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Starving Heat Controller (Comfort Aire) Furnace.

Kitra
Kitra Member Posts: 2
We have a 92.3% efficient 75K BTU (Input) Furnace heating a 1989 2650 square foot Home. Since our lows rarely get into the 20's, it does OK. Furnaces has the optional 3/4 HP 4-ton blower motor. So here is the issue. The Furnace only has a single return air upstairs for the 4-Ton Furnace Blower Fan. The return is 16" Flex. Sad but true. Several Contactors cannot believe it even works with the high static pressure. To add more return air is the problem. There is a wall in the attic which is the back of a bedroom. The 16" Flex is through the corner of the closet and then into the rafters. No one can figure out how to "add" more return air? Other walls in the attic are a Bathroom and Master Bedroom, and an outside wall. I wired the Furnace Fan to the highest speed on the Heat lug hoping to move more air. Did this to try and prevent the Furnace's Heat Exchanger from cracking as the installer said they wouldn't cover the warranty or the install. The Blower Motor sleeve bearings went out a couple of years ago. Got a new motor with sealed roller bearings (Century brand). So with the Fan running at full speed, the air coming out of the vent in the Kitchen is 116 degrees. On the next lower speed (Blue wire), the temperature from the vent is 125-130 degrees. Anyone have thoughts on what you would do in the situation? One Guy told us to jackhammer a hole in the garage floor and run a return under the House. The Furnace is in the Garage. I know this is a bad scenario but maybe someone here has an idea. We live in Beaverton Oregon. Thanks

Comments

  • Big Ed_4
    Big Ed_4 Member Posts: 2,687
    edited January 3
    Run a 20"x18" duct though one end of the closet
    I have enough experience to know , that I dont know it all
    Kitra
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,297
    Your air temps show your moving enough air. But you need to move 1600 CFM if you have 4 tons of cooling.

    There is no magic. Is this a new home? A replacement furnace? Need more info to point you in the right direction. 16" round is normally good for 1400 cfm but that is reduced especially if it is a long run of flex duct.

    Can you make a sketch and post it and some pictures of the furnace will help.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,687
    Add 20% more for the warm return air 2000....
    I have enough experience to know , that I dont know it all
  • pedmec
    pedmec Member Posts: 919
    You need to measure your temperatures at the furnace supply plenum (preferred 24" away from the furnace) and return, not at the diffusers and return grill, for proper readings. 75k x 92.3 = 69,225 btus. 69,225/ (46 x 1.08) = 1,393 cfm. 46 degrees temperature rise is assuming 70 degree return air temperature. Looks like your moving plenty of air. But these numbers would need to be confirmed to be accurate as stated then they don't mean anything if not measured at the furnace.
  • Kitra
    Kitra Member Posts: 2
    edited January 4
    Maybe I need to move the Blower fan back to the second highest speed for Heat and the highest speed tap back to Cooling? That would raise the temperature at the Registers back to the 125 to 130 degree mark instead of the 110 to 116 degrees? I have a bunch of HVAC tools and meters. I could also figure out how to measure the static pressure. I need to research if there is a way to add more return air. The House was built in 1989 and we get inadequate heating and cooling upstairs. I have a booster fan in the upstairs ductwork to assist.