I have a 1970 brick colonial with basement and a late-80s era forced air natural gas furnace and older AC unit (one for basement through 2nd floor). We would like to better control our air quality and energy use and are planning to replace the current system with a smaller one to serve the basement and first floor and a second in the attic space to serve the upstairs. In conjunction, encapsulating the attic will probably be a necessity to get the most out of the upstairs unit. Is it possible to size that load (or either of them, really) while the current system and the ventilated attic are still in place? We will be living in the house during our work, so we can’t just shut things down, gut everything for 2 weeks, and move back in.

Thanks.
Dan