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  1. #1

    return from one system into another...

    Hi Folks,

    As we're in the super hot part of the summer I'm sure a lot of people are finding that their AC can't really keep up. I can see that yesterday my upstairs AC unit ran solidly from 4pm until almost 10pm before it started to cycle again, but the downstairs unit maxed out at less than 80% duty cycle during the same period. And this is with the thermostat upstairs set 2 degrees above the one downstairs. I know that this is normal to a point, more heat gain upstairs but I get to thinking of solutions

    The downstairs return is in a joist space that runs open up to the second floor. It shouldn't be that big a project to extend the return up to the second floor and put in a second smaller filter and return duct there. I'm sure the guys that maintain my AC would be happy to do that. But would it help any? Or would it just make the downstairs that much less comfortable? The 2 areas aren't isolated both returns are basically in the 2 story open living room, but one on the left side on the second floor, and one on the right side on the 1st floor.

    Is this a good idea or a bad idea or don't you think it would make any appreciable difference at all?

    Thanks!

  2. #2
    Join Date
    Sep 2005
    Location
    Bay Area, CA
    Posts
    843
    If the goal is to be more energy efficent then no that wont help.
    Global Warming or: None like it hot
    No pleasure, no rapture, no exquisite sin greater... than central air. -Dogma

  3. #3
    In this case I'm wishing for comfort for the upstairs which simply can't keep up with the heat load on these 95 degree afternoons. I understand that will just place a portion of the load then on the downstairs unit which will cause it to work harder and use more energy. I pay for the comfort, if it's going to run then I would like it to actually work. A little more runtime on the downstairs unit I'd be willing to pay for if it helped.

  4. #4
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by jfs1138 View Post
    In this case I'm wishing for comfort for the upstairs which simply can't keep up with the heat load on these 95 degree afternoons. I understand that will just place a portion of the load then on the downstairs unit which will cause it to work harder and use more energy. I pay for the comfort, if it's going to run then I would like it to actually work. A little more runtime on the downstairs unit I'd be willing to pay for if it helped.
    Just a homeowner here, but I'm sure a PRO will be along to comment.

    Since the downstairs unit's ductwork is not feeding the upstairs, and cold air tends to fall, not rise, it likely would not be helpful. By expanding the return to the upstairs, it might possibly detract from your comfort downstairs.

    Amp

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,069
    It should take some of the load off of the second floor system.

    Weather or not it will. Will depend if your short on return on the first floor or not.

    If your not, you would need a damper to restrict the current first floor return. So the it draws from the new return also.
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  6. #6
    Join Date
    Mar 2005
    Location
    5,000 ft
    Posts
    2,192
    The r/a that is made via joist is bad news. They leak to the outside, communicating through the walls then to attic and crawl. You can never seal them.

    Many have other untilies passing through them such as water lines and other plumbing which = more penetrations usullly not sealed.
    If you can get your contractor to review if you can get a ducted r/a.

  7. #7
    Join Date
    Sep 2005
    Location
    Bay Area, CA
    Posts
    843
    I would be more concerned about making sure your upstairs system is running properly and at maximum capacity, ducts sealed, ducts sized properly, air conditioning unit working properly etc.

    Once all of that is verified to be working then you can think about something else, or have a load calc done and then you'd know your upstairs is undersized or your upstairs is malfunctioning.

    I would put money on leaky and undersized ductwork if it was put in by the builder.
    Global Warming or: None like it hot
    No pleasure, no rapture, no exquisite sin greater... than central air. -Dogma

  8. #8
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,842
    Is the up stairs unit installed in the attic, second floor closet, first floor utility or basement? Is the attic space above the second floor area properly ventilated.? Prperly insulated. If ducting is installed in the attic space, is it properly insulated to minimize heat gain. Power attic fan? How old is the equipment? Has it always had trouble keeping up? Has it been checked and serviced by a professional company recently?
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  9. #9
    hi folks, I do really appreciate the comments. Equipment has been regularly serviced up until this year when they upped their price and I know it's getting towards the end of it's life anyway. It's going on 7 or 8 years old I believe. It has not degraded in it's performance in any noticeable way, it's always been unable to keep up in the dead of summer. THe ductwork is in the attic, the attic is properly vented (or at least vented) and insulated but the temperature up there will easily exceed 110 on these difficult days. The ductwork is rigid and insulated, but almost certainly leaky to a certain extent. If anything the performance of the system has improved as I've had it serviced and have moved the thermostat from the hallway to a bedroom, since it's the bedrooms I was interested in cooling after all. I have also added a second layer of attic insulation a couple of years ago as well as doubled the insulation on the attic walls into one of the problem bedrooms. Each of these things has improved it a little bit, but I fear the system at the beginning was just not up to the heat gain. But if it was upsized then it was probably going to be oversized for the bulk of the season and I know that causes trouble too.

    So I guess I'm just looking to extend it's usefulness another season. We wanted to do a small addition on that end of the house and really dont want to put in a new system only to have to put in another new system a year later to be large enough for the new room. I think that a 2 stage is probably in the cards, but with things being what they are we've gotten scared to take out the money for the new room and are waiting a year to see what happens.

    If I did have the duct extended upstairs I'd make sure the joist space was panned in or whatever the term is. It really is just in the wall downstairs so even if they need to cut the entire joist space of drywall out to install it it's still going to be a minimal amount of repair work after the fact.

    Thanks folks, I think I've got enough info and questions to bring it to the HVAC company here and see if they can do it for a reasonable price and think it would do any good at all.

    Thanks again,

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