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  1. #1
    Join Date
    Apr 2011

    Spider system - cooler air off one mixing box

    I have a couple of questions to help me wrap my head around my next steps.

    We have a large house, two zones, and apparently a spider system in the roof. There are three mixing boxes off the main HVAC unit, each mixing box has between 5-6 flexible ducts of either 4" or 6". I've finally pieced together the three zones and how they trunked it after many hours of looking at the system and fixed many issues (cut ducts amongst others).

    I have one end of my house which unfortunately is north and has a number of windows, and it's always cold. I think part of the problem is the amount of outdoor walls in the room and a large window along with cupboards and a bathroom. As a result it's always 2 degrees cooler in that room(s). It used to be more than 2 degrees but i've been trying to create more air in the room by putting in bigger registers and also making the ductwork curve cleaner into the register rather than coming at it in some very angular way causing air to only exit one part of the register. So i've had some success in improving it, but want to do more.

    So my questions are two fold.

    1) The "mixing box" I asume is sheet metal and it looks like it has some kind of silver insulation on it. It's not particularly good and it's certainly not thick. Is it worth trying further insulate the mixing box(s) so as to not lose heat in that chamber? If so, are there any suggested methods? I may do all three of them if this makes sense.

    2) There are three main trunks coming from the blower. They are a pretty good size and are all flexible insulated duct (like the rest of the ductwork). I notice however, that two of them (which happen to go to the ends of the house) are entwined with one laying on the other over a rafter before heading off in their required directions. The lower one looked slightly crushed to me, so the other day I raised and strapped the upper duct in case it was pinching the lower one. I think this helped air-flow. I also notice that when they put the duct in, they tend to raise them about 3' from the insulation and run them along rafters. Is there a reason for this?

    The reason i'm asking is I'm considering re-running the two ducts that feed either end of the house and that have the longest runs. I feel like the air pressure isn't so great, in part due to distance but also because they seem to make these large runs take a few turns and they could be more direct. Am I right in thinking less turns and less hard angles is better for air-flow?

    Any advice is helpful.

  2. #2
    Join Date
    Oct 2010
    Anderson, South Carolina, United States
    The type of duct system you have is not great for airflow. Flexible duct doesn't allow air to flow nearly as good as metal duct especially when the flex is twisted and snaked around every which way. You want your trunk to be as straight a run as possible with all branch lines equall distance from the trunk with all branches as short and straight as possible. If it were my house I would rip all of it out and install a properly sized, sealed and insulated metal trunk with balancing dampers on all branch lines to balance the flow required to each room per acca manual j room by room and manual d requirements.

  3. #3
    Join Date
    Apr 2011
    Ok thanks for the reply. I'm going to re-route the two trunks that feed each end of the house to make them straighter and less angular curves.

    Do I also need to consider wrapping the mixing boxes with insulation beyond what appears to be some kind of silver backed insulation on there today?

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