Hi, we live in an old 3300 sq ft house near tristate nh-ma-vt intersection. We have many interventions planned to reduce air infiltration and switch to all electric heating. We have personally carried out room-by-room manual J and are looking at mitsubishi hyper heat ducted system. (Still have not learned how to do manual d nor s). Re: ventilation, we are looking at zehnder website but cannot tell if the right erv for us is comfoair Q350, Q450, or Q600. Each model moves, respectively, 206, 265, and 353 cfm. Which one is right for us? The licensed mech engineer we hired (there arent many near us) was real sloppy & inaccurate on our manual j so we are stuck trying to figure these things out on our own. Any input re: erv model is greatly appreciated. Thanks