unplug hose from furnace pressure switch, hook to magnehelic, then blower on high...

[batdude]

and as the blower spools up, the hose on magnehelic positive pressure port yields needle movement every time.

what's going on?

A. Wasting time.

B. Checking heat exchanger.

C. Other

[tlb]

does the needle hold when blower is running? I would go with heat xger

[batdude]

yes, steady needle until blower power is cut. then needle returns to zero.

[cehs]

Is the blower compartment OPEN to the room, SUCKING from the space and then down the flue?
Is this sealed combustion heating?
.

[batdude]

furnace in basement is an OLD upflow ninety percenter, one pipe out the rim joist and the brick veneer.

burner openings are like an eighty percenter, no sealed burner box.

the blower compartment door is a mite loose, it bangs audibly shut as the blower suction develops. (that would drive me nuts, and that's only a short walk from here.)

my helper said the furnace is a Magic Chef brand. ummm, BARBEQUE!!!!!!!!!!

[meplumber]

I would look for a crack on the HX.

Good Luck.