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Heating Load Calculations – adding ventilation loads – upsizing equipment?

orange_cat | Posted in General Questions on

I asked a question here :

Where for a 2,000 square feet (with 40% glazing, but decent u-value windows), the calculations were at 78MBH heating load. I used the program suggested to me in the prior thread and came up with my calculated 62 MBH heating load. I went back to the mechanical engineer asking them to recalculate (under the polite excuse that we have now ordered the windows so know exact u-values etc). They came back with 75 MBH. 

When asked as to why, I was told that the “design skin load” is 58 MBH (later revised to 61 MBH), but there is a ventilation load that brings it to 75 MBH.
I am attaching the revised calculations. I am having difficult time believing that a 2,000 square feet new build with above code insulation values requires 75 MBH heating.

There is no basement, it is slab on grade. There is unheated garage (but with same insulation as the house with extra insulated door as it it is intended to double as a workshop) and one room is above garage, there is no attick, and ceilings are under 9 feet on the lower floors and average around 10 feet at the top floor (roof slope).
Insulation values are 
Floor assembly slab on grade: R-10, 
Exterior walls: R-24 + R 7.5 CI
Floor R-30 +r10 CI or R 38 (different places)
Roof R-32 + R-10 CI.

Contractor purchased equipment delivering 58MBH. Upsizing to 75MBH at the moment is costly – but more to the point, is it even necessary?

I can see one way out of this stalemate – have someone else run the calculations for me. Ideally it would be someone in Ontario, Canada (familiar with the SB-12 and other local quirks should they matter). How do I find a mechanical engineer who would actually take the time to do it right?

Or even if your reaction is “no way you need 75 MBH for 2,000 sq feet in zone 5A” it would still be a data point…

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  1. Expert Member
    Akos | | #1

    What is the efficiency of your ERV/HRV? Looks like the heat load calcs are assuming no energy recovery thus the extra heat load.

    With a reasonable efficient ERV your ventilation heat load should be around 2000BTU. Not zero but not enough to need upsized equipment.

  2. orange_cat | | #2

    Sensible efficiency @64 CFM and 32 F 76
    Fan Airflow 80
    ESP inch 0.8
    Sensible efficiency @64 CFM and 32 F 58
    Fan Airflow 60
    ESP inch -

    Does it clarify? This is a bit over my head

    1. Expert Member
      Akos | | #3

      Those are above 60% to 75% efficiency depending on flow, lets use 70% for rough calcs. Looking at the design you have 85+42+42 CFM of ventillation, so your ventillation losses are:

      BTW, you heating design is still showing -10F, which is off for Toronto. OBC has it at -2F to 4F depending on how close to the lake you are. Lets use 0F for simplicity.

      (70F-0F) * 169CFM * 1.08 BTU/CFM *(1-70%)=3800BTU.

      The 169CFM is also WAY too much for a place your size, even if the units are capable of 170CFM, I would set them to run around 80 CFM to 100 CFM, so your ventilation losses would be around the 2000BTU I mentioned earlier.

      In either case, no need to upsize equipment.

      1. orange_cat | | #6

        Thank you!
        So two things I can ask the designer to "double check" is (1) the heating design temp.
        (2) the ERV btu?

        Thank you!

        1. orange_cat | | #7

          You know, using the software I used to calculate loads on my own - the temperature adjustment alone is 10,000 BTU difference holding all else the same. It is shocking.

          1. Expert Member
            Akos | | #8

            Using the wrong outdoor design temp is a pretty big thumb on the scale, common way for HVAC techs to skew Man J results to match the equipment they think should be installed. The other common one is including additional air leakage.

            This is less of an issue with a fuel burner but with a heat pump oversizing can create a efficiency problems.

            There is also no need for it, with a new build there are very few unknowns and a proper heat loss calculation accurately reflect building energy use.

            At the end of the day, right sized equipment (fuel burner or not) will be cheaper, quieter and provide better comfort.

  3. walta100 | | #4

    I hope it is a spectacular view given what it is going to cost to heat that wall of glass for the next 20 years.

    Don’t be stingy post a picture.

    We have to assume this building makes a statement and the cost to build and operate this folly are inconsequential in your world. I don’t see the point in arguing with the engineers for smaller equipment it is going to be expensive to build and operate. If you want to make this statement you need to resign yourself to the fact it will be expensive.

    If the costs do matter limit the glass to 15% of the walls.


  4. orange_cat | | #5

    The glass is for light and privacy, not views. Tight urban lot.
    The building has been framed and windows have been installed.

    But I can still size up mechanicals appropriately.
    Money matters -which is why insulation is above code and windows have respectable u-values. I am trying to keep operating costs within reason. Plus on a tight urban lot a giant air to air heat pump is going to be too noisy too. - or two of them side by side.

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