I am planning to install a whole house dehumidifier Ultra-Aire XT105H. The manufacturer recommends adding a separate return to the dehu unit and connecting the supply to the main supply line of the HVAC system. The back draft damper will prevent any back flow through the dehu unit when HVAC is on and the dehu fan is off:
My question is: When the dehu fan is on and the HVAC is off and the air is blowing from the dehu supply line into the supply line of the HVAC duct work, will the air travel both ways - to the supply registers and the the return registers, creating a counter flow in the main HVAC unit? The installation instructions do not mention the need for any additional back draft dampers in the main HVAC duct work and I am not sure if there is enough space for such a damper in my duct work between the HVAC unit and the place where the dehu supply will connect to the HVAC supply.