Vaillant EcoMax 236 non-ignition F21

[marchesexile]

Can anyone help with this one ? Vaillant EcoMax 236 model, 8 yrs old, at that time new installation in new house. Troublesome beast from beginning but that history. Serviced by authorised Vaillant engineer late summer 2004. Since early December has non-ignition fault - no fire up with F21 fault displayed. 3 weeks ago had new gas valve fitted by same engineer - OK for a week or so, then problems same as before. Attempt to start via reset switch - approx 5% success rate using this method. Problem seems to be failure to start from dead cold. When boiler warmed, will continue to fire on demand from CH or hot water system. Problem now is total non-ignition - cannot get it to fire at all. F21 every time = very annoying. Service engineer suggests boiler might not be economically repairable which doesn't sit well with me. Personally I think it might be a solvable problem if correctly diagnosed. A control system fault here ? Sensor fault maybe ? ECU problem ? Cold and desperate here now - any help or advice much appreciated !

 

[ChrisR]

Haven't got the manual, what does the code indicate? "Ignition fault" I suppose! What's it doing? Sparking but not lighting? If so it's annoying that an average engineer can't find the problem, unless it won't show the fault when he's there.
For a flame there has to be gas - well that isn't hard to find out! For gas there has to be a signal on the gas valve - ditto, etcetera. Such an intermittent fault is often a faulty wire connection leading to a poor spark or lack of volts where they should be, or may be a bad joint or crack on the pcb.
One parts list for it shows 5 pcb's - that's a lot of connections.

It might be tempting to start changing pcb's but that's an expensive game if you're guessing.

 

[marchesexile]

Chris,

Thanks for responding! That's right - no spark therefore no ignition. Fault code F21 refers to general "non-ignition", but isn't too clear as to what might cause it. The manual advises to check gas and water pressures. I first experienced the F21 problem soon after I moved in here - our gas supply is LPG from a communal and huge tank buried at the perimeter of this housing development. In the first couple of years the tank would run out before the supplier could refill it (difficult to monitor demand back then..) Zero gas pressure results in "F21", so I've seen this on the fault indicator panel many times previously. However, this time the gas is present and pressure is good. We have a gas hob, and the pressure seems perfectly normal. I believe the LPG supply regulator is also OK.

Non-ignition can be the effect of a few factors, including no gas pressure, water pressure - all the factors which have to be monitored to satisfy the control unit before it will allow start up. Gas pressure is OK, so is the water pressure. And you're right, true to form, it hasn't yet reproduced the fault in the presence of the engineer. He visited yesterday and has fitted a "monitor" to the flame sensor. But I would ask why, when the fault doesn't permit a flame to establish! But, I'm not a gas engineer.

I'm sure its an electrical fault - and yes, likely to be a PCB or the control unit, I think. Between just prior to Christmas (when the gas valve was replaced) and today, its ignition has been erratic, sometimes needing much patience pressing the reset button (wait 10 secs, etc...try again) whilst providing demand (turn up CH thermostat or turn on hot water tap). But now it just refuses to ignite, and I have given up havign tried nearly all day to get the b~@+*r to light.

Two things are definately true here - (1) it always occurs when a dead cold start is required, and (2) the fault seems more frequent when the weather is colder (seems barmy statement but the failure to start is definately more frequent when the ambient is lower - when the weather is slightly warmer (i.e last week it fired automatically from cold every time, unattended).

I have made located another Vaillant engineer locally and will hopefully speak with him tomorrow - maybe he has met this problem before.

Basically - I agree with your thinking - likely to be one of the points you suggest. I guess it needs to be tested wherever electrical tests can be made, and where not, by gradual substitution of PCBs.

Problem solving is one thing, and somehow I know there must be a practical solution to this. But to have it suggested that replacement of the whole boiler might prove necessary because of some contact/wire has failed......well, I have a major problem with that!

I appreciate your advice Chris. If anything else pops into your mind, please let me know.

Chris Curtis
Llanymynech,
Powys

 

[marchesexile]

Confidently diagnosed as defective IGNITION SEQUENCE PCB. When installed we will know the answers. I hate this contraption.

 

[ChrisR]

I hope someone has checked/changed the HT sparky lead. Breakdown of insulation could be temp/humidity dependent. Haven't mentioned that the size of the pilot (ie first flame to ignite) can also be critical - I assumed it would have been checked before the gas valve was changed.