Results 1 to 6 of 6

Thread: HVAC over tub Plenum replacement size question = mold

  1. #1
    Join Date
    Apr 2021
    Posts
    4
    Post Likes

    HVAC over tub Plenum replacement size question = mold

    Hello! This is a very common question on google, but there's never anyone out there who'll comment.

    I'm trying to help myself and the building's maintenance men at the same time here.

    My entire hvac system is getting replaced and want to solve the mold issue, caused I think, by the supply plenum configuration, etc. Diagram attached

    For years this apartment has had a 1998, 2.5 ton Trane unit which is waaaay too big for a 840 sq ft, 2 bed, 1 bath space.

    Live in Dallas area, 33 yr old bldg. Single pain windows, plenty of leaks - lol.

    The entire thing is above the tub. The duct board plenum interior is coated in thick, black mold and literally coming apart at the seams. Black scraps fly out of the vents.

    There are 5 runs off the plenum and a 6 x 4 inch hole in the bottom of it that feeds the bath vent (yes it's freezing in the bathroom & not enough air gets to the room supplied by duct 3).

    Ducts are all 6 inches and go like this:

    Duct 1 runs all of 14 inches
    Duct 2 runs 10 inches
    Duct 3 runs 9 to 10 foot
    Duct 4 runs 11 to 12 foot (its register is 4 feet over from the duct #2. They serve the living/dining combo)
    Duct 5 runs about 13 to 14 foot to the kitchen, but that area is also served by duct #2)

    I have the ducting all sorted out and return air is ample, but need advise on the supply plenum sizing. I'd like to get the landlord to put in a metal one, wrapped in insulation. I need to know what to make them do, or I'll just have a nightmare on my hands. I want this #*%^ fixed. lol

    View the attached image to see what I'm up against.

    To minimize the condensation forming in the supply plenum box I was hoping it would be okay to have it reduced from 22 or 24 inches wide to maybe 8 inches, something like that.

    I figure getting the air into the ducts asap will help reduce the condensation, along with an appropriate sized unit. They said they will put in a 2 ton. Being in Texas with crap windows, that's probably about right, but I want to make sure if that's too big. We get strong morning and afternoon sun (at least 6 - 7 hours), it's a brick exterior.

    I'd also like to do a 4 inch duct for the bathroom and reducing that 22 or 24 inch width is the only way I can fit one of those in.

    Because of that bath vent being so insane, duct 3 (9 - 10 ft, w/a single 90 degree turn) has a hard time getting enough air to that bedroom. Same with duct 5 for the kitchen.

    Duct 1 supplies the bedroom adjacent to the bath, so it gets super cold too. Not only that but there is a ( 8" x 24") RETURN 6 inches over from the register!! 😂

    What size plenum do you think will work in this situation? Am I right to want the air to get in the ducts asap or is that a bad idea?

    What would you experts do? Thanks in advance!
    Attached Images Attached Images  

  2. #2
    Join Date
    Jan 2020
    Location
    Winnipeg Canada
    Posts
    1,019
    Post Likes
    The root of the mold problem is probably in 2 parts.
    Part one the system is oversized. Part 2 lack of airflow.
    Both those problems together create an extreme low temperature area at your plenum. That plenum was probably 35 ,degrees. And that is going to likely be well below the dew point.
    The solution to the problem will have 3 parts.
    Part 1 is a proper load calculation for the space. And means properly sized equipment.
    Part 2 is ductwork. The reality is for 2 tons cooling you NEED to move 800 cfm MINIMUM. That will bring your plenum temperature closer to 55 degrees. And properly sealing and insulating the plenum and ductwork will help keep it dry and free of mold.
    The insulation is the part 3.
    Now having said you need to flow 800 cfm I'd actually add 20% and build the ductwork to flow 1000cfm.
    And the reason for going bigger on the ductwork is so that you have the ability to balance your air delivery without going below 800 cfm.
    You don't squat with your spurs on.
    And you NEVER put the torches away before pressure testing.

  3. #3
    Join Date
    Jan 2008
    Location
    Texas
    Posts
    5,956
    Post Likes
    I don’t know about yours but most furdowns units I see have the unit open for return air and a return grill cut into the wall, no duct. The units have a drop down metal access panel. Being above the tub/shower it needs a good seal otherwise being in a negative pressure state it can pull in hot air/steam from the shower which adds to the other issues.

  4. #4
    Join Date
    Apr 2021
    Posts
    4
    Post Likes
    Thread Starter
    I see what you mean now. I found this handy chart: FIELD DUCT SIZING CHART

    Wow!! Talk about inadequate air flow!! Problem is the height restrictions, can't get the size ducts needed.

    If they'd replace the dilapidated windows, they could use 1.5 ton units for sure and they'd still have to up the supply air flow a bit even for that.

    I cannot tell you how helpful you've been. Totally on the right track now. Thanks!!



    Quote Originally Posted by Restaurant mech View Post
    The root of the mold problem is probably in 2 parts.
    Part one the system is oversized. Part 2 lack of airflow.
    Both those problems together create an extreme low temperature area at your plenum. That plenum was probably 35 ,degrees. And that is going to likely be well below the dew point.
    The solution to the problem will have 3 parts.
    Part 1 is a proper load calculation for the space. And means properly sized equipment.
    Part 2 is ductwork. The reality is for 2 tons cooling you NEED to move 800 cfm MINIMUM. That will bring your plenum temperature closer to 55 degrees. And properly sealing and insulating the plenum and ductwork will help keep it dry and free of mold.
    The insulation is the part 3.
    Now having said you need to flow 800 cfm I'd actually add 20% and build the ductwork to flow 1000cfm.
    And the reason for going bigger on the ductwork is so that you have the ability to balance your air delivery without going below 800 cfm.

  5. #5
    Join Date
    Apr 2021
    Posts
    4
    Post Likes
    Thread Starter
    Thanks again! Anyone needing help with duct sizing, search for this - it's awesome! FIELD DUCT SIZING CHART

  6. #6
    Join Date
    Apr 2021
    Posts
    4
    Post Likes
    Thread Starter
    Quote Originally Posted by fliks View Post
    I don’t know about yours but most furdowns units I see have the unit open for return air and a return grill cut into the wall, no duct. The units have a drop down metal access panel. Being above the tub/shower it needs a good seal otherwise being in a negative pressure state it can pull in hot air/steam from the shower which adds to the other issues.
    Gosh darn it, I can't thank you enough for your response!! This is great information!

    Ah! True! So this access panel needs replaced too. All these years of having a full ton more than it was designed for, drain pan trying to drain UP!! Way too small of ducts, huge holes punched in the drywall in the mechanic's compartment, no return duct - for return #2 - just drawing from all over the huge bulkhead - this is a recipe for disaster.

    This access panel is rusting out and bent where it needs to seal.

    There isn't even enough height in the bulkhead area to put it larger ducts, you can't get a tall enough plenum in the space!

    If they'd replace the worn out, single pane windows they could do 1.5 tons, which it was designed for - even at that the supply side air flow isn't as much as required.

    To this day they slap these apartments together - not even meeting code, but evidentially the under paid city building inspector has little "presents" mysteriously appear and they pass the buildings anyway.

    That's a real shame and costly to us common folk.

    Thanks again! Anyone needing help with duct sizing, search for this - it's awesome! FIELD DUCT SIZING CHART

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •