In all vehicles the ventilation is very important. For a ventilation system to work it must have an opening to allow air in and a separate opening to allow the air out. If there is insufficient area at the outlet then the pressure will increase inside the vehicle. This pressure will cause the fan to start to stall and the airflow produced the fan will drop.
To allow air to escape from a vehicle exhaust vents are fitted at the rear on both sides. These vents have light flexible flaps that normally close but with pressure will open and allow the air out. On the Citroen Relay there are two sets on each side. One visible behind the wheel arch and another inside the C pillar. These vents are also visible from the outside under the bumper or side trim.
To allow air to reach these vents in a van with a bulkhead there are a series of slots in the bulkhead.
So, when you sit in the van and turn the heating or A/C fan the air is drawn from the outside below the windscreen by a fan and pushes out of vents in the dash. This air will only keep flowing if this air can go somewhere, so it leaks through the headlining and the bulkhead via the slots into the rear cargo area. The exhaust vents at the rear then allows the air to exit the vehicle.
If you remove the bulkhead you don’t need to worry about the slots but you should keep the exhaust vents open and allow air to flow through them. Blocking them up or boxing them in will only reduce the airflow when you drive and will cause issues when you slam the doors.
When a door is closed a volume of air is pushed into the van and causes a pressure spike inside. This needs to vent outside and will go via the exhaust vents. If you have blocked them you make see your roof vents or windows bulge with the pressure or have problems getting the door to close every time.
If you fit a roof fan to your van then it is important that you also allow this air to exhaust from the vehicle. There needs to be a flow path for the air. I'm fitting a fan at the front and a vent at the rear. The fan can either pull or push the air so that I can get flow in either direction. I'm also allowing the rear exhaust vents to be free of any blockage so that air can flow out of these if I only open the front vent with the fan.
