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Unsealed return air ducts costing big $

CleanCut | Posted in Energy, Heating & Insulation on March 8, 2007 06:45am

I was wondering if any HVAC guys would tell me why some installers don’t see the need to seal their supply and return lines. I just came home after seeing my first blower door/infrared camera test. We found the furnace was pulling air out of the floor system, which led straight to the attic via open web floor trusses. The vents in the bedrooms wouldn’t even suck a piece of paper up against them. The poor heat pump was running frantically, drawing in 15 degree fresh outside air, (story-and-a-half, open web floor trusses, no sheathing or air barrier on outside rim truss, fiberglass blown in insulation). As far as I can tell there’s a major gap in the return air lines. The homeowner remembers his 14 can lights above the kitchen counter playing havoc with the HVAC install. Maybe an electrician inadvertently knocked some panning off? Maybe the HVAC sub didn’t care where the air came from? Maybe he didn’t think the homeowner would notice a $900/month heat bill even though he paid $30,000+ for a geothermal unit? Somebody please tell me it was an honest mistake and I’m not surrounded by trades that don’t care.
CC

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  1. USAnigel | Mar 08, 2007 07:13am | #1

    The problem I see with most houses built for others is there is no incentive to only use the best! The builder (most, not all) is not interested in "running" costs and looks for the highest profit.

    Only built to perform and tested houses will give you lower running costs. A good comparison is the auto industry with the big shinny SUV looks great but drinks gas!

    With your problems, you should be able to call the HVAC guys back in to correct this, this must have broke a code somewhere! The insulation people have work to finish to from the sound of it.

    1. CleanCut | Mar 08, 2007 07:29am | #2

      The insulation people seem to think fiberglass batts stop air movement. Wherever the second floor cantilevered, they insulated the joist cavity with batts, and then the soffit guys put solid soffit on. We removed a 6'x6' area of solid soffit and found batt insulation stuffed into the floor system. Not 24" away was vented soffit happily doing it's job of bringing fresh air in.
      We pressure fitted XPS foam in between all the 2x4 sleepers, what a difference that made on the IR camera!
      I suspect different HVAC guys will be asked to install a central return grill, and they'll block off the existing runs.
      Fiberglass is a fine air filter, don't ask more of it than that.
      CC

      1. MAsprayfoam | Mar 08, 2007 08:19pm | #6

        Thank you for the honesty with the FG insulation. I sell foam insulation, both Icynene and Corbond. I hate to hear about your situation and this reinforces my advice to people... put the money into the thermal envelope and that investment many other things else less expensive both short and long term .With a super insulated home your geothermal probably would have needed much less loops and dirt moving. I've had customers where the properly sized unit for foam vs. FG saved enough $$$ in the heating system to totally cover the cost of the foam insulation ungrade!Secondly, now all ducting in inside conditioned space if you do the walls and roofline all over the house. No insulation reqd. Even sealing is less important but cannot be ignored as your situation proves. Finally, your monthly costs will be low enough to pay for the incresed cost of the foam on the mortgage each month, forever! Lower total cost every month. How can you go wrong? Good luck with your situation.Stu

  2. DanH | Mar 08, 2007 02:30pm | #3

    Look at it this way: Other folks pay thousands for air exchange ventillation systems. You got one for free!

    (We've spend 30 years tighening our home. Tightened by at least a factor of two and maybe four. Sad to hear that new homes are still being built so "loose".)

    So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin



    Edited 3/8/2007 6:32 am by DanH

  3. MisterT | Mar 08, 2007 02:51pm | #4

    My question is why is the an open pathway for air from your floor up to your attic???

    no fire codes in your town??

    I would expect this with balloon framing

    but that was daumped long before the advent of open web floor trusses.

    methinks your builder has more slainin to do than your HVAC guy..

    JMNSHO...

    Wars of nations are fought to change maps.

     But wars of poverty are fought to map change.

