Trane XR 80 furnace wierd problem
 

I have a 2 year old Trane XR 80 gas furnace that's been trouble free up
until I noticed something this morning.
When I call for heat. The combustion blower ( not the main blower)
comes on as ir should, but after 3 seconds it shuts off, then comes on
about 4 seconds later and then shuts off. It repeats this cycle
indefinately.
So I figured it is one of 2 things. A bad T-stat or some limit switch
is bad. When I opened the panel, I noticed a cylindrical limit switch
with a clear tube on it which leads into where the combustion chamber
is. My guess is that it senses if the combustion blower is on. So I
removed the clear tube and blew in it to clear any particles out of it.
I then jumped the T-stat terminals at the furnace and the furnace came
on with no problems.
When I put everything back together, the furnace operated normally. So
now I don't know what caused the problem.
What usually causes this problem? And also does anyone know what the
limit switch with the clear tube on it is for?

That limit your referring to is a pressure switch. You cleared the moisture
from it when you removed it. Not an uncommon occurrence in the spring or
fall.

Thanks, I figured it was a pressure switch of some sort.