GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Heating/Cooling Loads – Oversized and not correct?

tweeterisu | Posted in Mechanicals on

Hi everyone!
I am finally getting ready to dig and got back my system sizing and I have a few questions as it seems over sized and what homes normally receive without this amount of insulation and product.
What first led me to believe something looked off was that my heating load was higher than my cooling as well as the sqft area of the rooms were not looking correct.

If you wouldn’t mind taking a quick look and helping me understand if it seems correct or that I should have a conversation with the individual that I paid to size my system and perform the duct work design.

I am building in Ankeny, Iowa.
I am using the Hobbs VICF for basement, main floor and garage (R30)
2.5″ EPS underslab insulation
Zola Alum Clad windows U value 0.09 BTU/hr/sqft and an SHGC of 0.54
R60 Blown in Cellulose in the Attic
ComfoAir 550

Any insight is greatly appreciated.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Stu Turner | | #1

    Assuming this is fairly tight construction, the heating load seems to be in the right neighborhood. The individual room sizes don't factor in to the overall heating demand, so as long as your conditioned space on your approved plans is 5365 sq ft, then that shouldn't matter. I am building a 2200 sq ft house with similar R values and a design temp of -3F, and my heating load is proportional to yours (@ 19K btu/hr). I am not in a cooling climate so I can't speak to your AC loads.

  2. tweeterisu | | #2

    Thanks Stu! My sq ft for the house is not 5365 and is actually right at 5000. I was going through some of the rooms that were calculated and the area did not match actuals on the paper - for example LL Exercise Room is 12'1" x 15' 1 1/2" which is around 184 but the sheet says 223. That's where I get nervous.

  3. Stu Turner | | #3

    So it would seem 10%, maybe even 20% high to me... again since you have an HRV and very high performance, expensive windows, I am assuming you are going to build a tight envelope and check your results with a blower door test (~1.0 ACH50). Its a little late in the game, but i would bring in an energy consultant to create your own calculations, it would be a good investment considering the cost of the HVAC equipt and the annual energy use this decision will result in. Check the RESNET site for someone nearby. I would not rely on an HVAC sub to do all your energy calcs for you.

  4. dickrussell | | #4

    By all means, do your own calculations, based on what you know the dimensions and exterior shell construction to be, or have some knowledgeable do it for you. A good heat loss/gain model for a high-performance house cannot be done quickly, using any rule of thumb approximations. It takes hours to get the details right.

    For my own house, three separate installers of the heat pump equipment came up with similar results, calling for a 5-ton unit, while ultimately I installed a 2-ton unit, and which it turns out keeps the house at temperature without upstaging. My best calculation of design heat loss is 19,000 BTU/hr for a design minimum of -3 (central NH), for a gross square footage of almost 4,000 sqft. The house construction is in general very tight, R-40 walls, R-60 attic floor, triple pane windows typically U=0.17, R-20 subslab and foundation walls. For cooling load, which typically in NH is apt to be less than what Iowa will see, I figure worst case 72 F dew point and 95 F outside temp. The load was under one ton, and in practice the house is kept at temp and RH in summer while the heat pump loafs in first stage.

    You are right to suspect oversizing your needs for HVAC. Do your own homework to either verify the calcs already done or to show with a fair amount of certainty that something smaller is correct. You won't want oversized cooling equipment in summer unless you have provision for dehumidification, or you will wind up with that cold, clammy result from oversized cooling equipment.

  5. wjrobinson | | #5

    Sounds like you are getting some good help here, next I second the idea of hiring an energy consultant. Then you have someone who you paid to deliver the correct numbers.

  6. GBA Editor
    Martin Holladay | | #6

    Dick Russell,
    Thanks for your very specific and helpful answer.

  7. tweeterisu | | #7

    Thanks everyone for your helpful insights and suggestions. Unfortunatley, I did pay this gentleman to do the calculation and he is highly regarded around here. I am planning on sitting down with the individual and walk through his calculations as well as do my own. Never a better time to learn than now. I was just hoping to leave the expertise to those with it:)

    I'll post back what comes of everything! Again, thank you all!

  8. Expert Member
    Dana Dorsett | | #8

    The top of the short-sheet heat load page says:

    Construction Quality------------------Average

    Sounds like potentially a garbage-in-garbage out situation. Despite the relatively high quality of the WrightSoft tool set, if you don't enter all the data, it won't magically divine the details and give you the right number.

    The absolute R-value of the ICF is probably exaggerated a bit, since they clearly had to use the 25F R-value of Type-II EPS of about R4.75/inch to come up with R30 after factoring in the all the thermal bridging of the concrete beam structures. That 25F number is only valid when the average temp through the foam is 25F, which will not be the case at your -4F outside, 70F inside design temperatures- the coldest it would be is 33F at the concrete layer, which is half-way through the foam.. Using the 40F R-value for the foam (about R4.5/inch) would be more valid, and Steady state its probably closer to R25, but the mass effects of the concrete will also reduce peak loads (both heating and cooling)- and I'm not sure there is an adequate ICF model in Wrightsoft that corresponds will to the non-uniform thicknesses of both the foam & concrete in the assembly using the Hobbs product:

    The manufacturer's engineering report also (mis) uses the 25F R-value for the foam, but cooks up a whole-wall U-factor of about U0.030 for the assembly, including interior & exterior finish layers plus air films. That's probably going to be close enough for a steady-state approximation, but does not factor in the very real dynamic effects regarding peak loading:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |