Ducts outside of your living space is a big negative. When they leak, you blow your conditioned air outside. The make-up air comes from somewhere else, depressurizing your home somewhere else. Then you suck air in through random openings and cracks. In a humid environment this can lead to condensation and mold in some very interesting, sometimes hidden, areas. In my neck of the woods, you would seal the attic and insulate the roof deck to put HVAC up there. You, however, are in a very different climate and I don't know if that is recommended there. If you ask your builder, they might not know because some of them are very conservative. If you want to educate yourself, try the Building Science Corp. web site. You might be a little late to do that. I would also suggest that you work with an experienced energy rater (Resnet).

Here are the load calculations that I did on loadcalc.net (thanks for the tip, garden webber) for a Below Average construction regarding tightness (R30 in attic, brick exterior walls w/o insulation, single pane windows with storms) Total BTU's Cooling: 37006 Total BTU's Heating: 43690 Sensible Load: 33364 Laten Load: 3642 See original posting on 9/6/12 for estimates received, I am also waiting for an American Standard certified installer for a quote, any recommendations are appreciate. My house always smells stale and I noticed there are air filtrations system, however, I am on a budget and don't plan to stay in the house more than three years. Thanks in advance for any suggestions.

I would remove the filter at the return and add a Trane Perfect Fit Media Filter cabinet at the return side of air handler. Can't see job but for your airflow this would be the size you need. I don't like the idea of a filter at your 20 x 20 return, no filter on the 8 x 8, and then another filter at the air handler. Just curious, why was this issue not addressed at time of install? As long as you have good coverage/filter fit at air handler now, I would remove the filter at the 20 x 20 return. TFM215BOAHO 300-1,400 7.5 21.5 21 IMO

Well starting with a simple but most often overlooked "other than that",a proper trap. Within 6" to 1' of leaving drip pan the 3/4tr primary line often has an inverted T open at top to alow adding algacide or blowing out line. That access opening is better capped. The primary line is often not trapped and run's horizonally to dump into a sewer line or dedicated condensate line. It's best to have line trapped as near pan as pratical or line to emmediatly drop 6+ inches then turn horizonally into a running trap. Secoundary/emergency line is often untrapped,although it's best equally trapped as primary plus have means of being routinly charged. Alternatly,to reduce installation cost and ongoing maintainance,secoundary can be of sufficent leingth to dampen suction through it. Regardless how condensate drainage is handled,preventing reverse air flow through lines is desiriable. In a nutshell,air drawn in through line can distrurb water surface causing "spitting"droplets to be picked up and blown into discharge ducts. The condition is exacerbated if return is restrictive ( a condition Mr Heard and Mr Said both agree is very common). Competent techs will check out this"other than that"along with others to insure condensate drain's rather than blowing into discharge ducts.