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Community and Q&A

Lucky twice guessing heat/cooling load?

mikeolder | Posted in General Questions on

Hello folks.

When I changed out the furnace and AC of my 70’s split foyer, I guessed down from a 4 ton to a 3 ton by how it short cycled.  I also dropped down from 100k 80% to a 75k btu infinity furnace. Both capacity’s must be barely enough and I like how it runs and the comfort, but I lucked out because Ive installed many..  

But I’m going to build a new smaller home in a county with no code enforcement, and starting from scratch requires a load calculation because I want it to perform like my split system at home..

Ive studied manual J 30 years ago, but is there a simplified online version?

A little off topic, but can you do a reverse load calc?  Meaning, we know the smallest AC capacities are 5000 btu and 9000 btu etc.   Using “Pretty Good House 2.0” construction practices, what size tiny house would they barely cool to 71° in zone 5 iowa?  


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  1. Expert Member
    Akos | | #1

    In zone 5 your hvac design will be driven by heating. Any heat pump that can heat your place, will have plenty (generally way too much) capacity to cool it.

    You can do a rough calc based on areas of walls, roof and windows.

    Generally the surface area of walls tend to about about the square footage of the house.

    Heat loss through a surface (in F and BTU):

    (indoor temp-outdoor design temp)*area/(assembly R value).

    So a 2000sqft PGH (R20/40/60) with 0F outdoor design temp with 20% (high but makes the math easy) R4 glazing.

    Walls: (70-0)*2000/40=3500BTU
    Roof: (70)*2000/60=2300BTU
    Windows: 70*(2000*0.2)/4=7000BTU
    Slab ~1500BTU

    Add in some air leaks, HRV losses and take away internal gains (people, appliances, solar gain), you are probably looking at 15000 BTU.

    You can also run the place through one of the online calculators (loadcalc or coolcalc). Accuracy is not always the best, but a good start.

  2. mikeolder | | #2

    Thanks Akos.. Great information.

    I wont over size my air conditioner due to my heating load, and still skeptical that mini-splits can compete with propane efficiency.. Definitely not serviceability.

    But I'm considering down sizing to as little as 700 sq' from my existing 1700 which my 3 ton split pulls down 1/2° per hour. I usually run it manually when I come home and it may run 10 hours non stop to pull it down to 72° with one cycle. I want my new system to run similar (dry), as I was taught and agree a AC system is perfectly sized if it holds 72° on the hottest 100° day running non stop. My scenario, one ton of cooling was required for 567 sq'. Your scenario one ton of cooling is required for 1600 sq' if I'm not mistaken.. That sounds optimistic but also shows what upgrading from a 1970's 2x4 home to a 2020 2x6 PGH home can do. And if I go by your numbers, my cooling load for 700 sq' is 5250 btu. I think the best approach for me will be to modify a 5000 btu window air so that it's ducted into the home. A little undersized, but I think it might work because the home will sit under a large canopy and never see direct sunlight. And if I'm wrong, it would be easy and cheap it upgrade and replace..

  3. Jon_R | | #3

    > you are probably looking at 15000 BTU

    Note that a kitchen hood alone could significantly exceed this.

    1. Expert Member
      Peter Engle | | #5

      But only if the makeup air thoroughly mixes with the house air before going out through the hood. We've discussed inserting MUA close to the range so that you get a higher % of MUA going out the vent to reduce its effect on the overall heating and cooling. Plus, the range hood doesn't run all the time.

  4. Expert Member
    Akos | | #4

    I think the big change since the 70's has been air tight assemblies and dual pane windows with lowE coatings.

    The combination of these, plus good designing with window orientation and shading in mind, has significantly reduced cooling load, see the graph here:
    I comfortably cool 11000sqft with largish west facing window with a 12000BTU wallmount.

    I'm not sure a window shaker is the best, they are loud and not very efficient. Because most don't modulate, they are not very good humidity control during shoulder season. Mini splits have gotten so cheap that is just not worth while to put up with that.

    When it comes to maintenance, I would say a heat pump is much less maintenance and very little extra install cost vs AC only. Besides the filter change, there is not much more to do. Plus you don't need to buy and install a propane tank.

    As for kitchen hoods, most houses have enough thermal mass that the infiltration load from the hood airflow won't significantly budge the house temperature. Maybe if you are cooking sugo for 6 hours while running a commercial hood, you might have issues.

    1. mikeolder | | #6

      Thanks again..


      Was 11000 sq' a typo? Are we talking floor sq footage with 8' ceiling average?

      Summer comfort levels and run times continue to climb, as the tonnage goes down until you don't have enough capacity..

      A window shaker mounted in a window is out of the question. I'm talking about ducting it out the top with a transition (hack) so it sits isolated from the structure. Its a easy day project and cheap, but if I found a variable speed 5000 btu mini split, I would be tempted but their all heat pumps with the reversing valve I don't need or want.

      And I mentioned serviceability, not maintenance. I can change out many more, and mix and match parts with a furnace as long as the heat exchanger is good.
      How economical is it to change out a blower on a mini split compared to a furnace? Is the cost of replacing a reversing valve comparable to replacing a limit switch or gas valve of a furnace? How accessible are mini split parts? I can still get parts for my 18 year old carrier.

      Good points about a kitchen hood, but a kitchen wont be in this structure only a bathroom.. I'm building a tiny house village under a huge canopy roof. There's a existing mobile home to cook in now, which will remain as long as our children keep hanging around.


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