Worcester 28i Pcb, relay 1 burn out

[acgkplh]

Hi To everyone!
I'm going to cut this problem very short as I see from this forum loads of people have issues with the pcb on the 28i ... as I do. I have however notived it's always relay 1 that burns out on the 2 pcb's I've fitted. My question is what does replay 1 control please?

I fitted a new pcb last night and had hw and ch all evening (timer on combi set to conastant) Went to bed, set timer and put on auto. Woke up to noise from my combi (fan running) but no flame showing through spy hole. Powered of at mains, set to constant, applied power from mains .... fan comes on, click from combi, ignitors firing but no flame. I took out the pcb and compared to the old pcb .... both relay 1's burnt out .......

I'm thinking a component in the combi is being activated by the relay but for whatever reason the relay is not disengaging .... ?!?!?!?! I am assuming it is not the pcb or relay at fault but a component further "up the chain" causing the relay to fail/burn out because the component is continuing to draw voltage???

I'm not a central heating engineer but the ones I have called seem stumped ..... I've given this a couple of hours logical thought and also can clearly see relay 1 burnt out. Seems simple to me .. so whats failing?

Many Thanks
Phil.

 

[Agile]

You have not been very clear from your posting exactly what you have done and when.

It sounds as if you fitted a new PCB and the relay burnt out a few hours later ???

"Burnt out" is not a very good technical description. I can only guess that you mean the contacts are burnt ??? Not the solenoid gone o/c ?

Its easy enough to trace what that relay powers. I expect its the pump.

Pumps are inductive and when depowered cause sparking at relay contacts. In a good design a snubber would be used to prevent this but in a cheap boiler they save components and leave it out. The 1/2 amp taken by the pump allows a 5A or 8A relay contact to work for a few years in spite of not having a snubber and if they fail after a few years then thats a spare part sale for them.

So what else do you want to know? But I will need significant further detail to give a more specific answer.

Tony

 

[acgkplh]

I cant tell you how much I appreciate your reply!

Ok, series of events ..

1: system powered off for abt a year due to me not living in the house. powered on, no hw. turned screw on water pump, bingo. ch and hw even though fan was a bit sqeaky

2: few days later no ch or hw. Called engineer who replaced fan (I could hear it squeaking away for a month or two prior to failure). Then had HW but no ch. I was informed it was more than likely be a pcb issue as "sometimes a failed component can "burn out a board" .. his explanation, not mine). I couldnt afford to have a new pcb so agreed with engineer I would replace later in the month due to cost.

3: Despite leaving lots of voice mails for engineer I had no call back. I decided to take a punt and replace the pcb myself. BINGO! ch and hw so I left the 28i on constant for about 6 hours. Set the timer, went to bed and flicked it to auto.

4: Wake up to hear the normal noise from the 28i, looked through the chamber window to see no flame. rAN A TAP to find I had no hw. Also no CH.

5: When I flick 28i to constant or on timer you can hear the fan start, another couple of clicks and then the ignitors fire .... but no ignition.

6: Took out pcb and compared to the old pcb ..... both number 1 relay on both boards are blackend.

7: I'm assuming relay 1 controls the fan because if you follow the tracks on the back of the board they lead from the relay to the fan connector?

8: Called bosch/wors today but they did not have a scematic to hand but did indicate relay 1 controls the fan.

So, why would a fan blacken the relay 1? Relay 2 shows no signs of blackening on both pcb's. Is the fan the root cause and why?

Could the timer unit attached to the 281 be the culprit? The 28i was fine all evening on constant but when left on auto overnight the problem semed to materialise?

Any help would be very much appreciated.

Ta
Phil.

 

[Agile]

You say that you have had a new fan fitted very recently. Then you say the fan relay has burnt and blackened contacts all about four weeks after the fan was replaced.

The simple situation is that I cannot guess at whats wrong in your case without some measurements.

Are you confident that the new fan really was new?

A fan only takes about 0.18 Amps and is also inductive but they usually work fine on a typical 5A or 8A relay contact for the minimum 10 year boiler life.

I dont think that I can even remember a fan relay with burnt out contacts when the fan was operating correctly.

Due to the high impedance of a fan even if its stalled, the current only rises very slightly. They do get hot though but thats because they are no longer getting cooled.

It seems your mostly DIY repairs have now got you back to square one.

Was the PCB you fitted really new?

Sorry for all the questions but experience shows that we are often not told the whole story.

Tony

 

[basingwerk]

The white, schreck relays on my 28i boiler "burns out" every three years or so.

I don't know which one is the culprit, so I change them both.

 

[mattylad]

I too had this problem on an older WB combi, the fan control relay would burn out within 12 months or so.
After having several attempts at it's repair by RGI's I had to fix it myself, all it needed was a new relay of which I had plenty.
I also fitted a snubber circuit across the contacts and never had any more problems with the relays.

As has been stated above, it's something the manufacturers often try to save money on by leaving out, there may even be positions on he PCB for the components (ISTR usually a 0.1uf XY capacitor and a 100ohm 2w resistor should do.).

The RGI's were satisfied with my repair of the PCB so refitted it.

 

[ianmcd]

 

[mattylad]

Something funny?