same but covering a 12 month period.

My concern is that with it being an intermittent fault that when the Vaillant service engineer turns up it works perfectly. (I've had this with cars at the garage!) Equally, if all he does is fiddle with the wiring loom I'd be a bit choked on a bill for ~£300. On the other had winter is coming!

Best Regards
Dave B.

 

[mickyg]

I'd be very surprised if it were a wiring fault or loose connection. Also just because it doesn't happen when he's there doesn't mean you'll pay for nothing. Having been in the engineers position working for a manufacturer, if I came to a job where you gave a reasonably good explanation of the fault like you have but it didn't perform and all checks revealed nothing, I'd probably chuck a new fan, aps and pcb on it and expect not to get a recall!
I don't wanna sound like a killjoy, but playing about with the fan is really a job for an RGI as its safetly critical.

 

[Agile]

A British Gas fixed price repair is cheaper.

But make sure you indicate that the fault is intermittent.

Most of those fans have a high power supply and the fan then received a low power commutating signal from the boiler it then feeds back a signal from a Hall effect detector to indicate the actual speed.

Statistically if its intermittent it may well be a poor solder joint on the high power supply part of the fan PCB.

Some rectify the supply to about 300v DC so be prepared for a shock if you dont turn off the power ( and perhaps discharge any reservoir caps. )

Tony

 

[daveb0001]

Ok, thanks for the advice so far. Just one thing more though; do I wait for it to do it again - may be then as a hard fault -, or call out a service engineer now and hope that (as suggested) they'll replace all the electronics speculatively?

Thanks
Dave B.

 

[Agile]

Few people you would ever call out would understand the electronics!

Its probably the fan at fault but it could possibly be the main PCB as well or even both.

The danger for you is that you may get the agressive unhelpful engineer who just says its working so there is nothing he can do.

Tony