Boiler keeps blowing fuse

[stulew]

Can somebody please offer any advice on a problem with a boiler. The boiler in question is a conventional one that forms part of a gravity fed system, and has a 3 way motorised valve (which was replaced a few months ago). The boiler is a Concord WCF255A. The main fuse keeps blowing on my main fuse board, this happens at anytime between 2 minutes and 20 minutes.Any help would be greatly appreciated.
Thanks In Advance.

 

[oilman]

So how big is the main fuse?

It could one of a few things, and the only way to find out what is to methodically go through the circuit until you find it. With these symptoms, it may be a motor related problem.

 

[stulew]

The main fuse is a 3 Amp... What is the best and most methodical way to eliminate the parts of the boiler that require a feed from the mains. I have put different demands, H/W and C/H on it and it stilll blows. I take it I should test the fan etc with a meter but what is the right sequence etc.
Any Help would be greatly appreciated.
Many Thanks

 

[sterose]

The main fuse is a 3 Amp... What is the best and most methodical way to eliminate the parts of the boiler that require a feed from the mains. I have put different demands, H/W and C/H on it and it stilll blows. I take it I should test the fan etc with a meter but what is the right sequence etc.
Any Help would be greatly appreciated.
Many Thanks

When you say 'conventional boiler', I am assuming you mean a old bog standard boiler, without the all-singing-all-dancing features of modern combi- or system-boilers.

Anyway, in old boilers, the only parts that make demands on the electric, are the pump, the 3way valve, and the gas valve.

(I would be curious to know why you have a three-way valve in a gravity fed system)

You can see that these parts are used in any 'demand' situation, although if it is gravity fed, when using H/W you can eliminate the pump from the equation.

So basically you are looking at either the 3way valve, or the gas valve.

We have eliminated the 3way valve, because its new. (Unless your problems started when it was replaced, duh!)

So it might be a dodgy gas valve.

And 3amp fuse sounds a bit low, are you sure it doesnt just need 5amp.

I had a 'conventional' boiler at one point, it wasnt too big, but it drew 5amps (i.e. it had a 5amp fuse, not it actually drew 5amps).

 

[marios_kin]

The main fuse keeps blowing on my main fuse board,

Do you mean your spur fuse or your main fuse box, ie an RCD unit.

The way to trace it should be to isolate each component in turn. A new component does not guarantee its reliability.

It sounds like a y-plan system if you have a 3 port valve this can be disconnected and then manually put to a position still allowng flow to both heating and water.

 

[oilman]

stulew, if it's a 3A fuse, and it blows in 20 minutes, it will be passing much more than 6A, this will be 1.5kW, so what ever is faulty will be getting HOT and will probably have a burning or overheating aroma.

 

[stulew]

Sorry Gents.. How can I expect you to answer when I do not give you the correct information... Let me start again...
I was going on somebody elses description..
The boiler is a Concord WCF255A and it is not a gravity fed system, it is pumped and has 2 way motorised valves. The fuse that blows is on the main board under the stairs and that is 3Amp.
There is no smell of burning !! Should I still try to isolate the Motorised valves and pump and fire up the boiler?
Once again gents apologies for the bad original description.

 

[ollski]

possible that your motorised valve is leaking and shorting the micro switches or the same with your pump....any sign of a leak around either of those components?

 

[stulew]

I will check the motorised valves and report back.. As i mentioned it is not in my house so I will check it out ...
In the meantime... Thank you

 

[stulew]

Gentlemen.... Many Thanks... it was a faulty pump that was causing the problem. Changed and working fine now...

 

[stulew]

Well..... All the best made plans are designed to go wrong.. !!!! After the pump had been changed all appeared to be going well until, Guess What... The fuse keeps on blowing again, i have issolated all the electrical components in the heating system and they are all "sound" and I am pretty sure it is not them that is causing me all this stress.There is no burning smell (That I can detect) is there anybody with any further ideas of things i can check???.
Your help and guidance as usual is much appreciated.

 

[oilman]

You, or someone, should do some current measuring. Changing parts from left to right is not the best way of diagnosing faults.

 

[stulew]

OilMan... I understand that, But to measure the current across where.. i have eliminated the pump,and motorised valves, I do have a tester but where do i start.. should all the components inside the boiler have full mains voltage running through them or are they reduced through a transformer?
Could this be a Thermostat fault or a safety cut off coming into play?

 

[oilman]

You will need a current clamp or otherwise you will have to break into the wires as you need to know the current, not the voltage.

You might have eliminated some of the components, but you don't know what current is flowing or where it's going

 

[ollski]

got a feeling you will eventually discover condensation or a leak shorting a switch somewhere. Just one point... are you saying that your boiler is not wired to a 3a fused spur but directly back to the main fuseboard?