    1. CleanCut | Mar 09, 2007 01:36am | #7

      It's like this...
      Set a shoe box on the floor, that's the first floor of your house. Now set a smaller kleenex box on top of the shoe box, that's your second floor. Run a roof line from the first floor walls at a pitch that covers the second floor without even touching the "top plate" of your second floor exterior wall. That's this house in a nutshell. There's attic all over... above, beside etc. I entered through the soffit and easily crawled over every room in the house. Air under pressure has no trouble reaching all areas of the house. And since the framers didn't sheet the sides of those open web rim joists, it's no trouble for the air to enter the floor system through the fiberglass filter.. er, insulation that was blown up next to the rim joist. Actually, the framers didn't even housewrap anything that was inside the roof. (If it doesn't get siding why would we put tyvek up? must have been their thinking)
      To clear something up. This is not my own house, nor one that I had anything to do with building. I was called in to remedy the heat loss.
      It's been a challenge, but priceless education.
      CC

      1. MisterT | Mar 09, 2007 05:47am | #8

        sounds like it was design for superinsulation and they dropped the ball on the heat..also sound like a DIY job...Wars of nations are fought to change maps.

         But wars of poverty are fought to map change.

  4. RobWes | Mar 08, 2007 07:06pm | #5

    On my own project they HVAC guys not only didn't seal the returns, they didn't seal the supplys.

    My basement was the coolest and driest place in the house, we only use duct for AC. I called them back and they sealed up all the joints. It's much better now. I missed the non insulated ducts in the bid. That will be my own project.

  5. danski0224 | Mar 09, 2007 06:23am | #9

    You get what you pay for.

    Unless duct sealing is in the specs, or the codes, it won't get done because it adds time (cost, lower profit- whatever catergory you want use) to the job. That is why California now requires duct testing on residential replacement jobs- because the builder/homeowner did not want to pay for it to be done right the first time.

    Duct sealing makes a difference and it is worth it. 

    Problems with duct chases not sealed should have been caught at inspection.

    Homeowners and architects never want to give up their can lights (nor do they want to pay for smaller ones, or engineer mechanicals properly in the first place), so the HVAC installers get to "make it fit"... Then the homeowner complains that there isn't any air coming from the 1" wide ductwork smashed to make room for the can light...

    Electricians do not "inadvertently" knock off panning- they do it on purpose. The decent sparkys will tell someone.

    Hopefully they installed the GEO correctly....

    To put duct leakage into perspective...

    A residential "5 ton" sytem is capable of moving 2000 cfm of air. If your ducts leak 20%, then that is almost the same as sending 400 cfm outdoors- equivalent to about 4 standard 6" supply runs. Attic return ducts that leak a mere 10% will reduce the capacity of your air conditioner by 30%.



    Edited 3/10/2007 10:46 am ET by danski0224

  6. Tim | Mar 13, 2007 08:59pm | #10

    Why don't residential installers seal ducts? At less than 2/10 ths an inch (of water column) of static pressure on either side of the fan, any economical level of duct sealing would be fairly useless. In others words, its not done because its not necessary and not economically viable. I tape transverse joints, but that's all. No logitudinal seams will ever be sealed in a residence.

    If one is trying to heat a house that has such poor construction ("no sheathing or air barrier on outside rim truss, fiberglass blown in insulation") that the minor leakage in a domestic air system can pull in air from the attic, then the problem is envelope contruction, not HVAC.  And a floor system "which led straight to the attic via open web floor trusses...."?!? 

    Problems in the HVAC system may be worsening this sieve of an envelope, but you seem to have identified a symptom, not a cause.

    1. DanH | Mar 13, 2007 11:02pm | #11

      I'd have to argue with you there. Keep in mind that with leaks in both supply and return the furnace can be drawing in cold attic air, heating it, then discharging it back into the attic, without changing the static pressure in the house, so the integrity of the house envelope may not even factor in.Or, probably more common, the dynamics of the situation combined with the greater length and number of connections makes the supply side leakier, producing a negative pressure in the house which turns any leak in the house envelope into an unpleasant "cold draft". The actualy heat loss in $$ may be insignificant, but the loss in comfort is substantial.And I've seen some pretty sizeable leaks in ductwork. Holes you can stick a pencil through are commonplace (though granted maybe not in your work), and larger leaks aren't uncommon.But on a broader view you're right -- more can be gained in the average house by sealing the building envelope, and in general the most effort should be applied where there's the most payback per $ of materials and labor.
      So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin

      1. danski0224 | Mar 14, 2007 12:48am | #12

        Duct leaks are a loss of efficiency.

        If the ductwork is installed in a basement, then *technically* the leaks are within the living space of the home. However, significant duct leakage removes air from the duct and the ability to control where it goes.

        I have sealed accessible duct and rooms that had little airflow before sealing had significant improvement after sealing. I'm talking about sealing maybe 70% of the duct because the rest is inaccessible.

