Vaillant sine18 boiler packed in again, grrr

[sbeavis]

We have a Vaillant Sine18 combi boiler. Been in the house 4 years and it's just gone wrong for the third time. No hot water or heating. No main burners.

The first problem 3 years ago was the pilot light not staying lit, which turned out to be due to the barbed spring in the flame failure unit not catching, easily fixed, but took a week to find. Then last year, it blew the fuse on the control PCB, which I replaced and it worked for another 9 months until Sunday morning when the fuse blew again. This time a new fuse blows instantly so I've looked at the electronics a bit closer. I removed the control PCB (without disturbing the gas side of things...I am not CORGI, but am a qualified Electronics Engineer). The rectifier was pretty burnt so I replaced that, but still the fuse blows. I was reliably informed by "an ex Vaillant engineer" that this is a common problem due to moisture on the solonoid and the fuse should be 2A (seems very high to me for mains current! That's approx 500W!). The fuse I put in last year was 1A, and now a 1.6A blows. The pump is not downstream of the fuse, only the solonoid driver circuit. The fuse blows even with the solonoid disconnected, so I have diagnosed a faulty trainsformer. Anyone agree? The primary winding resistance is healthy (a can't remember what it is but prob 50ohms or so), but the secondary winding is 0.3ohms which seems a bit low. The solonoid is 23ohms which I'm pretty sure is correct. Anyone know what voltage the solonoid runs at? I seem to remember 15V from my probings last year.

I've ordered a new control board, but I'm worried that there is more to this than just the board. Why did it blow in the first place? Does that control board low voltage drive anything apart from the solonoid? Does the control board only power up when hw/heating is demanded, or should it be powered all the time? The fuse blows when hot tap opened or heating clicked on at thermostat.

The house is so cold my wife has taken the baby and cat and moved to her mother's! I need to fix this asap! Would appreciate any comments if you know these boilers, thanks in advance!

Scott

 

[Gasguru]

Can't remember the wiring on these but have you checked the pump?...try wiring it from a seperate supply fused with a 2 or 3 Amp.

 

[sbeavis]

The pump is operating fine. The pump can operate in 3 modes: 1) "on when burners on", 2) "on when heating on", or 3) "always on". I operate mine in the second mode (keeps water circulating in the radiators after burners die down). There is a relay on the control board which switches mains to the pump but I'm pretty sure this is only used in Mode1. Either way it is taken before the fuse.

The fuse only covers mains to the transformer, so this can only mean that either the transformer has shorted its windings, or something on the secondary side is pulling too much current. This current would have to be VERY high though to pull 2A from the mains. The rectifier was pretty black so I diagnosed that it had shorted internally, but after replacing that it still blows the fuse.

One possible way to get some warmth into the radiators would be to carefully use a separate 12V DC feed on the solonoid to open the main gas valve. I'm not keen on this though, but it would be a way of getting the burners to ignite.

The pilot light is on, and everything else like the water deficiency valve and thermal reset are ok. I've also checked the NTC temperature sensor. I don't think this is anything to do with water section or diverter as it affects HW and CH. I'm confident that a new control PCB will get it working, but still a mystery of why it failed in the first place.

Scott

 

[Agile]

If its the control PCB then its only about 100 mA fuse ont he PCB.

The built in rectifier bridge is about all that would cause the mains fuse to blow and its rated at about 1.6 A

I suspect the replacement you fitted has blown but you can isolate the DC op and see what happens then.

Its unlikely to be the transformer but have you measured the impedance instead of the resistance to see if its got shorted turns???

Tony

 

[sbeavis]

The original fuse was marked "0,2A" which I took to be a German "0.2A". I replaced it with a 0.2 and it blew instantly, then a 0.5 blew, finally 1A held. I was then told by a guy on ebay who claimed to be a Vaillant engineer that it should be 2A and it is a common problem due to condensation on the solonoid pulling more current. It worked so I thought nothing of it. Now I think it has been slowly overheating the transformer and rectifier. Both look burnt. Impedance @ 50Hz is similar to DC resistance. The primary is 30 ohms or so (can't remember exactly) and secondary is 0.3ohms (which seems a bit low).

I am confident that a new board will fix it, but also worried that whatever it was which caused the failure may get the new board too in time. Am really hoping the solonoid is ok. I can't really see why it should be fried though. It looks ok, windings shiny copper, but does smell a bit cooked!

Scott

 

[Agile]

I dont know what kind of electronics engineer you are but none I know ever fit increased fuse ratings!

If the impedance is the same at 50 Hz then the output is short circuit or the TXFMR has shorted turns.

The original rectifier seems to be a selenium or metal oxide and will run with a failed section and overheat severly with toxic fumes!

You dont say what you replaced it with but I suspect it was not suitable!

Did you check the 2N3055?

Tony

 

[sbeavis]

I was unsure if "0,2" was 0.2 or 2, and was told by a Vaillant Engineer that it was 2. I was not happy doing it, but I trusted him and it made it work so I worried not. The replacement rectifier was a 2A bridge device and should be fine. I agree with you that the transformer has shorted it's windings internally. I now have the new board and will fit it tonight and report back.

 

[sbeavis]

Received new control board yesterday, fitted it last night and all works perfectly again!

For the record the new transformer primary was 103ohms, old one 30. There's the problem. Melted primary winding enamel. Why it melted after working fine for 20 years I shall never know.

At least the house is warm and my wife/baby/cat have returned home.

 

[Nixt]

Why it melted after working fine for 20 years I shall never know.

Probably the same reason why people sometimes drop dead after living for a hundred years.

 

[heatingman]

Like the answer Nixt,
nothing wrong just plane and simple worn out and knackerd.

 

[Agile]

The built in rectifier bridge is about all that would cause the mains fuse to blow and the bridge is rated at about 1.6 A

Of course if you prevent the mains input fuse from blowing then the transformer will overheat and burn off the winding insulation.

Tony