My current plan is to hook up only one side of the flap valve to only one heat muff and split the heat front and rear. It sounds to me like there is no need for both heat muffs and two heat sources. Everyone I have sopken with seems to indicate that the heat will drive you out of the plane. That is how my current aircraft is and it is more than enough heat. Use the second flap to vent cool air down the tunnel during the summer and off during the winter by having it directly connected to fresh air. I can understand how Van's demo plane would be cold, they have no insulation whatsoever, but most of us are insulating the cabin where it will not only retain the heat, but keep it cooler in the summer. If I find it is not warm enough in the winter I can always hook up the second muff.
In addition, I have layered the floor and the firewall with 1/8" ceramic insulation covered with 1/2" black insulation using the 3M ceramic cement. I tested this with a 1500 degree flame for 5 minutes and could still touch the insulation side. My firewall will have Koolmat on the engine side.
It is my belief that the type of temps that do not allow a person to touch the metal, are far to high for fuel lines, and just plane don't make good sense. If the fuel selector is hot, just think how hot the valve and the aluminum tubing is!
Bill
Bill