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calcomp | Posted in General Questions on

I had a Manual J calculation, equipment sizing and duct design done by an energy mechanical engineer for my single family new construction. I have energy efficient windows and 2×6 exterior framing. The family room is a 2 story open concept. My energy consultant came up with a 1.5 ton heat pump ducted system upstairs and 3 ton heat pump ducted system downstairs (conditioned basement and unconditioned attic). Now I am trying to get quotes from HVAC contractors and I am getting all kinds of crazy tonnage quotes. Some is calling for two 5 ton system and some are saying my designer is wrong and some wants to oversize by 10%. This has caused me to question my designer but I had him recheck his numbers and he stood by his calculation. I am kinda stuck and afraid which contractor to use. Any suggestions or help would be greatly appreciated. Thank you.

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  1. Expert Member
    Dana Dorsett | | #1

    Trust the engineer who took the time to run the numbers- most HVAC contractors are total hacks when it comes to sizing things correctly- they rarely do the real math, for lack of time &/or skills. (That's why you hire an engineer.)

    If you have hard copy of the Manual J to show the contractor you can let them know up front that you won't accept proposals significantly bigger that specified, saving them time from having to take their own WAG based on chicken guts or crystal balls or whatever methods they use to come up with that crazy stuff.

    Don't let them bulldoze over you. If you run into a contractor with the express attitude, "I won't install anything smaller than xxx here" it's good to tell them straight out that they surely won't, since they won't be installing anything at all, saving you both a lot of time & frustration. It's an effective filter for getting rid of arrogant heating contractors that way! Many of them are often wrong, yet never in doubt. :-)

  2. Expert Member
    BILL WICHERS | | #2

    I will second what Dana said about trusting the engineer. At work, I AM the engineer, and I have contractors question me frequently. Usually it’s curioisty, but sometimes the contractor doesn’t understand the reason for doing something a particular way so they think it’s “wrong”.

    Contractors are professionals, but their profession is to install things properly. Code books are like their rule books and are full of rules that should work in most cases. The engineers job is to understand WHY things are done a certain way and to know when the code doesn’t apply, and how to do things safely in those cases that the code doesn’t cover.

    In the construction world, engineers can overrule contractors since engineers should have a better understanding of the “why”. I would still ask your contractor why they disagree with the engineer though as a sanity check (we engineers are not perfect after all :), and tell your engineer what the contractor says. As long as the engineer is able to address the contractors concerns, which may be a “your contractor doesn’t understand this particular case” type of response, you should be ok.


  3. calcomp | | #3

    Thanks for the reply. The contractor said they ran the same software (Wright suite) and did their own Manual J and S calculations. Even though both my engineer and the contractor ran the same software, there would still be that much of a discrepancy?

    I did found another contractor that said he will do whatever my engineer design.

    1. Expert Member
      BILL WICHERS | | #4

      Chances are your engineer was more thorough and precise than your contractor, but it’s also possible the engineer missed something. It’s worth a double check.

      The results produced by the software are only as good as the data that is input to it. If a wall of windows is missed, you’ll get different results. It takes some skill to do things properly which is why there are specialists in this field.


      1. Yupster | | #6

        In addition to what Bill said, I run Wrightsoft's software in our office and I can tell you that if I wanted it to give me the result of a 5 ton system when it really only need a 1.5 ton, I could do that with a few clicks. It's extremely easy to manipulate the results to be whatever you want. A contractor might do that because he doesn't trust the software results, he believes bigger is better, higher profit margins, bigger/more ductwork with bigger equipment so bigger job/$$$, etc. It's pretty difficult if you are being reasonably careful with your inputs to make a mistake that would result in differences of a few tons. I would go with your engineer on this one. If you want to give the engineers report a sanity check, just check the number of windows, orientation of the house (north, south, etc), window U-value and SHGC compared to what you are actually installing/building. These few things have some big effects on the load.

  4. Jon_R | | #5

    Make sure that eventually you have the full set - Manual J, S, T & D. Toss out any installers that won't provide their part (with you getting a copy).

    Be aware that some over-sizing (up to 40%, 15%, 25%, depending) is OK. Multi-speed helps.

    1. Expert Member
      Dana Dorsett | | #7

      I suspect the consulting engineer already ran those numbers and included some amount of oversize factor in the equipment recommendations. (That's why you hire an engineer for this, after all...) But DO look over the documentation and try to understand it.

  5. calcomp | | #8

    I will try to look it over but its all Greek to me.

  6. MattJF | | #9

    Go ahead and post what area of the country you are and some details about your house several members here will give good feedback.

    You can even post the manual J if you are up for that, blank out any personal info if you care.

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