Boiler Conundrum

[securespark]

Parents in law rang the other day with a query.

Their boiler had gone off overnight, and following my previous advice,they discovered the boiler MCB had tripped, so they reset it.

There was still no power, so I talked to them on the phone and said I could point them towards changing the 3A fuse in the FCU, but I said that if it had blown, it had blown for a reason and would probably go again.

So they called the maintenance company and they came the next day and changed the fuse, proclaimed it fit and went away.
He didn't change any parts.

IDK what tests they did, but I guess they must have done some.

Can anybody who knows more about boilers than me shed any light on why the fuse might have gone and could it go again?

It's a 20 month old Worcester combi.

Cheers.

 

[Harry Bloomfield]

IDK what tests they did, but I guess they must have done some.

Can anybody who knows more about boilers than me shed any light on why the fuse might have gone and could it go again?

Probably no tests, just replaced the fuse, and it worked.

It could have been a faulty fuse, not unknown, but I would be smashing the fuse open, to investigate whether it was, or had actually blown.

 

[Jurassicspark]

A blown fuse, not caused by any fault can happen but it's not very common.

As you say the MCB tripped, it could have simply been a loose wire and one of the wires touched another, causing the fuse to blow.

 

[JohnW2]

It could have been a faulty fuse, not unknown, but I would be smashing the fuse open, to investigate whether it was, or had actually blown.

A blown fuse, not caused by any fault can happen but it's not very common.

Whilst those statements are very true 'in general', in this case an MCB also operated so one has to assume that there really was a high 'fault current' - so, as Jurassicspark wrote (and given that the boiler is apparently now working OK) .it seems most likely that ...

As you say the MCB tripped, it could have simply been a loose wire and one of the wires touched another, causing the fuse to blow.

However, if, as it seems, nothing has yet been done to rectify any such 'intermittent short', there is presumably a fair chance that the problem will recur?

 

[CountryFan]

Maybe better in the plumbing section?

I know the circulation pumps can take out a 3A fuse, but normally does it again straight away. Obviously if a part was replaced and mains live wires touched can do it too. Leaks don't help either.

 

[securespark]

Whilst those statements are very true 'in general', in this case an MCB also operated so one has to assume that there really was a high 'fault current' - so, as Jurassicspark wrote (and given that the boiler is apparently now working OK) .it seems most likely that ...


However, if, as it seems, nothing has yet been done to rectify any such 'intermittent short', there is presumably a fair chance that the problem will recur?

Which is what I said to the PiLs... The fuse blew for a reason.

Without investigation and rectification, surely there is a risk it will happen again?

The fuse has been disposed of by the maintenance guy.

 

[JohnW2]

Which is what I said to the PiLs... The fuse blew for a reason.

Indeed, and the fact that the MCB operated reinforces that conclusion.

Without investigation and rectification, surely there is a risk it will happen again?

Quite - that's what I wrote.

What was the rating of the MCB which tripped?

 

[securespark]

Ah, didn't ask.
I'll find out.

 

[Harry Bloomfield]

As you say the MCB tripped, it could have simply been a loose wire and one of the wires touched another, causing the fuse to blow.

I missed, that the MCB had also tripped. So that suggests there was an actual fault.

 

[JohnW2]

Ah, didn't ask. ... I'll find out.

Thanks. It would obviously be at least 6A, but presumably much more likely 16A or 20A, maybe even 32A - so I don't think that it is at all likely to be due to a 3A fuse blowing because of the normal load (as was suggested may sometimes happen {presumably at 'start-up'} with a CH pump).

 

[thomp1983]

Is the mcb that was tripped definitely the one for the boiler or is it maybe one that's off as that circuits no longer in use and the 3a fuse was just blown?

 

[securespark]

I wasn't there, but it was labelled boiler, and the bungalow was rewired when they moved in, so I think it will be a dedicated circuit.

My guess would be a 6 or 16A type B.

 

[JohnW2]

My guess would be a 6 or 16A type B.

Although 6A would clearly be adequate for a dedicated boiler circuit, I have to say that (although my experience is limited) I don't recall having seen that - 16A (or sometimes 20A) seems much more common. Is your experience different?

 

[ebee]

Yes I think that either 16A (or 15A) MCB but possibly 20A would be the most common or 15A if it was a rewireable fuse (but you could get 20A in rewireable fuses back in the day - not unknown to put more than one fusewire in rewireable fuses by some folk either) and I can not say I have never seen a strand of flex or a copper wire in the place of a fusewire either.

Some folk have a strange fixation with fusewires too - the 5A has blown so that put a 15A in then blows so they put a 30A in, once they get to they point where it does not blow "They Have Fixed It!" What a mindset !!!

 

[Harry Bloomfield]

Although 6A would clearly be adequate for a dedicated boiler circuit, I have to say that (although my experience is limited) I don't recall having seen that - 16A (or sometimes 20A) seems much more common. Is your experience different?

Mine is on a dedicated 6amp MCB, since installation in the mid-80's. I replaced it with a new CU a while ago, plus a 6amp MCB. It has never ever tripped.