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Community and Q&A

Heat Loss Calculations for Design and Component Selections

| Posted in Energy Efficiency and Durability on

I had issues posting last night to GBA, so please excuse if this is a duplicate. 

Planning a house build for this spring in zone 7 (8960HDD). 2,000 square feet with a 1093sf unfinished basement. 
I am attempting to get a reasonable sense for heat loss in our house design to aid in mechanical and component selections (heat system, windows). Here is what I have found:
Free Excel Sheet from Marc Rosenbaum
I found a free Excel spreadsheet with the typical inputs that is meant to give you a quick and basic calculation. I’ve attached a screenshot. I am able to pull the volume and surface specs directly from my SketchUp model. 
Any sense for how accurate this tool might be? Is it possible to have a reasonably accurate calculation with such basic inputs?
I like this tool as it allows for easy tweaking to your building components and factors. There are some inputs that drive the results in a huge way (like airtightness of course) but it seems that things like wall insulation have a much, much less dramatic effect. 
I also ran all of our specs through REScheck (front page screenshot attached) since we’ll be required to do so by code anyway. I was surprised that REScheck did not provide me with a heat loss calculation but rather a UA (building load coefficient). It appears that I can calculate a heat loss using UA (I think by multiplying it by design temp difference). 
How do I get a heat loss in BTU/hr from UA?
Is REScheck (UA to heat loss conversion) an accurate tool for heat loss calculation?
REScheck shows a UA much lower (259 vs. 414 in Excel sheet), I assume that makes more sense and would be more accurate, right? More detail input, more accurate outcome?
Decisions to Make
There are two main drivers to this: 
1. I need to make a window selection and would like to understand how different window specs impact overall performance and heat loss to aid in my decision. See recent post:
2. I would like to confirm whether or not we can heat with air-source mini-splits and if it will actually be comfortable. 
Does anyone in zone 7 have experience heating a home with air-source heat pumps? We just had a period that remained between 0 and -25 for roughly 7-10 days and these are the periods I am most concerned with.
Any other insight on heat loss calculations, window selections and use of air-source heat pumps in zone 7 would be greatly appreciated! 

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  1. mpsterner | | #1

    Well, I've posted this 4 times now and keep getting a Wordpress error upon submission. I am hoping that by commenting on the thread, it will help it to actually show up and I can get some feedback from folks. Thank you everyone.

  2. Expert Member
    Michael Maines | | #2

    Michael, Marc Rosenbaum is highly respected in the field of high performance building and I'm sure his spreadsheet is reasonably accurate. Energy models are always approximations and the more inputs they have, the more accurate they usually are. Rescheck's UA is a quick-and-dirty calculation meant to ensure basic code compliance, not to fine-tune energy needs. I would trust values you get from Marc's spreadsheet over those in your Rescheck report. You might want to try BEopt if you want a second opinion. BEopt includes a lot of options for window specs and you can easily add more to see the result.

    Regarding heat pumps in very cold climates, here's a recent question from someone in a similar climate:

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