HVAC Sizing
I have a “difference of opinion” with an hvac contractor on the size of the equipment to be installed in a 4800 sf house, 2 story, 2 hvac systems (16 SEER and 95AFUE) with all metal ducts in the conditioned space through dedicated truss chases. Location is in Dallas, TX, design temps at 70/75 indoor and 22/100 outdoor, and expected 1ACH50.
The Building Analysis from the Manual J is:
1st floor: HL 35,327 Btuh and CL 17,636 Btuh. 2nd floor: HL 22,873 Btuh and CL 18,688 Btuh
The Project Summary calls for:
1st floor: HL 35,367 Btuh and CL 20,050 Btuh calling for 2.2 ton @ 70 SHR.
2nd floor: HL 22,873 Btuh and CL 21,726 Btuh, calling for 2.3 ton @ 70 SHR
Can you please tell me what size of equipment I should install in each floor? I don’t want to give our ideas, so not to influence your response.
Thank you, AC
I just thought of a couple of questions to add:
1. Should the SHR be 70 in residential high efficient equipment? Maybe 85-90?
2. Should the multiplier be higher than 1? as in 1.05, 1.1 etc, etc.
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Replies
Armando,
What are the R-values for the various elements, walls, ceiling, foundation, etc.? A 58k Btu peak heating load for Dallas, TX sounds like a bunch. I would consider higher R-values, a single furnace and AC unit zoned for each floor. The 7 to 10k savings on equipment would buy a lot of insulation and cut the yearly energy costs as well.
Sorry, my whole comment is not showing up... OK, I found out you can't use the "less than" sign. I guess it conflicts with the HTML tags... La-Di-Da...
Thanks, Doug. The house is in an old subdivision, where the existing house was torn down and a new home is going up. It’s a small lot with N-S long axis (lots of western exposure). Windows are Low E, less than U.30, less than SHGC.30, 24” overhangs. Wall assembly is 3-coat stucco, WRB, R3 ½” rigid foam, OSB, R21 cellulose, sheetrock. Roof assembly: Tile roof, WRB, R5 1”rigid foam, OSB, R25 7” open cell foam. The manual J has close enough wall, window footage and building volume; also it has the right design temperatures. I disagree on the multiplier, SHR and size of equipment the hvac co. wants to install. And, yes... in my own MJ, my total heating loads are 60% of theirs... but I didn't want to influence the thread in any way.
I’m trying to find all the “typical” places I know hvac designers “used” to upsize the loads and therefore the equipment.
Armando,
You may find this blog interesting....
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/38069/Oversized-AC-Screwed-up-Manual-J-ENERGY-STAR-HVAC-Tirade
Also... you might post your question/comment at this forum/thread
http://www.linkedin.com/groupItem?view=&gid=1110797&type=member&item=53950278&qid=86ca83be-100d-4026-9c4f-ff6d3f9bb003&goback=%2Egmp_1110797
John,
Thanks for those links. I've been through this many times before and it does continue to amaze me how far our building industry has yet to go in this day and age, and one the hardest minds to change are the HVAC contractors.
I had contacted Allison last week about reading our MJ, but I'm waiting on my client to decide if he stays with the current house design or making a last minute additional changes.