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do i need a third-party HVAC designer to review the calculations done by my HVAC contractor?

beedigs | Posted in General Questions on

Hi.  We are building a one-story all-brick home in CZ 2 that we are hoping to be a “tight home”.  We chose Zip R3 sheathing and Zip roof deck, with open-cell spray foam cavity wall insulation and under the roof deck to achieve a conditioned attic. For mechanicals we are going with a variable speed unit with a dehumidifier and an ERV.  This is an all-electric home as well.

We found our HVAC contractor through the Mitsubishi website of certified contractors with their designated “badge” that assures customers of their expertise and received training.  We do like our HVAC contractor but wondering if we should still shell out some budget to have a third-party HVAC designer review our HVAC guy’s calculations (manual J, S, T, D, etc)?   Just for peace of mind, as I understand when we are trying to build a tighter home that the HVAC performance is very critical to making it all work and hopefully lessen the chances of mold issues and such.  We have been having so much more expenses due to increase in prices left and right with every trade that it’s eating into our contingency, but then if this is a critical step to ensure good indoor air quality and comfort we will probably take that step.  Thanks in advance.

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  1. walta100 | | #1

    The truth is most of the HVAC contractors have done enough of the run of the mill houses that they have a rule of thumb that get them close enough for them to end up with working systems.

    If you build the same code minimum house with the HVAC in the attic and 4 inches of spray foam on the rafters(or whatever the local norm is) the rule of thumb will work just fine. If you choose to be different and keep the duct work and equipment below an R60 ceiling you will cut the electric bill in half but you will need a manual J to get the correct size equipment.


    1. bLu2021 | | #8

      We’ll have 3.5” of open cell spray foam cavity, plus the R3-something from the Zip R3 for the walls, plus 5.5” under the roof deck.

  2. kyle_r | | #2

    In a cooling dominated climate with a conditioned attic and a dehumidifier, I can’t see too many opportunities for major errors. I doubt you will see a noticeable improvement in comfort or efficiency by having it designed by a third party, at least not enough to justify the cost. If your contractor is reputable I would trust them.

  3. chicagofarbs | | #3

    A Mitsubishi rep would probably review their calculations for free and confirm system sizing.

    I have provided load calcs to Mitsubishi in the past to have them size the systems and it is a free service.

    1. bLu2021 | | #9

      thanks for the tip!

  4. Expert Member
    ARMANDO COBO | | #4

    So here is REALITY!!! I recently designed a 5k sf house that the homeowner declined to spend $2,500 to have the HVAC system designed by an independent ME.

    My Specs: CZ 3, Conditioned 5k sf house, 2x6 @ 24 o.c. walls with R20 DP cellulose + 1” R6.7 outsulation and R49 ventilated attic.. 882 sf of glazing (U=.28, SHGC=.25). 2-2 ton AC and 2-10HSPF units with all metal ducts in conditioned space with furr-down hallways for ducts and closets for equipment. 2 ERVs. Energy Star appliances and lighting. 1 HPWH, 65 gal. with an on-demand recirculating pump. Goals of 1ACH50 and HERS ≤45 w/o PV. 100% high-performing house. Plenty of details and specifications on drawings.

    HVAC contractor Summary: Conditioned 5k sf house, 2x4 @ 16” o.c. walls with R13 insulation, and R38 attic. 1245 sf of glazing (U=.35, SHGC=.50). Ventilated attic. A 50 gal gas WH, 58% EF. Nothing on appliances, lighting, plumbing, building tightness nor HERS rating.

    HVAC contractor bid: 3 HVAC gas systems with a 10 ton. total (3,3 &4 tons). Flex ducts in attic, no ERVs. All for $24,375.00. Take a guess how much discomfort and upfront money my client is wasting, and for the rest of that house’s lifetime.

    Learn from experience, these cases are more the norm than the exception… I just wonder if the HVA contractor looked at the drawings and read any of the specs, or he just looked at the square footage and figure it from there... Hummm, you can't fix stupid!

    1. chicagofarbs | | #5

      Oof that’s brutal.

    2. Expert Member
      ARMANDO COBO | | #6

      ... Ahhh, but the best part is that the Builder picked this contract because "it was the best". Really????? This happens 9 out of 10 times everywhere you build homes. Code enforcement is non-existent, HERS raters skip by it, NAHB, ACCA, RESNET, etc. are blind and deaf, and Builders are mostly ignorant.

      1. chicagofarbs | | #7

        With you, 100%.

        Original poster seems to have some basic knowledge to at least check the envelope assumptions used by the HVAC contractor.

        That would at least be a start, at least. If the assumptions seem right, I’d have Mitsubishi double check the sizing. I’ve used Michael Schaefer in the past for heat pump sizing.

        1. bLu2021 | | #10

          so perhaps ask for a copy of manual J from the HVAC contractor?

          1. chicagofarbs | | #11

            Yep! I’d ask for a copy of their calcs/report and all their assumptions if they aren’t in that report.

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