737s when sitting on the ground the pressurization panel would normally be set to ground mode which should put the outflow valve full open unless manually selected otherwise, so when air is coming in from either the APU via the packs or from the heat cart via the ground connection there will be a pretty much equivalent flow of air going out of the outflow valve instead of the cabin pressurizing. However, as mentioned farther up, I would also think that when any of the doors are open then a bunch of the flow would probably go out that instead based on path of least resistance.