Is the short run by design? Or is it a bad thermal relay that is

I assume that the fan is blowing on it and cooling the Fan switch faster than MY liking.
I did talk to the furnace guy back then about it and he said it worked ok

shutting it down? You can hook it up to run as long as (a) certain
temperature is reached.

I'll double check the Fan switch settings.

Is the fan taking heat from a heat exchanger or directly from the burner
where it could cause an overdraft? Many possibilities here.

hmm, hard to answer this one since I can't see the entire inerds of the furnace.
it goes something like this. in a u shaped process;
•‘•‘
•š•�


•‘†“Air flows by the 1 pipe flue
•‘†“, then through the filters,

•š †’ here sits the blower and motor
•�†‘then the unknown wood box
†‘ •‘that holds the electric furnace coils.