Help reading Manual J Calcs for new construction

10 years ago

We're in the process of building a home just outside of Atlanta. It was surprisingly difficult to find a HVAC contractor willing to spend the time to actually run these calculations rather than use rules of thumb. Now that we have results, as I suspected, the proposed HVAC needs are essentially half what the "rule of thumb" crowd was proposing. I can tell from other posts on here its ok to trust the numbers but I'd really appreciate some help looking at the attached results to make sure there weren't any assumptions used I didn't give or that don't appear obvious.

I can tell from looking at this, they used the correct info on the windows I gave them, the R values and lumber type for all exterior walls and insulation and the open cell spray foam info for the rafters we're using.

It looks like I can't post a pdf and the report is 10 pages long including the graphs. I've attached a one page jpeg with the second floor data but don't see how to post multiple pages. Any advice for a newbie on how to get the other information on here for everyone to see? If not, anything stand out on the page I attached? Thanks very much!

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Comments (5)

mike_home
10 years ago
I am happy to see you have received a real Manual J calculation. Check the area sizes, insulation values, and the indoor and outdoor design temperatures for summer and winter. If they all make sense then you should be fine.

How many systems have been calculated? You could post a summary for each system (size, heating loss, and total gain). From this information you will get suggestions on equipment configuration.
fsq4cw
10 years ago
These calcs look very professional, serious and Ã¢ÂÂseemÃ¢ÂÂ OK. Based on what I see so far, this contractor might be someone IÃ¢ÂÂd want to work with rather than Ã¢ÂÂshopping aroundÃ¢ÂÂ his hard work and professionalism.

Sometimes Ã¢ÂÂThe advice is worth the price!Ã¢ÂÂ

IMPO

SR
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samg23
Original Author
10 years ago
So it's not crazy to see a final number where you're at 1500 sqaure feet per ton? I realize we have some energy efficient choices (i.e. foam in the attic, upgraded windows) but there's a lot we didn't do that others here have (i.e. closed cell in the walls) and I've seen lots of reports where the final numbers were 1000 square feet per ton. To be at 1500 or needing 1/3 of the rule of thumb seemed shocking.
mike_home
10 years ago
Something is not right. I see the latent heat gain is zero. You expect it to be small but not zero. I would not expect 1500 square feet per ton for the second floor.

You need to talk to the contractor about the calculation. You house is well insulated, but these calculations are too low in my opinion.
weedmeister
10 years ago
The assumed temperatures are important. Perhaps they assumed that 80F was the appropriate indoor temperature with an outdoor of 90F. I like cooler and I know it gets hotter than that in Atlanta. It makes a big difference.