What size fuse for an Oil Boiler isolation switch

Hi,

I have an isolation switch for my Horstmann timer unit which controls an oil boiler. What size fuse should be used in the isolation switch? It is currently a a 13amp, i would have thought I should have been 3 or 5amp.

And could the above 13amp fuse have anything to do with a new circulation pump failing after a week?

Thanks for any help!

Most likely a 3amp but best that you look at the boiler manual for the info.
If you don't have the manual then a quick call to the manufacturer should yield the answer. A correctly rated fuse is a balance between protecting all circuits downstream of the fuse and not blowing under normal operating conditions.

An incorrectly rated fuse won't cause an initial fault in the pump but it may make things worse by not disconnecting (blowing) fast enough to prevent further damage or (in an extreme case) even fire.

Fused Connection Units (FCUs) are usually shipped with 13A fuses in them. Many installers (especially of heating systems - sorry plumbers!) do not think to change the fuse to the size required by the installation.

I expect that is what has happened, or the original fuse blew and somebody just put a 13A one in

Most boiler systems require a 3A fuse. Do put the correct one in; heating programmers will fry (at substantial cost) if a short occurs and the fuse is 13amps!!