ANNUAL, 2005 Redux...

Speaking of being toasty warm, the last trip I made in my Aztec was at the
first
of January at night with the OAT around 20 degs F. The heater was not
working.
After a 6 hour round trip in that, even with heavy clothes, jackets, gloves
and blankets,
we were miserable. It is hard to tune the radios with ski gloves, so my
right hand was in
and out of the glove a lot, and the result was numb fingers.

That finally provided the motivation needed to get the heater working. It
is a Southwind
Model 940DB12, which is rated at 15,500 BTU/hr on low and 27,500 BTU/hr on
high. I used to wonder if it were even capable of keeping the cabin warm,
but after
looking at my old thermo book and doing a little calculation, I determined
that if a heater
that size can't keep the Aztec cabin warm, something is really, bad wrong.

Anyway, after studying the service manual on the heater for a while to
understand how it
was supposed to worked and doing a little trouble shooting, it was revealed
that the heater
overhaul shop had incorrectly wired the heater. I don't know how it ever
passed a burn
test. I'll bet it didn't. It would light from the prime fuel charge, open
both fuel
values, burn very rich and smoke migthly until the high temperature
thermostat opened which
closed both fuel values. Only the high heat fuel value was suppoed to be
controlled
by the thermostat. Controlling both fuel values with the thermostat caused
combustion
to stop once the heater exceeded the 240 deg F outlet temperature. Once the
flame was
out, the heater had to be turned off, allowed to cool, and reprimed before
it would ignite again.

It was supposed to ignite, burn on low, and only open the second fuel value
if the operator
selected high heat. In high heat, the increased fuel flow requires
additional combustion air
for the proper mixture. Proper combustion airflow depends on an increased
pressure head
from the slipstream. The airplane needs to be flying to provide this 1" to
2" of water pressure
head in the intake. There is a very small ram air scoop on the air intake
for this purpose, which
has been "calibrated" with a couple of small holes drilled into the flage,
at least I'm guessing that what
the small holes are for. This is why the airplane's Approved Flight Manual
advices against using
high heat during ground operatiions and cautions that doing so will result
in excessive smoking
from the heater exhaust.

Once it was wired to match the wiring diagram in the service manual, it
works like a champ.
We have heat!!!!!!!

I'm still thankful for you guys educating me about C&D and I will use them
for
parts and/or a rebuilt should the old Southwind hang it up, but for now, it
is going
strong. Now I need to add a CO detector in the cabin to warn is the heat
exchanger
springs a leak.

I also need to get motivated to relace the side windows like. I need Jim
Burns for
my partner :-)

Ronnie


"Denny" wrote in message
oups.com...
That's true Jim... I am toasty warm and heven't even run the heater on
the highest setting yet. Subzero wx is comin so I will have the chance
I'm sure... The carb seems to be running just fine, now that we put
all the parts inside that the designer intended...

denny