Heating Plumbing

If there's one thing that is embarrassingly bad about production airplanes, it's their cabin heating and cooling. I don't understand how Wichita was able to justify, for 40 years, building expensive airplanes whose cabin heating and cooling was worse than the cheapest car. So, I'm going to find out...I've decided to install good, controllable hot and cold air intakes in my RV-7. Or try to, anyway...I may discover why the Cessnas and Pipers that I rent suck so much!

Some of the things I hate about the 1974 Warrior I fly, for instance, are that a) I can't shut off the overhead vents, so I have to endure cold air blowing on me no matter how cold I feel, and b) I can't shut off the hot air that bakes my feet more or less year-round.

I'm installing two cabin heat boxes. One will be fed by one heat muff on the left exhaust branch, and one by another on the right exhaust branch. They are individually controlled. One feeds the triangular Van's heat box, installed in the "stock" location under the firewall recess. The other is a T valve, to allow either hot or cold air to be selected. The T valve also has a shutoff to close all the air, hot and cold. That one will be ducted to provide either defrosting or cabin heat, through another T valve. Confused yet?

This is the location for the cabin heat valve, as shown in the drawings.	I built the cabin heat box from an optional kit provided by Van's. It's pretty easy to build.
It's actuated by a cable from inside the cabin, requiring one less firewall pass-through A seal made from high-temperature RTV is added between the valve and the firewall.	It's no fun cutting a big hole in stainless steel! I bought a hole saw at Sears that did the trick...but first I dulled the blade in my fly cutter in the attempt. I clamped a wooden board behind the firewall to both support the material and act as a bushing for the hole saw's pilot bit.
	Good thing the Christmas shopping season is behind us...this stuff isn't cheap. The 90 deg. elbow was about $30, and the T valve over $100. Note that this T valve has two butterfly valves: the one closest to my wrist is the hot/cold air selector, and the one farthest (which goes on the cabin side of the firewall) is the air volume control (and shutoff valve.)
Now I have to find space for all this claptrap, not to mention the 2" scat hoses that connects them. And the Y-duct (not shown) that spreads the hot air around the cabin. I'm an engineer...I can't make anything simple!	This thing sticks out a good 5" into the cabin, if you include the elbow. That limits where it can go on the cabin side of the firewall, since it mustn't interfere with the pedals. Even worse is the need to remain clear of the as-yet-unbuilt stuff forward of the firewall. I've looked at several builder's web sites, and finished airplanes, and of course, the drawings, but I can't say with certitude that it won't interfere with the engine or an engine mount tube. So, I decided to mount it high up.
I think this uses otherwise unoccupied spaced, on both sides of the firewall. I'm sure I'd have put other stuff there, things like electrical busses, ground busses, etc., but that stuff is easily relocated. So out came the trusty hole saw again.	It's a start...the hard part of the heating plumbing is behind the firewall, and I'm not there yet.
The bay closest to you will be full of SCAT tube, valves, their control cables, etc. The other side will probably end up full of fuse blocks, ground busses, and their firewall pass-throughs.	I finally broke down and bought the Experimental Air eyeball vents. They're expensive, but well-made. Unfortunately, they DON'T fit in the holes made for Van's vents, so I had to make new subpanels...another weekend lost!