I'm curious as to the proper way to do this.
Outside air through ERVs to bring it up to a minimum temperature for ventilation air to heat pumps which recirculate some of the room air. Room air gets mixed with the ventilation air prior to going back to the heat pumps. Exhaust air going back to the ERVs is equal in volume to the ventilation air.
How many systems are needed? I'm thinking ventilation (conditioned OA), Supply, Return, and Exhaust at a minimum.
Would you make a separate system for that mixed air (ventilation and return) going back into the heat pump? It's basically a plenum that has both ventilation air and return air going through to a common output.
How would you mix the systems, ventilation and return? Does it need to be a plenum (or something else) that mixes the systems or can it be a duct?