Poor Original Design? Do I need a duct booster?

Discussion in 'HVAC Heating & Cooling' started by Walter H., Feb 21, 2021.

  1. Walter H.

    Walter H. New Member

    Joined:
    Feb 21, 2021
    Location:
    80123
    Our split level main floor has poor air flow to one of the levels. This level has is ducted by a 9" dia duct positioned 1/2 way up the 30" vertical by 20" sq. plenum. At the top of this 30" vertical plenum are 2 - 8"x 18" ducts coming out the sides. One of these feeds the basement and the upper level of the split level the other feeds the upstairs. Both of these runs feed by the 8" x 18" ducts function fine with noticeable air flow.

    The 9" round duct halfway up the vertical plenum immediately 90's down then 90's out to exit the basement wall. It now is under the garage floor for a couple of feet before it 90's again to pass through the basement wall into the crawl space below the level in question. Once in the crawl space it transitions into a 8" x 14" duct 8' long. At the transition there is a 5" dia. run for 4' to a 2" x 12" register and at the other end of the duct are 2 - 5" dia. 8' runs that terminate at the corners of the room with 2" x 12" registers.

    My professionally uneducated guess is that there is not enough pressure in the plenum to push air into the 9" dia. duct especially with all of those 90's. I tried to build a diverter out of cardboard to direct more air towards the 9" but that did not seem to help much.

    Do you think I can install a 8" booster inside the 9" duct before the 90's to help pull and push air into this run?

    We are only having this issue with HEATING. This is the lowest level above the basement so it tends to stay cooler. I'm hoping if we go with a booster I can tap into where the booster would only run when heat is called for.

    Sorry for the long post, I'm hoping someone can provide some advice. Please let me know if you need any addition info.

    Walter
     
    Last edited: Feb 21, 2021
  2. Dana

    Dana In the trades

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    Location:
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    First off, how tight are the ducts? If there isn't duct mastic on every joint & seam a fraction of the flow goes somewhere other than intended.

    Secondly (or perhaps the second "first off") is there an adequate RETURN duct for the underperforming zone? Without a properly sized dedicated return path adding a booster would simply use more of "the great outdoors" as a return path. There are many possible solutions to inadequate returns, but what makes the most sense depends on the configuration of the house.

    Third, is it a vented crawlspace? If not vented, are the foundation walls insulated to the current IRC code minimum (=R15 continuous insulation for IECC zone 5B, which would includes the Denver area)? A vented crawlspace would have nearly zero effect on AC in your area, but a potentially large impact on heat loss, especially if the supplies &/or returns are leaking in that vented crawlspace. If the ducts are all inside the insulation & air pressure boundary of the house that's less of an issue.
     
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  4. fitter30

    fitter30 Well-Known Member

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    Retired service tech
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    Furnaces normally have muilti speed motor heat is set to run medium ac on hi. Turn fan to on at thermostat see if theres a difference in flow. If it works it's a simple wiring change. Duct sealer can be purchased at Lowe's or home depot and pick up a couple of 1.5" cheap chip brushes to apply it.
     
  5. Walter H.

    Walter H. New Member

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    Feb 21, 2021
    Location:
    80123
    Thanks for the quick reply!
    1) All joints are tight. The duct work itself is uninsulated and I was wondering if heat absorption was occurring because of that.
    2) There is a 30" x 6" return on a floor joist dedicated run, no restrictions. Only 1 - 90 turn back to unit
    3) The two perimeter crawlspace vents are sealed and there is a insulation blanket around all foundation walls. There is a 6" x 12" register in the crawlspace at the end of the 8" x 14" duct that I leave cracked to allow some heat to the crawlspace. The crawlspace never gets cold the floor of it is lined with very HD plastic and I have moving blankets covering all of that as I store stuff in there.

    I forgot to include in my original post which I just edited is that the vertical 30" section is 20" x 20" if that matters
     
    Last edited: Feb 22, 2021
  6. Dana

    Dana In the trades

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    Even the seams of hard-piped ducts need to be mastic-sealed (or taped, if the tape is applied when it's shiny-clean galvanized surface).

    Uninsulated ducts would be a code violation under many local codes (and the current IRC- see section N1103.3 ), even when the ducts are fully inside the thermal boundary of the house. The insulation requirements under current code for ducts inside of conditioned space (inside the insulation & pressure boundary) is R6, (even return ducts)and R8 if in a ventilated crawlspace or in an attic above the insulation layer. Return ducts also need to be insulated when outside the thermal boundary of the house.

    The meaning of "...return on a floor joist..." isn't clear. Panned joist returns don't meet current code either, partly because they're impossible to reliably seal or insulate for the long term.

