high GFA plenum temps?

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??) causing air flow restriction and the high temps at the plenum and near registers? How to check? How to clean?
2) Calling in the HVAC guy is on the to-do list, but they're really booked up right now. What minor stuff can we do (cleaning, examining) until the HVAC guy comes to do the winter inspection/tune-up??
3) Is it dangerous to have such high plenum temp? Furnace is not short cycling, or acting wacky in any other ways..
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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.
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John Gilmer wrote:

Excellent advice, John.
When filters get too loaded they begin bypassing dirt and lint, therefore the blower wheel and the E-Coil are both due for a thorough cleaning. I doubt you will find a tech that will do it for 200.
http://www.udarrell.com/air-conditioning-seer.html
Thirty years is long enough, I would have a new unit installed asap. Also, have the duct system and airflow cfm checked, and if it were me, I would install a new 12 SEER high efficiency matched AC system. Darrell - udarrell
--
Get the Cooling Capacity and Efficiency you Paid for -
http://www.udarrell.com/air-conditioner-capacity-seer.html
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Actually....
When a filter is brand new, dirt will tend to pass right through it....filters do not start to filter at capacity until they load up a bit... That old of a furnce I am sure cant take the added static that a 3M or AirBear filter would add, but pleated paper, as all of us in the trade know is the way to go...and I am sure that the A coil, or slab is clogged, and the blowers prob toast as well in regards to being full of crap.
A proper filter fit, wont bypass a thing...much less air....unless the blower is strong enough to allow for sucking the filter from its mounts...and thats more rare than it is common.
BTW...I have done an evap coil, blower, AND HE clean for under $200..recently...all shes got to do is shop around...it depends on more factors than we need to get into here, and her area, but in some parts of the country, $200 might be considered high, while I know in others it wont get the tech to the door.

Only 12SEER? We stopped offering 10SEER a while back..stopped selling 80% units as well. Now, a savy shopper can find that the 14SEER units are running what a 12 was a couple of years ago....but higher SEER, in SOME parts of the country, will not always equal a savings.

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