You should contact the manufacturers of furnaces, and tell
them that they are doing it wrong. They will thank you.
Don't take no for an answer.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
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wrote in message
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So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so
use the next
bigger fuse; let's say 1.5 A.

We don't need to guess here.
The transformer has a rating. NEVER fuse above the
transformer rating.
If the transformer is a 40va 24 volt transformer the MAXIMUM
fuse
rating is 40/24=1.66 amps - so a 1.5 amp fuse will protect
the
transformer - a 2 amp fuse will NOT.