Actually guys (male and female) after re-reading my manual I think we aren't quite getting it right with regard to what valves are where, and when they do what!!
There is:
Main Outflow Valve at the rear, underneath.
Forward Outflow Valve at the front, side (LHS).
Exhaust Port at the front, underneath.
The forward outflow valve closes automatically to assist in maintaining cabin pressure when the main outflow valve is almost closed or when the recirculation fan (right recirculation fan on B737-400) is operating.
The exhaust port (also known as the Flow Control Valve!) opens to exhaust the cooling air from the E & E compartment overboard during ground operation, unpressurised flight and pressurised flight below a cabin differential pressure of 2.5 psi.
When the "flow control valve" closes, air is directed around the forward cargo compartment liner for inflight heating.
So it seems we are all a little confused. The forward outflow valve position has nothing to do with cabin pressure differential, and all to do with the position of the main outflow valve or recirc fan. I suspect the connection with the recirc fan has to do with smoke removal and the like (as Flight Detent has already mentioned). That is, if you turn the recirc fan off, you are obviously trying to refresh the cabin air as quickly as you can. Opening the forward outflow valve will assist this. This is also assuming the main outflow valve isn't "almost closed".
Also the connection between the Forward Outflow Valve and the selection of High Flow is an indirect one through the Recirc Fans. The Recirc Fan(s) may or may not be operating, depending on whether or not one or both packs are operating in HIGH or not, and depending on which B737 variant you are flying.
The
Exhaust Port is the one that opens and closes, depending on the 2.5 psi cabin pressure differential. It's also called the Flow Control Valve because it controls where the flow of warm air leaving the E & E compartment travels.
The Exhaust Port is either OPEN, allowing the air to leave the aircraft immediately, or it is CLOSED, which forces the air to pass around the Forward Cargo Compartment Liner for inflight heating (so the pet pooches and pussycats don't freeze their little paws).
I think from memory, the Exhaust Port is angled at about 45 degree to the aircraft skin, and is pointing rearwards, making the hole an oval shape.
Now back to the original question.
[Edited because I said something I shouldn't have.]
