Glowworm Boilers blows fuses

[Paulie]

My ex has a Glowworm Fuelsaver F that I fitted a new fan to last year. (another story) Some months later, it started blowing the fuse on the PCB and every time I tested it, everything was ok. No shorts & no earths. The last fuse lasted about 4 months until today when the RCD tripped just after the boiler was switched on. The fuse had blown again. I'd left my meter at another job (I'm an alarm engineer) so couldn't test the boiler apart from replacing the fuse so I switched on and noted when it tripped the RCD. Fan starts ok, pressure switch operates, then when the ignition starts to click the RCD trips. Is this the gas valve that's causing the RCD/fuse to trip & if so, can I replace the electrical part without touching the gas pipes? Your help would be appreciated.

Paul.

 

[ollski]

It is usually only the fan and the pump that have enough juice to blow the fuse.

 

[Paulie]

Thanks for coming back so quickly. I'm sure it's not the pump because the pump starts runs ok when the controls (timer & stat) are turned on. Like I said, the fan starts ok and runs for a few seconds before the ignition stage. One click, then it trips. That's what makes me think it's the gas valve. If I disconnect the wires to the valve & switch on I can see if the RCD trips. If it doesn't then it's a new valve. Not too keen on touching the gas pipes, though.

 

[Paulie]

Had another go at it this morning and it ran ok for a couple of minutes then the RCD tripped. Disconnected the fan & it's ok in as much as the pump runs. Recon fan and pop! RCD goes again. Took fan & sequence board to supplier & tested ok. Put fan back on and it ran for about 25 minutes. RCD went again. Dis fan - all ok. Recon fan - RCD goes. Removed the fan & wired from a plug top - all ok. Replace flex from sequence board to fan - ok for 2 minutes then RCD goes. Ran the fan in place via a socket - RCD goes. By now I'm not making sense of it so I put everything back & give up. The boiler's now been running for an hour & I'm non the wiser. I'm getting a sparky mate to look at the electrics.

A plumber told me: 'You need a new boiler, mate'. I can see where he was coming from.

 

[Paulie]

I think I've sorted it.

A mate told me to try an isolating transformer on the mains supply to the boiler. This had some strange effects. The boiler would light but the ignition kept going after the burner lit and the pump only ran when the control switch was OFF. Disconnecting the tranny it went back to it's usual tricks so I decided to try replacing the pilot/flame sense jobby as the RCD only tripped when the HT spark was firing. This seems to have done the trick and it's been running fine for over a week now. I can only assume that the tranny wouldn't work because the boiler needs a live and neutral to work properly. The ignition spark possibly had a bit of a leak to earth somewhere, which tripped the RCD.

 

[DP]

With isolating transformer, boiler metalwork will be free floating in relation to operating voltages.

When the boiler is ready to ignite burner, ht is offered at the spark electrode and gas at the burner for spark to ignite. Once the gas has flamed, ht travels, via the flame to earth. At this point a circuit see this happening and disconnects the spark and ignition moves to next stage i.e. full flame. Isolated voltages is 240 volts hogher than 0 volts on the transoformer. Mains voltage is 240 above N as well as earth terminal.

 

[ChrisR]

Have heard of, but admittedly not witnessed, some darned odd effects with trips because the current to earth is (I wos told) something like 10mA. That isn't enough to trip the trip (usually 30 mA) but if something else is contributing it LOOKS like the ignition is at fault.

By pilot/flame sense jobby (!) I guess you mean the electrode?


Otherwise, it's pump, fan, gas valve, ....

 

[Paulie]

DP, thanks for the explanation. Makes a bit more sense now, although I don't get the part where the flame acts as a path to earth - I thought the pilot burner was that path.

You may be right, Chris, about the electrode CONTRIBUTING to the earth current if something else has a bit of a leak but not enough to trip the RCD.

Hopefully, my sparky mate will test the electrics for me soon.

Thanks for your interest,

Paul

 

[Paulie]

Put a Megger on the fan when it was bolted in & it had a bit of a leak to earth. When I tested it on 1000v I could hear it tracking. Bizarrely, it was ok when it was tested by the suppliers afew months back. Got a new one off them FOC and it's running fine now.

Fingers crossed.