On Thu, 8 Feb 2007 07:38:47 -0800, "Martino"
wrote:

Many local HVAC companies just guess the size of the AC unit and therefore I
decided to run my new house data through a Manual J compliant software.
Depending on design summer temperature I obtain the following AC size (in
tons) recommendations

Summer Temp Size 1 Size 2
90F 4.76 6.02
95F 5.04 6.46
100F 5.32 6.89
105F 5.61 7.33

The house is being build near Seattle where the max summer temp rarely
exceeds 105F but gets to 100F for a week or so during summer. 90F can stay
with us for few weeks per summer.

I am a bit confused about two sizing recommendations:

Size 1 is based on sensible and latent heat gain
Size 2 is based on 75% Sensible Capacity

I do not want to oversize my A/C unit but I do not want to significantly
undersize it either. What do you think? Thanks.

Since the standard size residential unit has a max of 5 tons, your
solution is simple. You need two units, a 5 ton and and 2 1/2 ton
unit.

You will find it most economical if you run the highest SEER unit 95
percent of the time and only kick in the other unit when it is
extremely hot and the single unit can't keep up. And you do that by
setting your thermostats appropriately.