"Barb Grajewski" wrote in message
...

30+ year old GFA furnace, hoping to have it squeak through one more
winter. New to this home, old owner wasn't very good about routine
maintenance. Replaced the furnace filter right away; it was clogged
with dog hair and dust...

At the pre-sale home inspection, the inspector noted that the
temperature in the plenum was running high. I don't have the exact
figure, but the registers in the room directly above the furnace get
REALLY hot to the touch, way too hot to me. My impression is that the
blower isn't really blowing all that well, either.

Questions:
1) could there be dirt and crap further up in the system (A-coil??)

Yes!

If you even suspect that the furnace was operated without an air filter for
a time, you should try to gain access to the upstream side of the "A" coil.
Often, this requires you to party removing the coil (the line set has some
flexibility.)

BUT another cause of over heating in the ducts is a leak in the heat
exchanger.

IF you have a leak you call in the HVAC guys for replacement estimates. If
you just have a very dirty A-coil and don't want to move the line set to
clean it be prepared to pay $100 to $200 for a pro to come in and do it for
you.

IF you have such a leak a digital CO detector on an outlet MIGHT detect a
problem (you should have a digital CO detector whenever you have gas burning
in your house.

Frankly, if you have already planned to change out the unit, you might as
well do it before the start of the heating season.

How hot is too hot?

Some 30 years ago, the fan control was set to not turn on until the
temperature reached 130 or more and shut off when the temperature went down
to about 110. If you have a thermometer (digital) you can insert in the
duct (drill a hole and cover it with alluminum duct take afterward) take an
accurate measurement.