4 or 5 ton systems should be multi-stage...
Despite the passing of Regional Efficiency Standards, International Code requirements for Load Calculations and Equipment Selection, tighter homes and duct systems, I cannot see the likelihood of grossly over-sized equipment going down! The significant problem at hand is that HVAC Contractors are either going to use the system design process, or not.
In the meantime, Regional Standards are pushing Manufacturers into finding ways to make their systems more efficient. Other than variable refrigerant flow (VRF), I am not sure what a manufacturer can do to increase the Energy Efficiency Ratio (EER). In fact, some manufacturers and distributors have reported their variable capacity systems have actually tested, in a laboratory, lower EER than their two-stage and even single-stage counterparts. We have seen the increased use of Electronic Expansion Valves (EEV) to maintain the lowest possible superheats in the industry. Even increasing the physical size of Evaporator and Condenser coils in attempts to pick up/reject more heat. This is all putting the burden on the contractor to find the room for installation, as well as finding systems that can deliver the latent capacity needed in the replacement market.
So, I propose this: Why don't we require any 4 or 5 ton system be multiple stage or variable capacity? This can address the significant over-sizing issue so often seen in the HVAC Industry. After all, a study completed in 2006 by the Florida Solar Energy Center: "Measured Impacts of Proper Air Conditioning Sizing..." found little electrical savings for homeowners when replacing systems 47 - 65% oversized. There is still significant peak energy savings to Electric Providers, hence the enforcement of proper sizing with most Utility Rebate Programs. I wonder what those savings to the homeowner would have been if they just installed a multi or variable capacity system? This code enhancement could change the industry significantly in future years, keeping the focus away from bigger coils and higher efficiency, but still pushing the contractor to proper sizing and equipment commissioning. Yes, two-stage equipment costs more! So you should size the equipment correctly to avoid these extra costs, increasing your bids. Trust me, I have inspected as much as hundreds of A/C installations a year, for almost five years now, in a local utility rebate program. Almost never do I see an installation or replacement system larger than a 3-ton. Less than 10% of my inspected systems are larger, and almost always multi or variable capacity. Customers still report increased comfort and reduced energy use. Let's push efficiency using correctly sized, multiple capacity for the 4 or 5 ton systems, not physically larger units!