    If it's a hard-piped dedicated 30" x 6" ( 180 square inches) return duct for room/zone supplied with a round 9" (64 square inches) connected to an 8" x 14" (112 square inches) the supply and return aren't well balanced. A booster fan MIGHT be needed, but verifying duct tightness on both the supplies and returns would still be in order prior to taking that step. With the configuration as-described any leaks on the return side would be drawing in crawlspace air, and depressurizing the crawlspace relative to the outdoors, pulling outdoor air into the crawlspace. Opening up the supply register to the crawlspace with no corresponding correctly sized return register pressurizes the crawlspace relative to the outdoors, blowing conditioned air outside.

    The wall-blanket approach to sealing foundations is legal but really sub-par. It doesn't adequately air seal the wall from outdoor air infiltration. Insulation the floor is much less critical as long as the plastic vapor barrier is air tight and well sealed to air tight insulated foundation walls.

    Actively heating the crawlspace may be doing more harm than good from a system efficiency point of view. If there isn't a corresponding properly sized return register in the crawlspace, opening up the crawlspace register is using leaks to the outdoors as it's return path, creating an air-handler driven outdoor air infiltration problem. The tighter the walls & vapor barrier, the smaller the problem, but it's never zero.

    With uninsulated hard-piped ducts you can often find leaks by following the sound by temporarily plugging the supply registers (to both the crawlspace and room). Infra-red cameras don't usually show the leaks in shiny metal ducts very well, but can easily spot warm wood where the leak has created localized warm/cold zones. (Bare metals have too low an emissivity, and always read colder than reality.)

    The duct boots also need to be sealed to the subfloor (if it's a floor register) or wallboard (if wall register), as well as all seams of the duct boots.) This is sometimes easier to do from the room side when performed after the fact. Combinations of polyurethane caulk and housewrap tape (for larger gaps) tends to work more reliably than can-foam (which can have adhesion and shrinkage issues) when mating dissimilar materials such as sheet metal and wood. Fiber reinforced duct mastic usually works pretty well too, but it can be pretty messy when working from the room side.

    Retrofit insulating ducts in a tight crawlspace is difficult at best. Sometimes it can be done adequately with 2-part DIY foam kits, but usually not when the ducts are running in joist bays. Straight runs can sometimes be insulated in-situ with a triple-wrap of aluminized bubblepack.
     
  7. Walter H.

    Walter H. New Member

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    Feb 21, 2021
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    80123
    Dana, thanks again for your input. I'm going to re-examine the duct joints and seal properly.

    The house was built in 1981 and since then and prior to us purchasing it in 2006 the basement has been finished. The full height basement is 1/2 of the floor plan, the other half is assessable by the crawl space. In the finished portion of the basement there is 3 - 4 x10 ceiling registers.

    Nowhere in the basement or crawlspace is there a return! I wonder if this is the problem?
     
  8. Dana

    Dana In the trades

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    Don't know if it's THE problem but it's definitely A problem, if all return registers are separated from the lower level by doors! If there are common returns upstairs there may be jump-duct solutions to make it all work better, but how that's done or whether more involved ducted returns are necessary depends on the house design and where return paths might be created.
     
  9. Walter H.

    Walter H. New Member

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    None of the returns in the house are separated by doors.

    In the split level main floor, the lowest (underperforming) level has the 30" X 6" return GRATE in the base of the wall at the floor. This is a 24" tall wall that separates the split levels. From the basement I can see this same return is a ducted 14" x 8" on the other side of that wall for 3' then 90 along the panned floor joist run to the unit.

    The upper split level has 26" x 7" return in the wall at the ceiling located roughly in the middle of this level. This return runs vertically down between wall studs and is then panned through the floor joist back to the furnace.

    The 2nd floor has 1- 26" x 7" return in the wall at the ceiling located in a hallway.

    Upon closer inspection I see gaps and open joints with ZERO sealant on the exposed section of the lowest split level return in the basement. I also see in the crawl space duct tape was used on the 5" ducts and transition to the floor registers. The duct tape is old and brittle however I feel no heat escaping.

    EDIT Additional note - The basement is the only floor separated by a door.
     
    Last edited: Feb 23, 2021
  10. Dana

    Dana In the trades

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    You are saying that there are no interior doors in the house? (Hard to believe!) Or are you saying there are no rooms with supply registers that don't also have a corresponding return register? A gap at the bottom of the door is almost never anywhere big enough to provide an adequate return path, but transom grilles or stud bays in partition walls employed as "jump ducts" can be, if done correctly, sealing the section of stud bay used as duct off from the remainder of the stud bay with tight air dams.

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    Is the crawlspace completely open to the basement (bad idea for a vented crawlspace) and that there are no doors on the staircase from the basement to the return floor?


    This grate is just a pass-through to the other level, and not ducted? 30 x 6 is 180 square inches, but the grating cuts into the free-air cross sectional area.

