Oil boiler central heating - tripping fuse

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Hi,
I have a Warmflow 50/70 boiler outside my 1st floor flat (where I've lived for 6 years). For the last 2 years I've put up with it tripping out the 5 amp fuse on occasions. The time it works for can vary, sometimes 10 minutes then trips, sometimes half an hour the trips, sometimes I have it on for an hour and it runs through fine. My dad has a bit of electrical knowledge and was able to separate it off so that it's on a circuit by itself. It still trips on this circuit.

If I run a 13 amp extension cable out to it, I can have the boiler and pump running off the extension cable with no issue for an hour.

I had a boiler man look at it about a year ago and he replaced an electromagnet part in the burner unit in front of the boiler. Worked fine for about two weeks but started to trip again after that.

The pump and boiler have a 3 amp plug each in the boiler house outside my flat. If it is an issue with the boiler or pump, why would the fuse in the plug there not blow?

I know it's a best guess scenario but is it an issue with the pump? Is there anything else I can look at or get checked?

macman

When you say 'tripping', is this a RCD protected circuit or a standard 5A MCB?
An RCD will detect earth leakage that an MCB will not, and that would not necessarily take out the plug fuse or an MCB.