The recommendation is 2 sq ft of exhaust per 1,000 cfm if the exhausts are screened, which mine are. It takes hours of running the fan to get the interior temp from 80 down into the low 70s when the outdoor temp is in the low 60s. Anyways, I've been looking around the neighborhood and there's some houses identical to mine that have at least doubled the size of their attic vents (they're huge!). So they may have had the same thoughts that I'm having.

"We have no vents in place leading from the attic to the second floor" The whole house fan would go in the attic and draw air from the second floor. Our fan draws air from the hallway centrally located on the second floor but there are options for fans in each room. The installer would determine whether you need more attic venting to the outside of the house. We have a 2700 cfm whole house fan and LOVE it. When it cools down in the evening we open the windows on our 2nd floor and turn the fan on. It's amazing how fast is cools the home. Since the whole house fan vents to the attic it also turns the air over in the attic helping to cool the attic. We have been very happy with our whole house fan. There are some things to consider though. I've read comments from experts on here about fire hazards due to the suction from the fan. I believe this is primarily a concern for those with water heaters in the home. Our water heater is in our garage and so the flame is not subject to the pressure variation caused by the fan. Hopefully some experts will chime in on the things to look out for.

Almost a fifth of the price involves allowances. Allowances are not prices they are estimates. You will be expected to pay the actual cost of hiring an electrician and plumber which could be a far greater amount than estimated. Make sure the allowances are only for materials and subs; his labor should be in the base bid price. Ask if he will mark up materials and subs covered by allowances. You will need an exhaust fan with a duct to the outside. You didn't say where the project is located but you will probably have to provide heat for the space. It seems pretty obvious that this room will be used as a sleeping room so one or two hard wired smoke detectors will be needed. A carbon monoxide detector MUST be installed. I would guess the project will cost well over $20,000 not including what you buy unless the contractor is uninsured, and ignores licensing and permitting requirements. As for minimum space and height requirements, if the local code is based on the 2009 IRC, the 70 s.f. minimum floor space for a habitable space (bathrooms are not considered habitable spaces) includes the space where a sloped ceiling is 5 ft or higher and a level ceiling is 7 ft. or higher for a minimum of 50% of the required room floor area. The minimum height of a habitable room level ceiling is 7 ft and 5 ft at a sloped ceiling. A bathroom must have a ceiling height of 6-8 in front of each fixture. The required "means of egress" from all spaces in the house must be 3-0 wide for corridors and stairs and lead to an outside door without passing through a garage. Habitable attics and every sleeping room, in addition to a means of egress, must have an "emergency escape and rescue opening" of 5.7 s.f. (min. 24" high and 20" wide) with a max. sill height of 44" above the floor. You need to decide if the habitable space will be used as a sleeping space because the building inspector will either ask you or just assume it is one. As for insulation, some jurisdictions have different requirements for an existing structure which may be contained in the International Existing Building Code or state amendments to the IRC. Where I live the wall, floor and roof cavities must be filled with insulation equivalent to fiberglass batts however you might want more depending on your climate and cost of heating fuel. As for the structure, the framing seems fairly new so if it was really labeled as a "bonus" room on the original permit drawings it would have been designed for a habitable space load instead of a storage load. All of this should be covered by the tables in the building code so an engineer should not be needed unless the knee walls are intended to transfer rafter load to the floor joists. The worst case is you would need to open the garage ceiling to add some joists but it might be wise to open the ceiling anyway because the best way to seal the floor is to spray an inch or two of foam insulation from below, in fact I don't know of another way that works. Putting a sleeping room over a garage requires great care. I would have two CO detectors in the new space, one hard wired and one battery operated. CO is a dangerous killer. I would have lost a client if newly cut flowers had not wilted soon after delivery.

"Also have checked the drain lines - no clogs, and in fact, the drain pan has been dry." " So when I say there is not water in the pan, I was referring to the overflow pan. There is water in the pan inside the unit under the coils." Did I mention , you are overlooking, omitting or mis-stating some fact(s). ??? And there she is plain and simple. With that straightened out I'll stick by my initial knee jerk,leaking return. Useing a sensitive and accurate thermometer,record air temp at intake GRILL and then record temp immediatly before air enters blower. If you find more than 1 deg differential,investigate for return leak and/or poorly insulated return. You will get advice about load calculations and such more suited to trained pros but do the simple things that better fit your wheelhouse before getting that deep. There's a recent trend of pulling all the stops regardless what the complaint is. Here's something to consider. It is plenty hot and humid in Houston Tx where many systems are intentionly oversized. It's hard to find people complaining because their ac is too large. Until you reach about 30% oversized,the biggest difference is increased operating cost. If your place call's for 3.5 tons cooling you will not complain about being uncomfortable with a 4.5. If it were so simple anyone could tell you the problem, techs wouldn't have to come out in person to figure these things out.