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A/C balancing with aeroseal and obd register swaps

nik_p | Posted in General Questions on

Hello everyone, I’ve gotten great information here while I was in western Mass and a job change has brought me to Tucson Az. My new 10 year old production house of 3900 sqft, has 2 floors and my upstairs office sometimes has a temp delta of 6 degrees to the hallway thermostat 10 feet down the hall. On hot days, upstairs will hit 78 with thermostat set at  73. There is a 3 ton 16seer unit for upstairs, 3.5 ton unit down. Mechanicals are in attic, natural gas for heat.  I finally found a place to get an energy audit that does blower door test and it was 4.5 ach50. They found a few warm walls with thermal cam on opposite side of house from office with a/c off. They used a lighter to show airflow from ceiling vents upstairs with a/c off. They said that was air leaks from attic and I needed to seal my vents. They said I needed to swap out my registers to obds to balance the system. Also said I needed to get an electrostatic filter to increase airflow from my MERV 13 regular filter.  There is a lot of airflow from my outlet covers on windy days. A previous a/c contractor taped over my fresh air intake to the return box, because the flap would bang continuously whenever the wind would gust. It’s Tucson, so that’s nightly. The energy auditor, read a/c company sales person, also said I should buy gaskets for all of my outlets. Big data dump there so here are my questions:
1. Will the registers actually help balance my system and “mix air better”? Other articles here suggest it’s not good to balance with registers.
2. Does the aeroseal make sense? Or is this smoke and mirrors with a stack effect? No airflow was noted at downstairs registers.
3. Is 4.5 getting tight where if I air seal attic and outlets, I need to consider manual ventilation? What sales guy told me.
4. Is the variable speed fan upgrade to the system at about 2.5k worth it for comfort/energy savings?
5. Are there any other suggestions to make the system more comfortable and energy efficient? There seems to be a lack of contractors here that are concerned with efficiency or thermodynamics.

Thanks for the help!

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  1. walta100 | | #1

    See if you can find someone with a “Duct Blaster” to test and repair your duct and burying them in insulation should be ok in you dry climate


  2. Expert Member
    PETER G ENGLE PE | | #2

    With one room behaving differently than the others, you start looking for differences between that one and the rest. The problem is either too much heat getting in or too little cold getting in. The heat in part is air sealing - between duct boots and ceiling, around outlet covers, etc. Lack of insulation in the walls is an unlikely issue, but certainly possible. Air leakage into/through the walls is far more likely in a production build. Fixing that sort of thing is DIY work with a caulking gun, gaskets and canned foam.

    The cold part is your A/C supply. A good HVAC company or energy commissioning company will have a flow hood that can be used to measure the airflow from the ducts. In a production build, anything can happen. Ducts folded back on themselves and kinked so bad that little air gets through. Ducts not connected to anything in the attic at one end or the other. A careful inspection from the attic side can often detect serious issues with ducts, especially with flex ducts. If this room is the farthest from the air handler, then additional duct insulation, duct sealing, and burying the ducts in insulation could help. As Walter mentions, a "duct blaster" test can help identify leaky ducts that rob the entire system of capacity.

    Again unlikely, but it might just be that there isn't enough supply in that room to deal with solar gain and gain through the walls. In that case, another duct from the main trunk to that room would help.

    I don't think a variable speed fan will help much, unless it is matched to a variable speed compressor. Operating the fan without the compressor just circulates warm(ish) air through the system.

    If rebalancing the system seems like a good option, I prefer to use adjustable dampers where the ducts come off of the main trunk rather than adjustable registers.

    You can also start with what should have been the beginning but rarely is: have a competent person perform a Manual J (heat loads), S (system sizing), D(ductwork design) and T (air distribution). Manual T is the one that tells you whether you need different diffusers/registers/grilles, but it can't be properly done before the others are done, so get the full suite. This should cost at least a few hundred $$ because it requires actual measurement and calculations. Most HVAC systems are designed by the installer sticking his thumb in the air and trying to measure the size of your wallet.

    This sort of thing is sometimes pretty easy to fix. Sometimes it takes a little bit more analysis and inspection. Sometimes you try the easy stuff first and if that doesn't work you have to dig in more. Sometimes you just learn to live with a ceiling fan running in very hot weather.

  3. Expert Member
    PETER G ENGLE PE | | #3

    I just noticed a few other things in the OP: You said the energy auditor showed air coming in from the attic around your vents. Was this while the blower door was running, or with blower door and HVAC all shut off? Because of stack effect, air usually goes out through 2nd floor ceiling holes, not in.

    Also: Is the office on the southwest corner of the house? Solar gain can easily overheat a room that otherwise works pretty well.

    And: From reading the post, I think you are providing two different data points about behavior. First, that the office 6 degrees hotter than the thermostat location. Second, the upstairs gets up to 78 when the t-stat is set at 73. My long post above is mostly about the delta T between office and t-stat, not the inability to hold the proper temperature. Inability to hold the proper temperature means either that the system is undersized (very rare), or there are unexpected heat gains from solar gain, ingestion of super hot attic air into the return side (common), air leaks from the attic (also common), etc. The Manual J will show if the system is undersized, and the whole-house blower door test should be able to tell you if there are major air leaks and where they are.

  4. nik_p | | #4

    Thanks for the replies.
    I haven't been able to find anyone that uses the duct blaster. All the contractors just want to aeroseal and be done. My "energy auditor" was a salesman.
    The air was coming out of the upstairs vents with a/c and blower door test off. He said that meant the air had to be comming from the attic through leaks. I figured it might have been through the stack effect, but wasn't sure. The office has a South and east exposure over an unconditioned garage. Figured it was just solar gain. The thermal cam did not show any specific hot spots. I may just have to keep the door open in the afternoon when not in meetings in the summer. I'm worried extra vents will cause it to be too cold in the mornings.
    The upstairs temp was on a very hot day (for me) 105ish. I think the downstairs thermostat may have been accidently set a degree warmer than upstairs, but didn't think it would have been that much of a difference.
    I will call around, but no one seems to be interested in doing those calculations around here. At least with real measurements.
    Maybe I'll try airsealing the attic floor seams with spray foam same with the outlet boxes(around not in the boxes). I also noticed can lights were installed and no insulation touching them. In MA they put foam boxes around them so was thinking of doing the same. The blown in insulation is pushed away from them right now.
    At what point do I need to add mechanical ventilation? A previous contractor blocked off the passive vent to the a/c return due to the flap banging.

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