Help with Alpha boiler and john guest Room stat

Hi all, I'm after some advise please, I've wired a John Guest programable room stat to a Alpha boiler (can't remember the model), everything has been fine for a week, but now, the fuse in the boiler just keeps blowing. I replaced the stat today, it worked fine, got home and then had a text that the fuse had blown in the boiler. When I put the link back in the boiler, everything ok, any suggestions, thanks in advance Scooterboy