How much AC tonnage do I need for a 6,700-sq.-ft. house?
I am building a 6700 sqft house in Brownsville, Texas. The house is not the usual wood frame but is being built from Areated Autoclaved Concrete also called Hebel Blocks. This is a very efficient insulating material. The inside walls are 6″ thick and the perimeter walls 8. The ceiling heights vary from 10′ to 18′. The living room has 6 windows that at 9’x6′ and 2 windows 9’x2.5′. The rafters will received 6″ open cell spray foam.
I have received 3 airconditioning quotes and all three have different tonnage. One has it at 8 tons with Tappan Brand A/c. The other 2 have the carrier infinity 21 SEER but one has it at 9 tons and the other has is at 11. All claim that they used the Manual J to do the calculations. They are not willing to show the manual J.
I am confused as to who to pick.
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There are many issues that affect Manual J, S, D & T calculations, but ASSUMING you have done a good job of air sealing the envelope, attic insulating and installed good windows (and/or shading); your house should not need more than 5-6 tons of AC. You may need to hire an independent mechanical engineer or HVAC consultant to run all your Manuals properly; all of them!!!
There is plenty of information on this website, and on the web, about proper Manual J calculations and HVAC design… read on.
I would also stay with 16 SEER, dual speed compressor and variable speed air handler. The efficiency of 18 or 21 SEER compressors is not as good as 16 SEER. Also, install all metal duct system, sealed with mastic AND inside the conditioned space.
Omar,
First of all, Hebel block walls have a low R-value. Hebel makes three types of 8-inch blocks; they range from a low of R-8.9 to a high of R-10.85. That's not very impressive.
For information on cooling load calculations, see these articles:
Calculating Cooling Loads
Saving Energy With Manual J and Manual D
If they're not willing to show the Manual J, my guess is they haven't actually done it.
My advice: Check out http://eec.ucdavis.edu/ACEEE/1994-96/1996/VOL08/207.PDF before you go any further with any of these guys.
My recollection is that Manual J does not handle concrete wall systems very well. I would suggest picking up a copy of this: http://www.cement.org/bookstore/profile.asp?itemid=CD044
It's amazing how well AAC disperses heat. I used to blast the side of a block with a blowtorch, wait about 20 seconds, and then hand it to an audience member. No hands were burned. (I can't take credit for coming up with that stunt; I saw someone else do it first.)