To actually arrive at a true "temp rise" of a forced air furnace where is the supply temp taken?
The incoming COLD return air location is no problem, as it can be taken just about anywhere just before the blower's compartment,(approx 66 F.)
As for the location of the HOT supply, if you take it approx 8" above the top of the exchanger you get maybe 160 f.(or more)
If you take it higher up in the plenum it may be 140 F
If you take it 24" (downstream) from the supply plenum it could be 120. (or less)
These variations can make quite a diffence in the actual "rise" calculation.
Does anyone know precisely where this location is on the outgoing 'hot' supply air side?