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Date: Fri, 03 Feb 2012 08:02:36 -0500
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From: Gary Casey <casey.gary@yahoo.com>
Reply-To: Gary Casey <casey.gary@yahoo.com>
Subject: Re: Cabin heat
In-Reply-To: <listdigest-5381153@logan.com>
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Peter,=0AWhat 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 bee=
n configured to route the hot air overboard when the heater is turned off, =
not shut it off completely as you describe. =A0Will that portion of the exh=
aust pipe overheat if no air is flowing through the system? =A0I doubt it, =
but I'm not sure. =A0How much hotter would it get? =A0I don't know, but I'm=
 sure not a lot. =A0After all, the rest of the exhaust system doesn't have =
deliberate cooling - it just exists in whatever air flow goes around it, an=
d I've never seen any one worry about that. =A0I would think it more likely=
 that the flex hose attached to the heat muff would overheat and fail prema=
turely. =A0Yes, the air dumped does reduce the air available to cool the en=
gine. =A0I find it ironic that many (including me) go to great trouble to r=
educe all leakage around the shroud, including the use of RTV sealant, and =
then
 ignore the air flow through the heater valve. =A0Maybe I should put a rest=
ricter orifice in the "dump" line to reduce, but not eliminate, that air fl=
ow. =A0The answer to your question is that I'm not sure anyone knows the an=
swer to your question.=0AGary Casey=0A=0AFrom Peter:=0AI am routing my cabi=
n heat hoses and the follwoing was braought to my attention.=0A=A0=0APresen=
tly I take the pressured air from my plenum, than to the heat exchanger mou=
nted on the exhaust and routing the heated air to the valve which I can ope=
n for cabin heat or close it. If I close it, the air gets blocked at the va=
lve 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 dump=
ing it to the plenum reduces the available air to cool the engine and reduc=
es the pressure difference between the plenum and the cowling.=0A=A0=0ANow =
a friend mentioned that I might get a heat blocking at the heat exchanger w=
ith undefined consequences. So to say better dump the air to the cowling ra=
ther=A0than getting a cracked exhaust.=0A=A0=0AWhat is your experience=A0/ =
opinion ?=0A=A0=0AThanks in advance,=0A=A0=0APeter
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<html><body><div style=3D"color:#000; background-color:#fff; font-family:bo=
okman old style, new york, times, serif;font-size:12pt"><div>Peter,</div><d=
iv>What you mention is certainly the "common knowledge" with respect to cab=
in heat and every plane I've seen since the dawn of (my) time has been conf=
igured to route the hot air overboard when the heater is turned off, not sh=
ut it off completely as you describe. &nbsp;Will that portion of the exhaus=
t pipe overheat if no air is flowing through the system? &nbsp;I doubt it, =
but I'm not sure. &nbsp;How much hotter would it get? &nbsp;I don't know, b=
ut I'm sure not a lot. &nbsp;After all, the rest of the exhaust system does=
n't have deliberate cooling - it just exists in whatever air flow goes arou=
nd it, and I've never seen any one worry about that. &nbsp;I would think it=
 more likely that the flex hose attached to the heat muff would overheat an=
d fail prematurely. &nbsp;Yes, the air dumped does reduce the air
 available to cool the engine. &nbsp;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 val=
ve. &nbsp;Maybe I should put a restricter orifice in the "dump" line to red=
uce, but not eliminate, that air flow. &nbsp;The answer to your question is=
 that I'm not sure anyone knows the answer to your question.</div><div>Gary=
 Casey</div><div><br></div><div>From Peter:</div><div><span class=3D"Apple-=
style-span" style=3D"color: rgb(69, 69, 69); font-family: arial, helvetica,=
 sans-serif; font-size: 13px; "><div>I am routing my cabin heat hoses and t=
he follwoing was braought to my attention.</div><div>&nbsp;</div><div>Prese=
ntly I take the pressured air from my plenum, than to the heat exchanger mo=
unted on the exhaust and routing the heated air to the valve which I can op=
en 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 a=
nd dumping it to the plenum reduces the available air to cool the engine an=
d reduces the pressure difference between the plenum and the cowling.</div>=
<div>&nbsp;</div><div>Now a friend mentioned that I might get a heat blocki=
ng at the heat exchanger with undefined consequences. So to say better dump=
 the air to the cowling rather&nbsp;than getting a cracked exhaust.</div><d=
iv>&nbsp;</div><div>What is your experience&nbsp;/ opinion ?</div><div>&nbs=
p;</div><div>Thanks in advance,</div><div>&nbsp;</div><div>Peter</div></spa=
n></div>  </div></body></html>
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