24 volt transformer fuse

Im replacing my 24 volt transformer for my boiler it is a honeywell AT72D1683/U. There was no inline fuse on the 24 volt side outputs. I noticed on my dads boiler that he has a fuse to protect the tstat wiring in the house. What terminal on the transformer should have the fuse inline and what size fuse for 18 gauge tstat wire. There are three zone valves to supply 24 volts to. The aquastat takes care of the boilers gas valve 24 volts.