F tube.

[Daniel Macmillan]

Can someone share some light on why they are pulling inside and outside air into the gas heater on a 73 Thing. I am referring to the combustion air intake.

[mondshine]

But the "F tube" is for combustion air.

[Daniel Macmillan]

Thanks. Missed that. So what is the reasoning behind it.

[mondshine]

All of the Eberspacher heaters draw their combustion air from some source of fresh air.
The heated "cabin air" is usually recirculated by the heater's inlet and outlet.
(On some older Type 2s, the heater draws its cabin air from the smelly engine compartment, just to let you know you're driving an old bus.

The F-Tube seems to be unique to Things.
For some reason, some of the combustion air is drawn from a baffled inlet in the front wheel well, while some is drawn from an opening in the body cavity next to the fuel tank.

It's a mystery to me why the VW engineers found it necessary to use this complex combustion air inlet arrangement, but they did.

In practical use, my own BN4 runs well with the F-Tube connected as it should be, but truthfully, the heater is happier with the flexible hose disconnected at one end; drawing a portion of the combustion air from the trunk. It's just less restrictive, and the fan runs noticeably faster.

Good luck with your project, Mondshine