Your gut instinct is dead-on about the AC guys and unfortunately, many companies don't size AC systems properly.
- I struggled to find a company in the central Florida area that really looked at sizing the systems well, but I finally found Certified AC.
- It is harder for homeowners to find the right contractor though -- especially when doing a "simple" system replacement.
AC units are commonly oversized
Many AC contractors will use the obsolete square footage rule of thumb to calculate the tonnage you need, while others simply quote out the same size unit that was there before.
- They will typically oversize the units to ensure "comfort."
- Luckily, improvements in variable speed air-handling have reduced the humidity/comfort issues created by oversized units, but this is not really an efficient solution.
Furthermore, neither sizing technique will consider improvements in AC equipment and changes in the home that affect the cooling and heating load. These changes could allow you to reduce the size of the AC equipment -- thus saving energy and money. For instance, does the home have newer/more efficient windows, CFL or LED lighting, or added insulation?
Hire an independent consultant
The safest bet is to hire an independent company that specializes in HVAC system design and load calculation.
- I have personally used Calcs-Plus for several projects for both front-end independent calculations and also performance testing.
- There are several other companies in the state that complete the same type of work, however -- like Two Trails.
- Hiring an independent company to do your load calculation will cost a few hundred dollars, though (depending on the size of your home).
Or, find an AC contractor that can do proper load calculations
If I were in your shoes, I would call around to a few more AC contractors and try to find one that has experience with new construction installations and homes that utilize foam insulation.
Companies with this type of experience should have more background doing proper load calculations and will include the service as part of their pricing to you.
If you can't find a company that you feel comfortable with, then hire one of the independents for your load calcs. I like to say that you always need to go with your gut in the building business.
For more information:
Read "What size AC unit should I use for a 1,986 sq. ft. home in Kingwood, Texas?", a Q&A answered by Danny Kelly.