Peter,
What you mention is certainly the "common knowledge" with respect to cabin heat and every plane I've seen since the dawn of (my) time has been configured to route the hot air overboard when the heater is turned off, not shut it off completely as you describe. Will that portion of the exhaust pipe overheat if no air is flowing through the system? I doubt it, but I'm not sure. How much hotter would it get? I don't know, but I'm sure not a lot. After all, the rest of the exhaust system doesn't have deliberate cooling - it just exists in whatever air flow goes around it, and I've never seen any one worry about that. I would think it more likely that the flex hose attached to the heat muff would overheat and fail prematurely. Yes, the air dumped does reduce the air
available to cool the engine. I find it ironic that many (including me) go to great trouble to reduce all leakage around the shroud, including the use of RTV sealant, and then ignore the air flow through the heater valve. Maybe I should put a restricter orifice in the "dump" line to reduce, but not eliminate, that air flow. The answer to your question is that I'm not sure anyone knows the answer to your question.
Gary Casey
From Peter:
I am routing my cabin heat hoses and the follwoing was braought to my attention.
Presently I take the pressured air from my plenum, than to the heat exchanger mounted on the exhaust and routing the heated air to the valve which I can open for cabin heat or close it. If I close it, the air gets blocked at
the valve as I thought it is better to block it rather to dump the heated air to the cowling. Reason for that is that I get the air from the plenum and dumping it to the plenum reduces the available air to cool the engine and reduces the pressure difference between the plenum and the cowling.
Now a friend mentioned that I might get a heat blocking at the heat exchanger with undefined consequences. So to say better dump the air to the cowling rather than getting a cracked exhaust.
What is your experience / opinion ?
Thanks in advance,
Peter