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Community and Q&A

High Performance Home HVAC Needs Balancing, Contractor Limitations

mpsterner | Posted in Mechanicals on

Hello all,
I built a high performance home in Zone 7 Northern Wisconsin, finishing it in summer of 2022. 

We installed a Mitsubishi SVZ-KP30NA Air Handler and a SUZ-KA30NAHZ condensing outdoor unit. The air handler is ducted throughout the house with a duct plan provided by a 3rd party mechanical design firm. 

I have Renewaire EV-L Premium ducted into the air return for fresh air distribution throughout the home and then dedicated exhaust. 

Here’s what’s going on: 
1. The main floor is fine, even with many registers closed. The temperature is very equal since it is such an insulated house. The upstairs is too warm in the summer and winter and never seems “fresh enough.” The air is stale. When you put your hand over ducts upstairs, it seems like almost no air is coming out. The contractor acknowledged upon completion that “it was hard to get the air up there” but of course, I didn’t know what that meant at the time. 

3. We’re over-ventilating with the ERV (see, but I don’t want to dial it back yet until the overall HVAC is balanced since fresh air is carried via the same ductwork. 

I’ve tried dialing back the balancing dampers a little bit and closing off registers to get more air to the upstairs. That doesn’t seem to make a big difference. 
I don’t have confidence that the original contractor can get this balanced properly given that they were puzzled at the original commissioning. 
The ductwork is generally pretty close to the design. There are some cases where there is some “can of worms” arrangements going on. See attached for examples. There are a few additional bends, turns and such, but it isn’t excessive. 
They didn’t follow the register and boot sizes–nearly everything is a 2×10. The plan had called for different sizes all over the place. 
Originally they didn’t even put balancing dampers in. I required them to add them. They installed most balancing dampers near the boot in most cases though and not near where the duct breaks off from the trunk. 
They added (2) 90s and then 8’x2 at the end of the entire system run (the upstairs bedrooms) where the registers were supposed to be right off the trunk with no additional lengths. So that didn’t follow plan. 

What should I look at? 
– Just play with balancing and dialing back vents that are closer to the air handler to push more air? I’ve tried this a little bit and hasn’t seemed to make a big difference. 
– Duct tightness/leakage–should I go over all of their joints and seal better?
– Eliminate duct and branches in rooms that haven’t had any temp control issues? For example, our main space has 6 registers and I could probably close a bunch of them and have no heat/cool issues. 
– Move balancing dampers closer to the trunk?

I admittedly don’t know a lot about HVAC, however, I did build the entire house myself and am generally handy. 

Do it myself, find a contractor, work with someone from afar?
– I don’t have any experts in my area but there are people who have vent hoods and a general knowledge. 
– I dont’ have the equipment but if the best solution is to learn this myself, I will. 
– I did make contact with an expert several hours away (who writes for Fine Homebuilding) that said they could come and help, but they wouldn’t be a long-term contractor solution. 

Thank you so much for your help, as always!

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  1. kyle_r | | #1

    Is the air handler fan running in auto? Have you tried setting it manually at high speed?

    Did you try changing the programming for the static pressure?

    1. mpsterner | | #5

      Kyle, I've tried running the fan on both auto and setting manually at high speed. Setting it at high speed seems to get a little more air to the most distant registers, but not enough. It also then increases the sound substantially throughout the house and makes other ducts blow a ton of air.

      I haven't done anything with static pressure programming.

      1. kyle_r | | #6
  2. Expert Member
    DCcontrarian | | #2

    It sounds like the ductwork to the upstairs is undersized. There's diminishing returns to closing off the downstairs, what you probably need is to boost the fan. The first thing to find out is whether it has multiple speeds and whether it's on the highest if it does. From there, there are ways to add booster fans.

    Upstairs too hot in winter has me a little puzzled though. In a high-performance house you tend not to get much stratification. Is there a lot of solar gain?

    1. mpsterner | | #9

      Hi DC,
      I've gone through the HVAC to figure out what is in line with the plans and what is not.

      The main trunk is basically the size the plan called for. The trunk to the 2nd floor (where I have the problem) is an 8" square instead of 8" round.

      Beyond that, at the end of the line where the 2 upstairs bedrooms are (and the most significant problem), there are (2) hard 90s instead of angled take-offs and then a 6' run and a 10' run to the duct boots where they should have just been right off the main trunk. So, some additional distance.

      Through the whole system though, there are a number of things that are different. Nothing major, but just extra 45s and 90s here and there. There are some locations where duct work is larger than called for, but not really smaller. There is (1) 180° turn that is really weird and some additional flex pipes where it should have been rigid.

      Overall, can those issues closer to the air handler cause excessive drag in the system, resulting in the relatively accurate ductwork of the 2nd floor just never getting enough air?

      - Would moving balancing dampers closer to trunks instead of boot/register make any difference?
      - Would sealing ducts better make a difference?

      I've ordered an anemometer per Akos' suggestion below and will get measurements to start diagnosing, then start cutting ducts closer to the air handler to see if I can get more air up there.

  3. Expert Member
    Akos | | #3

    I think you are on the right track by looking for a tech with a flow hood.. First step is to figure out what the register flow rates are. A flow hood is good, you can also get close enough results by making your own. You can get a 4x14x6 straight boot with a 6" coupler to extend the round section a bit. A budget anemometer from your favorite online retailer for a couple of dollars and use this get a rough ballpark flow rate. You want the larger boot opening so it fully covers your register and the small outlet so there is high enough velocity to get good flow readings.

    Set your air handler to fan only, high speed and measure all flows.

