I thought you were worried about optimizing the loss through the exhaust/extractor systems, i.e. the intermittent fans used to remove steam or bad odors by dump

I thought you were worried about optimizing the loss through the exhaust/extractor systems, i.e. the intermittent fans used to remove steam or bad odors by dumping conditioned air to the exterior. With a central HVAC, there would of course be some kind of return to take from inside the house and send it back through the system again. I think others in the thread are taking it for granted that any system will be an inherently recirculating design. Some modern houses might have return ducts in many or all rooms. But, it is relatively common to just have a few centrally located returns, relying on open floor plans and/or leaky interior doors to allow circulation from rooms back to the return vents. For example, our ~45 year old house has a gas furnace with heat vents near the outside walls of each room, and air flows back through hallways to reach centrally located return vents on each level. The return vents are in a stack quite close to the furnace which is also centrally located.