We've had a curveball thrown into our HVAC choice: to be eligible for more than a two (or three?) star rating from the city's Green Building Program, there can be no more than 1 ton of cooling for 600 square feet of house. Our current system is spec'd at 4 tons. Our square footage (2,281) divided by the spec'd tonnage equals not good (570 square feet per ton). We could go with a 3.5 ton system (652 square feet per ton), but the variable speed compressor we covet only comes in 1 ton increments, so we'd have to go with a 3 ton system, but that's 760 square feet per ton. Given the amount of windows we have, the architect's concerned that might not be enough cooling.
My understanding of a variable speed compressor is that it runs at just the right speed to deliver just the right amount of cooling. If that's the case, it seems to me that it doesn't really matter how big the system is (as far as energy efficiency is concerned; system size impacts cost). Unfortunately, it doesn't appear that the city's rating system takes this into account, probably because these variable speed systems are relatively new. Ironically, the rating system may "force" us into a less efficient system (we're still thinking about this; not sure that a 5-star "false" rating outweighs real efficiency).
We've requested a manual J analysis (an HVAC sizing analysis required for any rating; the 4-tons mentioned above appear to be a shoot-from-the-hip estimate) to see what the actual fit is, and I will be verifying my understanding of how a variable speed compressor works. Based on those answers, I may try to talk to our rater (the person at the city in charge of overseeing our green rating). Also need to investigate whether or not variable speed compressors from other vendors come in half ton increments.
We're starting to lose our cool... ;-)