    Once you have the flow rate you can compare them to the HVAC design plans and see where things need to be adjusted. Usually you end up closing down the damper on the register closer to the air handler to reduce the flow there to design values which increases the pressure in the trunk and the air flow to the longer runs.

    If this doesn't get your upstairs better, you might have to do some duct tweaking or install an in-line duct booster fan on the trunk to the 2nd floor to improve air flow.

    The rigid to flex joint there is pretty ugly, I hope that is a one off. That joint alone would probably cut the airflow to the branch by about 2/3.

    The Mitsubishi units have a configurable external static pressure setting in the installer menu. Check that it matches the pressure across the air handler once you are done with adjustments.

    This type of adjustment can be a bit iterative, but the first round will get you 75% of the way of where you need to be.

    1. mpsterner | | #7

      Thanks Akos. Per some above comments, I am doing a detailed audit of the ductwork to start. What is "to the plan" and what is not.

      As far as flow hood testing. Since this is going to be pretty iterative, I am wondering if I wouldn't be better served by getting the equipment myself and learning this process. Does that seem feasible? Are there any hoods on the market for $500 or less that I can get away with or do I have to drop some serious $$$?

      I am concerned that I'll have a tech out and then have to change a bunch of ductwork, have them back out, etc, etc.

      For this: "The Mitsubishi units have a configurable external static pressure setting in the installer menu. Check that it matches the pressure across the air handler once you are done with adjustments."
      - Will I need any special equipment beyond the manometer that I already purchased for the ERV work?

      Thank you!

      1. Expert Member
        Akos | | #12

        You don't need anything fancy for flow measurement. My suggestion above of a register boot + coupler and a budget anemometer works well enough. Pretty much what I use except for a pitot tube based flow meter. Have one with a 6" outlet for higher flows and one with a 4" outlet for lower flows. Gets the results close enough to not matter.

        Fore measuring the air handler pressure, the manometer you have should work if it can read up to 0.8" WG or so. To do the measurement, you need to connect it a small hole drilled between the air filter and the air handler and one on the supply plenum just after the air handler. You connect the gauge to both ports to measure the delta across the unit.

        1. mpsterner | | #13

          Hey Akos,
          I received my anemometer today. Quick question, it has reading in ft/min and such, but what is the calculation I need to use to get that into CFM?

          There must be a volume component to the boot size or something to take the ft/min into CFM...

          I have to make a couple different custom boots to get my under cabinet ducts read, so I just want to make sure I understand the math here. Thanks!

          1. Expert Member
            Akos | | #14

            FPM*area in ft^3 = airflow in CFM.

            So assuming 500FPM, 6" round duct (28sqin, or 0.19sqf) = 95CFM.

            For a buried register like under cabinet, you can hold the anemometer above the register and estimate the airflow based on the FPM reading and the open area of the register.

            So for example in case of a 2"x9.5" register opening(0.13sqft area) a 380fpm reading is about 50CFM. Not as accurate as with a hood, but close enough for a starting point.

  4. user-5946022 | | #4

    Not sure what we are looking at in the photo, but I see multiple issues just in that photo:
    1. Looks like the flex takes a 180?? That never works.
    2. The inner duct of the flex is visible at the bottom and the fiberglass insulation is pulled up and exposed just above the attachment to the hard boot. There is all sorts of exposed fiberglass on the other end. The fiberglass insulation should be sealed tight around the rigid boot.
    3. The inner duct of the flex is attached to the metal boot with a zip tie. This is standard, but there should also be tape to reduce leakage.
    4. At the rigid to rigid connections, there should be mastic, not tape

  5. walta100 | | #8

    It seems to my you did the math and have a plan for how much air you need to deliver to each room. Since you know the duct size all you need to do is measure the speed of the air flowing in the duct with an anemometer. It is a $30 tool on Amazon. Then you can decide if the fault in in your design or in your ductwork.


  6. deucevantage | | #10

    Am curious, did you have an experienced person do a Manual J,D and S ? If so why didn’t the contractor follow them? If not, it’s all guesswork, even in hands of experienced HVAC contractor. Proper HVAC is so hard even with accurate manual j, d and s, with i joists and flex duct the norm. And who has 10k to get a full mechanical design ?

    1. mpsterner | | #11

      Hi deucevantage,
      Yes, I had a high performance home mechanical design firm create my plans.

      While I wish it were a surprise, what I've found in my rural area is that most contractors do things "the way they've always been done / the way they were taught." Many are/were willing to try new things, follow plans at the outset and when the job was sold, but as time passes and they realize that they really have to follow the plan (especially when that is inconvenient for them or they estimated the job wrong because they didn't look close enough) and following the plan can be more work.

      It's infinitely frustrating but I've run into this countless times.

      While incredibly clear on the plans, our HVAC contractor once told me that "no one puts in balancing dampers around here, you just use your diffuser/register for adjustments." That was a real red flag. They ended up essentially being willing to do some stuff and kind of just checking out because they were so tired of being held to the plans by me. I also became tired of hounding them.

      Unfortunately, my entire house build was a battle to get outstanding work from a building culture that is all about "good enough." I am proud to say though that while it nearly killed me, it did turn out incredibly well. Just some things to iron out still, like this HVAC.

      Sorry for the novel!

  7. Jason_K | | #15

    "Unfortunately, my entire house build was a battle to get outstanding work from a building culture that is all about "good enough." I am proud to say though that while it nearly killed me, it did turn out incredibly well"

    I've had the same issue here in middle TN. Can very much relate to the "they were so tired of being held to the plans by me. I also became tired of hounding them".

    Glad to hear that you were largely successful in the end!

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