Results 1 to 7 of 7
  1. #1

    What causes temperature differential inside the plenum?

    My air handler's plenum has 3 flex ducts connected to it: 16" main supply to 3 rooms + hallways, 10" duct to a room at other side of the house, and 8" duct to utility rooms.

    I disconnected the 10" and 8" ducts to check the air flow and noticed that the air coming out of the 10" is warmer than the air coming out of the 8" and the 16" ducts. The temperature differential is about 4 to 6F.

    What would cause this to happen? I would thought with the air velocity and pressure inside the plenum, the temperature of the air should be uniform when going into the ducts.

  2. #2
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    air will go where it wants. That's the simple answer.

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,789
    Improper charge.
    Type of metering device.
    Position of the tap may not be allowing the air to mix in the plenum.
    The coils are not the same temp from one end to the next.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Even if all the items beenthere mentioned are normal, there will always be a bypass factor in a supply plenum, and uneven mixing of air due to turbulence within the air handler and plenum. There is always a certain amount of air that passes through the coil that is unaffected by the coil. This is known as bypass factor.

    Also, when you removed the two ducts to observe this temperature difference, removing the ducts changed the dynamics of the airflow in the plenum and across the coil. You would have been better off leaving the ducts in place and placing a temperature probe through the ducts to observe what differences there might be in temperature. I would remeasure the ducts with them in place, not removed.

  5. #5
    Quote Originally Posted by shophound View Post
    Also, when you removed the two ducts to observe this temperature difference, removing the ducts changed the dynamics of the airflow in the plenum and across the coil. You would have been better off leaving the ducts in place and placing a temperature probe through the ducts to observe what differences there might be in temperature. I would remeasure the ducts with them in place, not removed.
    Actually, I only opened one duct at a time and measured the air temperature from the mouth of the collar, so this should not affect the dynamics of the airflow too much, would it?

    What is the best way to achieve this? Do I have to poke a hole through the duct and then insert the pen thermometer's tip through it? Thx
    Last edited by toohothere; 07-30-2008 at 01:35 PM.

  6. #6
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Like I sai. Air goes whrer it wants is the simple answer

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Quote Originally Posted by toohothere View Post
    Actually, I only opened one duct at a time and measured the air temperature from the mouth of the collar, so this should not affect the dynamics of the airflow too much, would it?

    What is the best way to achieve this? Do I have to poke a hole through the duct and then insert the pen thermometer's tip through it? Thx
    Removing just one duct affects the static pressure of the entire system. Where that duct originally offered resistance to airflow, with it removed there is no resistance. Therefore the entire system is affected as the lack of resistance on the take-off with the removed duct will flow air more freely.

    Go a few feet away from the plenum and drill a hole into the duct just large enough to insert a temperature probe. Patch the hole with foil tape when you're done. The few feet away will assure you're measuring the mixed air properties entering that duct run, but minimize heat gain should these ducts in question be in the attic.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event