I have a 4 ton unit on a simple 3 BR 1 bath one story house.

On the plenum I have a 12" metal duct supplying the living room, 10" metal supplying dining room and kitchen (2 vents off the same line), 10" metal ducts (2 vents off the same line) supplying 2 bedrooms, 9" flex duct for the 3rd bedroom, and 7" flex duct for the bathroom.

When the heat is on, the air coming out of the 12" metal in the living room and the 10" in the dining room and kitchen is extremely warm >90 deg.

However, the air out of the 10" metal for the 2 bedrooms and the 2 flex ducts is cold <65 deg.

All of the heating elements are good.

Connections at the plenum are snug (even had the plenum rebuilt). No discernible difference in air pressure from the vents in any room.

I've had several A/C repair specialists out and everyone seems stumped.

It's difficult for me to get my head around b/c all of the air entering the plenum is roughly equally warm having passed over the heating coils right?

There's no leak in the ducts and even if there was it'd be quite a coincidence since the vents blowing cold air are on 3 separate lines (7" flex, 9" flex, 10" metal).

The house is basically rectangular and the plenum sits in the middle of the attic. The hot air vents are north, the 2 flex ducts (cold air) are east and west of the plenum and the 10" metal cold air blowing duct runs south off the plenum.

Any ideas?

  • I'm sorry correction...I do not have a 10" metal for the dining room and kitchen. It's one 12" duct that runs north off the plenum, supplying 2 vents on a straight run to the kitchen and dining room, then elbows at 90 deg and terminates at a vent in the living room...this is the one duct supplying all of the hot air. The 10" metal for the 2 bedrooms and the 2 flex blowing cold air
    – Anthony
    Jul 17, 2017 at 11:50
  • 1
    Please edit your post to make corrections. It's much clearer to the reader. Also consider some paragraph breaks and other formatting using the toolbar.
    – isherwood
    Jul 17, 2017 at 12:06
  • Can you show photographs?
    – SDsolar
    Jul 18, 2017 at 3:55
  • Hello, @Anthony, you can edit your question to insert corrections. If hey are significant they can be marked as so, but keep in mind that a lot of people skim the question and may not make it to the comments.
    – SDsolar
    Jul 19, 2017 at 0:32

1 Answer 1


Could you add some pictures of the furnace and connected duct work. Are The duct sizes you list round ducts? How long are the ducts from the furnace to the register connections? Are the ducts insulated? Where are you located since a 4 ton A/C unit is quite large for a 3 bedroom 1 bathroom house. I would have guessed a 2 to 2 1/2 ton unit would be large enough.

  • I can't get you pics at this time. But to answer your other questions, the ducts are round and the length of the 2 flex ducts are approx. 15 ft, the metal duct blowing the really hot air is somewhere around 30-35 ft I would estimate and I would also say the same length for the other metal duct. Yes it is quite large for that size of house but it's an older house with virtually no shading on the west side and the 3 ton the house had when I moved in would not shut off during the day in the dead of summer and never cool the house below 78 deg at that.
    – Anthony
    Jul 17, 2017 at 15:43
  • I would assume the newer flex ducts are insulated and yes the metal ducts are insulated. Funny thing is as I said the 2 flex ducts spit out cold air along with the smaller (10") metal duct but the 12" duct carries air with temps over 90 deg. With all of the elements working I would expect the same temp coming out of each duct even it were cooler than it should be. So essentially during the winter we burn up in the front of the house an freeze to death in the back...
    – Anthony
    Jul 17, 2017 at 15:43

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