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Assuming that both wing tanks are vented, and assuming those wing tanks are plumbed so that they feed into the header tank, then the header tank is in fact vented – through the wing tank vents. Any air bubbles in that fuel will flow to the top of the header tank, then back to the wing tank through the feed line. So instead of having to plumb the return line all the way to a wing tank, it should be OK to plumb the return line to the header tank, which is effectively vented through the wing tanks.
I learned this at an Oshkosh seminar conducted by an engineer who designed fuel systems for a living. His take was that simpler is better, and that adding a vent line to the header tank was unnecessary. Ditto for returning fuel to a wing tank in a system that already has a header tank.
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This sounds like an accident waiting to happen.
As long as here is fuel coming DOWN the Fuel line into the Header tank, there is no opportunity for the air to vent back UP to the main tank.
The vapor can not go Up while the fuel is comming Down: and unless there is fuel comes down to replace the vapor going UP, the Vapor isnt going to go UP in any event.
Given enough time between flights, the header tank might refill itself as temperature changes caused it to “Breathe”, but in flight the vapor bubble would have no place to go.
Unless the feed line had a HUGE cross section, a dedicated Vent Line plumbed slightly higher than the fuel feed line would be needed to have enough “Differential Head” pressure to start the vent flow.
The vent line cross section can be significantly smaller than the Fuel line if needed.
I would mock up the tanks to confirm that what you expect to happen actually does happen before committing to flight.
