I'm designing a system for a 3-story new home in Charleston, SC, with ducted mini-splits (Mitsubishi). Approx. s.f.: 3,000.
There are 4 indoor fan coils: 2 serving the first floor, 1 serving the each of the second and third floors. The house is being Designed to Earn ENERGY STAR, version 3.0, and will be built to have a very low infiltration rate. (projected: between 1 and 2 ach50).
My infiltration strategy is to provide enough ventilation to meet ASHRAE 62.2, and connect all bath exhausts (50 cfm each) for additional recovery (and fewer penetrations...bonus!). This brought the cfm capacity, including bath exhaust, to 270cfm for the whole house. So, I have specified (2) ERV's, each at 130 cfm.
With that, I was planning to put one 130cfm ERV on the top floor system and one 130cfm ERV on the first floor. But, when I laid out the equipment and ducting for these ERV's, I found it VERY constrained in the first floor ceiling. Almost too tight to make it work with the ducted fan coil in the same area. There is more than enough room for the 3rd floor ERV, so this is not an issue.
The theory behind this approach is that I'm essentially creating a fresh air sandwich. The second floor will get fresh air through exchange via the open stair in the center of the house.
MY QUESTION: If there simply is not enough room in the first floor ceiling, does it make sense to connect the second ERV to the system for the second floor (where I have more room for the equipment). Will the first floor still draw an appropriate amount of fresh air from the upper floors.
Alternatively, I could just connect a single 300cfm ERV to the middle floor system, and rely on exchange to the upper and lower floor from there.
Any thoughts would be appreciated