        If the ductwork is in an attic, then that is a whole different ballgame. Minor leaks, especially in the return system, will have a major impact on the efficiency and capacity if the equipment. Cut a 3" hole in the return duct (simulating the sum of leaks in the duct run) and see what happens to the performance of the system. Put a 3" hole in the supply in snow country and watch the ice dams form.

        A leaky and expensive 18 SEER system may have the actual performance of a well sealed 13 SEER system.

        There is a little extra labor and expense to seal ess and drive connections and tape round pipe joints at the time of installation. Yes, it does take longer, and it does cost more. A whole lot less than trying to do it later. I have done it both ways.

        The benefits are well worth it. If you have seen typical slam and bam residential work, then you know all about the "quality" of installation. Commercial work outside of a school or hospital isn't any better than tract residential.

        Municipalities around here must agree, because taping all joints with UL listed foil tape is code in my area. Chicago code requires the sealing of all joints- even longitudinal seams on rectangular duct and round pipe- not just medium pressure commercial stuff. Manufacturers are releasing snaplock pipe with factory applied sealant (Ductmate Green Seal (Seam?) is one). California requires duct leakage testing (and repairs) when equipment is replaced to ensure it operates at the rated efficiency.

        There is a big write up on duct sealing in the latest issue of Snips magazine.

      2. Tim | Mar 14, 2007 08:44pm | #14

        I made an assumption, since unless I missed it, the unit is not installed in the attic, but in side the conditioned space somewhere. IF you choose to install equipment in or run duct through unconditioned spaces, my comments do not apply.

        I'm sure there are lots of installations out there that would make me believe that what you say is true. Hacks would not seal any better than the "tin".

        If the framing, insulation, sheathing and sheet metal in this house are obviously hosed, what kind of shape are the plumbing and electrical in? Pitiful, all around.

        1. DanH | Mar 14, 2007 08:52pm | #15

          Yeah, I would never buy a house around here with the furnace in unconditioned space, but they are built from time to time. And certainly quite common farther south, especially in areas where AC efficiency is the bigger issue.
          So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin

    2. CleanCut | Mar 14, 2007 02:13pm | #13

      Unfortunately, this leak is not in the 2/10ths of an inch range. There is so little pressure at the return air grills that the fan can't pull a piece of paper against it. Where is the air coming from if not through the return air ducts?
      Framer failed, yes. HVAC install failed, yes. Who's more responsible? Hard to tell, if either one had done their job right we wouldn't see as big of a problem.
      I looked at a return air run yesterday with a "gap" big enough to put a shoebox in. Today it's got sheetrock over it. That's a problem. Should I call in the GC, the HVAC installer and the homeowner? Or should we go the way of California and require duct testing? I say yes, we need the testing because none of the three listed above will ask for it.
      Ben

      1. Tim | Mar 14, 2007 08:58pm | #16

        0.2" is static pressure, not a measurement. The average residential furnace will produce about 0.4" inches of static pressure exclusive of the filter and the AC coil. Half of that will be appearant on the supply and half on the return in a relatively well balanced and properly designed system. Those descriptors do not apply to the mess you described.

      2. danski0224 | Mar 14, 2007 10:32pm | #17

        Some "gaps" are designed to allow the sheetrock to act as part of the ductwork. This is common when a return goes up a wall then travels along the floor joists above to the return on the upper or lower level. Sheet metal blockers are installed at the ends of the joist "ductwork". Therefore, some of the gaps between the metal ends may not be a problem. Installed properly, it functions adequately.

        However, sloppy workmanship resulting in big gaps around the panning or joist blockers is a problem.

        It is cheaper to use the framing as the ductwork compared to installing all metal pipe and fittings for return ductwork.

        The homeowner gets the HVAC system the builder allows the HVAC contractor to bid. The HVAC technicians must install the system in the timeframe allowed by the boss or they are looking for a new job.

        That's why duct sealing and leakage testing codes are being adopted by towns. It's also the reasoning behind the 13 SEER changes with air conditioners, and some places here are talking about requiring 90+% furnaces in new residential construction.

        Things will not improve for consumers until proper licenses and load calculations along with duct sizing are required for HVAC work. Then consumers/builders will be forced to pay for the work that they should be getting/delivering in the first place.... instead of taking the cheapest bid and then wondering why it doesn't work. 

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