    A 14 x 8 has a cross sectional are of about 112"square inches.

    Are the floor joists 2x10s (9.25" deep) or bigger, 16" on center (14.5" internal width), for ~134 square inch cross sectional area?

    Is this an exterior wall? If it's an external wall it's an energy disaster.

    Is it ducted inside the stud bay, or is (are) the stud bay(s) being used as the duct? If there isn't a separate sealed duct inside the stud bay there needs to be a well sealed air barrier isolating the section used as a duct from the portion of the bay above and below the stud-bay-as-duct portion.

    A 2x6 stud bay has a ~80 square inches, a pair of them ~160 square inches. A pair of 2x4 stud bays comes in at square inches, which is probably quite a bit smaller than the net free area of a 26 x 7 return register grille.

    Cloth or fabric duct tape should never be used on ducts. Product name notwithstanding it's not really designed for that purpose. Temperature rated aluminum duct tapes (eg Nasual 324A) can be used if the duct is shiny-clean, but it'll never be clean enough if it has crumbling duct tape adhesive on it. Duct mastic would be the right retrofit. Just because you can't feel the hot air leaking with a hand doesn't mean it isn't a problem, or isn't going to BECOME a problem.

    With a door between the supplies & returns all the supply ducts to the basement and crawlspace are pressurizing those spaces when the air handler is running. If there is a convenient location on the upper level to install adequately sized floor grilles, a ceiling grill in the basement or crawlspace could be added to use the ceiling/floor joist bays as a jump duct (with well sealed air barriers to isolate the jump duct section of the joist bay from the rest of the joist bay, just as with stud bay jump ducts.)

    As a retrofit it's not possible to fully seal framing bay jump ducts, but as long as they are not attic or exterior wall framing bays the consequences of those leaky quasi-ducts aren't terrible, despite not being up to current code. The consequences of inadequate return is an order of magnitude larger than having a somewhat leaky section of return "duct". See if you can verify that the existing panned joist sections have well sealed air dams isolating them from the rest of those joist bays, since leakage at the ends of the bays that terminate at exterior walls/band joists can be a much bigger energy/heat loss issue than what happens in between.
     
  11. Walter H.

    Walter H. New Member

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    Feb 21, 2021
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    Dana, I appreciate all of your help but I feel all for naught. Long story short, nowhere are any of my duct runs on an exterior wall and all of my returns are run vertically in double 2 x 4 bays with no interior duct. All horizontal panned supply and returns are on run in 16" O.C. bays with no interior duct. There are no doors on the split levels and although there are doors in the upper floor they are rarely closed. The house is only 1860 Sf.

    I have been removing the old "cloth" duct tape which was only used on the 3 adjustable elbows where they join the 5" rigid and the register boxs in the crawl space. Don't laugh but I took a stick of incense with the blower fan going to find leaks. I have found a big 1" x 6" gap in a panned return where 2 floor joists are staggered where the end on a beam. I have also found leaks in the duct metal work that I will either use aluminum tape or use joint compound on.

    I don't see how I can create a jump duct between the pressurized basement and the split level above. The basement to the crawlspace is closed off by a door to a storage room which has the opening in the wall to crawl through. I can open up the ceiling of the basement to the split level return duct but I don't think that is a good idea as it would be diminishing the usefulness of it in the underperforming room. Another thought although ugly would be to put opposing grills on the door down to the basement.

    Thanks again for all of your help!
     
  12. Dana

    Dana In the trades

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    Don't count on an incense stick or smoke pencil to find all the duct leaks. On unsealed seams the leaks will come and go at different operating temperatures and air handler speeds as the ducts expand & contract with temperature & pressure. Its best to just go ahead mastic-seal any seam or joint you can see, even if it's not apparently leaking. Unless it's metal to metal on shiny clean metal mastic is by far the better choice. Aluminum tape works great on cleaned air handler cabinet seams, and on clean hard piped ducts. To use aluminum tape even on new hard pipe ducts & duct boots they need to be first wiped down with isopropyl alcohol (or cheap gin :) ) to clear off any light-oil the manufacturer's use to inhibit corrosion during storage.

    But big fix all the big leaks (like 1x6" gaps) first.

    A grille cut into access door or pony-wall stud bay in the partition wall between the crawlspace & storage room should provide a big enough return path, provided the storage room has sufficient return path to the system elsewhere.

    The jump duct for the basement need not be in a wall or door. Basements are by definition below a floor, and a simple floor grille pass-through can work, or a grille in the basement ceiling into a joist bay, with a floor grille on the first floor at some other location along the joist bay. If there is enough headroom above the door a transom grille might work too. ANY deliberate return path is better than counting on random leakage return paths, which is where you are right now.
     
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