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Pro-Generated Manual J Calculations

kwoolfsm | Posted in General Questions on

Good morning folks, I am located in Calgary Alberta. We have a new home under construction, through a home builder. It’s been difficult getting much support to make the home tighter, more efficient, beyond the basic builder expectations around building code, with the exception of using AeroBarrier a at the drywall stage (there only option).  We had some limited inputs with the HVAC contractor, but is limited by the contract with the builder.

After possession, my wife and I would like to look at heat pump installation.  There are a “FEW” HVAC contractors in the area that appear to have some experience with installations. I’ve reached out to a couple, limited response, with only vague equipment configuration offerings.    Given the climate, we are looking at the dual fuel systems as nat gas is the street connection.  I have run my own calculations using https://hvac.betterbuiltnw.com/ and came out with a VERY low number (heat load)  compared to the HVAC contractor who used WRIGHTSOFT.   My rough number was 54000 btu/hr, vs 98000 btu/hr which aligns with the furnace choices already offered.  Can anyone recommend a local to Alberta firm that would do/assist with a manual J to ensure a proper calculation.   Behind all this is out intention to use a heat pump for AC/HEAT as much as reasonably possible given the winter cold temps.  Eventually (2-3 years) we intend to install a full solar array to more fully offset the heat pumps electrical consumption.

Home Basics:
2700 ft above grade, 1100 below grade, typical 2×6 walls, batt insulation, insulated slab, roughed in for hydronic in the basement. Low-e – triple pane windows.

Thoughts? Helpful guidance here.  Thank you.

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Replies

  1. paul_wiedefeld | | #1

    I don't have a recommendation, but I'd trust the 54kBtu or lower based on how contractor calculations usually go. It's hard to run a manual J at this point, as the infiltration number is just a wild guess. However, with the intended hybrid set-up, an oversized furnace isn't the end of the world. Just make sure the heat pump is sized well and you'll easily be covered for ~90% of all heating: for example, if the true heat loss is 90% of 54000, then you'll just need about 36,000btu/hr at 0F, so a heat pump with those specs can cover a huge portion. And that's energy use, in terms of hours, the heat pump will be running even more, so the short cycling furnace shouldn't be too noticeable.

  2. Expert Member
    Akos | | #2

    A lot of equipment selection will come down to your local utility rates. In lot of places, heating with a properly sized cold climate heat pump is about the same cost as natural gas. If that is the case, going for dual fuel doesn't make much sense as it adds complexity and really limits the equipment you can get.

    As for sizing, builder installed HVAC equipment tends to be 2x to 3x oversized in a lot of cases. I would trust the betterbuiltnw tool. In Wrightsoft it is very easy for the HVAC contractor to fudge inputs to get the result they feel comfortable with. If you have the full report from them, post it here and we can see where the 96k came from.

    For a proper man J, you want to find a local HVAC engineer whose job it is to get the design right.

    1. kwoolfsm | | #3

      Thanks for the thoughts here folks.

      For context our fixed electricity rate is 5.99c/kwh, and nat gas is $4.09/gj. The local rates are growing higher, but I was able to lock in for 3 years last fall. I don't see the economics ever getting us to the return on capital investment, but my wife and I feel committed to contributing to the reduction of our gas usage. We've also taken into account an air conditioner would happen in this home, no matter what, so that portion of the equation is a wash. HVAC contractor work attached. Thanks again for the help here folks.

      1. Expert Member
        Akos | | #4

        Your natural gas is very cheap. Burned in a 90% furnace, that works out to 1.66c/kWh of heat.

        If you get a heat pump setup that can hit a yearly average COP of 3, that would get you 2.0c/kWh of heat, so not that far off. Going off gas completely can be made to work.

        I would recommend using the house as is for a season and see what your gas bill is like, than run through the calculations here to figure your actual heat loss:

        https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

        Once you have that, you can dial in exactly the size of heat pump you need.

        The nice thing is your 96k furnace has some pretty big ducting which means you can most likely swap it out for a right sized multi position air handler heat pump (Mitsubishi Zuba or equivalent Fujitsu/Daikin/Carrier unit) without any issues.

        The Man J is not the full report so it is hard to see what the exact issue is. Right off the batt, an insulated basement will never have 16000BTU heat loss. At best it is 1/2 to 1/3 of that number. My guess lot of the insulation details are off or the air leakage is too high (ie use the [email protected] instead Ach(Nat) ). Quick google says Calgary has a -15F design temp not -24F as in the report.

        1. paul_wiedefeld | | #5

          The 15,000 for humidification catches my eye too. The method Akos describes is great- if the designer is accurate, they should be able to match their previous manual Js to actual usage.

  3. kwoolfsm | | #6

    Slight update: recently had aerobarrier completed after drywall went up. ACH50 was 0.3, with the hvac sealed off, estimated to be 0.7 with hvac opened up. I’m still trying to find a local hvac engineer and re-do on the man-j. More to come, as we’ve moved into a short term rental awaiting the home to be completed, so it’s been hectic.

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