Why does my Central Heating Boiler main fuse keep blowing?

ifeelfine

We have a (system) Worcester Greenstar 18 Ri Central Heating Boiler, fitted 8 years ago.

The motorised valve is leaking in the airing cupboard,
which is inconvenient, but in the current CV climate it was bearable,

as the leak wasn't leaking onto any of the electrics of the timer, pump, or valve motor.

The other day the boilers power went off.

I pulled it apart, and followed the fault guide in its installation booklet, which said to start by checking 2 fuses.
The first mains fuse inside the boiler, had failed, so I replaced it with a spare that was included inside the boiler.

It ran fine for about 5 days, then because the weather was hot,

I reset the timer to stop the heating coming on in the morning, so now it was just heating the water.

Later that day, (when the water had gone off) I went to turn on the heating, and the fuse failed again.

Some have suggested that its the motorised valve that has seized,
but I can move it by its 'manual' lever on the back of the valve head.

I've also tried it with the valve head removed from the valve body, but it still blows the fuse.

The system is fed by a main 3 amp fused switch,
which feeds the boiler, timer and the valve, and that fuse has not blown.

The pump is fed from the boilers PCB (which is fed by the boilers fuse)

So if the valve motor is faulty, then I cant see how it would blow the boiler fuse,
as I would expect that to blow the main fuse, in the fused switch.

So any idea as to what's causing this.

Many thanks

fenwick458

the fuse in the boiler is likely to be smaller (1 or 2A) so it will blow before the 3A one in the SFCU.
i'd try disconnecting the zone valve from the circuit, fit a new fuse,  and just operate the valve manually for now. if the problem goes away, you just need a new zone valve.
if the fuse keeps blowing, you have another problem

ifeelfine

fenwick458 said:

the fuse in the boiler is likely to be smaller (1 or 2A) so it will blow before the 3A one in the SFCU.
i'd try disconnecting the zone valve from the circuit, fit a new fuse,  and just operate the valve manually for now. if the problem goes away, you just need a new zone valve.
if the fuse keeps blowing, you have another problem

Thanks for the reply:

Yes, I understand that, but my point was that the valve current doesn't go through the boiler fuse, only the main fuse.