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Heating and cooling load calculations

user-5527914 | Posted in General Questions on

I wanted to get a 2nd opinion from the knowledgeable readers of this site about what an accurate heating/cooling calculation would be for my new house. Both programs I have used(BEOPT and RemDesign trial version) say 18-24,000 btu heating load and 12-14,000 cooling load. I made a spreadsheet and calculated about the same as well.

The builder has received the heating/cooling calculation from his HVAC sub-contractor with a heating loss of 35,000 btus. I was not shown the cooling calculations which I’m more worried about as I’m going with a Hydro-air system with a Phoenix Light duty water heater. All household ventilation will be provided by recoupaerator ERV with a range hood fan while cooking. Heating will be forced air with a fan coil with ECM blower fan. AC will be seer 13. My calculations show 1.5ton of cooling.

The house specs are as follows:
1232 sq ft on each floor(basement-1st-2nd)–all conditioned for this calculation.
8’8″ basement, 14″ floor truss, 9′ first floor, 14″ floor truss, 8′ 2nd floor
Energy heel roof truss with R-60 cellulose insulation

Walls are 2X6, 16″ on center with flash and batt(Proseal ECO and denim batts) with rain screen gap on outside of Tyvek. R16.5
Location- Milwaukee- 53154
2″ of exterior insulation at R-10,
4″ of exterior foundation insulation at R-20
2″ of sub slab insulation at R-10

Windows are at or above R-5( Wasco triple pane,LSGH, Passive house rated windows)- about 220 sq-ft
Patio door is Wasco Tilt-turn triple pane LSHG- r-5 at 40 sq-ft
Two other entrance doors are standard 36″ entrance doors- R-5 at 40 sq-ft

The builder’s most recent Focus on Energy houses have an average of .15 cfm50. Focus on Energy standard is .2. For my house, at .15, would be about 1ach50.

If I missed anything, please let me know, otherwise thanks for the help. This site has provided me a world of knowledge for building my house.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brad,
    It sounds like you have been conscientious. I think that your heating and cooling load calculations are much closer to the truth than the calculations of the HVAC contractor.

    -- Martin Holladay

  2. charlie_sullivan | | #2

    Yes, a good assumption would be that if they are not willing to show you the details, the details are not likely to hold up to scrutiny.

    It sounds like you are using XPS insulation on the foundation and exterior. If so, you might want to consider changing to EPS or graphite-infused EPS (aka neopor) because the gas used to blow the bubbles in XPS has two limitations: 1) It eventually diffuses out, and the R-value reverts to the same as EPS, and lower than Neopor, and 2) The gas has a huge greenhouse gas impact, >1000 times that of CO2.

  3. user-5574861 | | #3

    Brad,

    To give you a data point, the PHIUS climate specific peak heat load for Milwaukee is 4.9 BTU's / sf. So at 3,696 sf, that would be 18,110 BTU's. Look's like you have done a great job of spec'ing the components so I would expect your finished product to be at least a "pretty good house," and much better than code. Maybe not Passive House, which many believe is too much anyways. My guess would be that your calculation would be closer to your needs than what the HVAC contractor is saying. I would also guess that your HVAC contractor is sizing your system based on his typical BTU's/sf and not Manual J. If he is not willing to provide his work, then he probably did not do it. As you know, there is nothing proprietary about a Manual J and even if he did do one and does not allow anyone to check his work or his assumptions, then he could have easily made mistakes that you will have to live with, not him.

  4. user-5527914 | | #4

    Thanks for the information. I received the email yesterday from the builder so I haven't had time to request more details. I received the rescheck form with the heat loss in the notes. The insulation values were spot on except for the windows, which were a little low. The only data not on the paperwork I was sent was air infiltration and mechanical ventilation, which becomes a large percentage of the heat loss with higher levels of insulation. I will inquire this week. I just wanted to make sure I wasn't way off base.

    My other issue is the cooling calculations. Does 14000 but seem out of line?

    The rigid insulation I requested was a higher density EPS that has an r value about R5 at the temps when the heat loss is the greatest.

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