"JOHN F" writes:

I have a forced hot air system and I have had to replace the limit switch 4
times in last 11 months.

Hrmm.

Part number HH 12zB and it has 200 degree rating. All power feeds
measure correctly. This is in town house with 3 floors of
ductwork. System works fine between limit switch replacements. I
have lined out limit switch for short durations ( middle of night
and heat needed), but I consider this a fire hazard over long term,

Wise!

plus unit runs continually in that mode. With defective limit
switch, fan runs continuously when themostat calls for heat, but
naturally the unit doesn't actuate the pilot, so get cold air only.

I'm curious about the root cause of these recurring failures.

It is possible that the limit switches are doing their job and that
there is a real temperature rise issue in your furnace that needs to
be addressed? Clean filters, fan running normal speed, etc?

Best Regards,
--
Todd H.
http://www.toddh.net/