Yes, that's how I figured it to work. It has a flapper to close the fresh air channel, doesn't it.
I have a chassis with the inlet in the cowl as seen above and a heater box without the channel in top of the box ( from a different car ). And I haven't decided yet should I open the top of the box or make some sort of duct to the suction side of the fan. I guess they both are more or less maginal ways as the possible duct can't be sealed tightly to the intake of the fan.