Paul,
          Did read on this forum about someone who used
the cooling tube from the muffler to heat the cab. I am
currently trying to see if I can get rid of my cab water
heater which is really quite heavy when the water is added.
So I can either take heated air from the muffler area which
I do not like due to the possibility of CO gas, or somehow
get hot air from the rad exit.  Does anyone have
experience / advice on this matter ??
                            Neil.


Neil,
Bill brought this point up long ago.   First it is hard to beat the freely
available high quality 950°F · plus heat  that can be grabbed from around
an exhaust pipe.  Insulating, and cooling the exhaust is also a good thing
to lower inter cowl ambient temps. This implies the heater muff should be
after any pipe joints and  in a place least likely to corrode or leak.
Properly made it is safe but of course a C0 monitor is wise to have in the
cockpit.

The alternative choice, like you mentioned is an additional water heat
exchanger. It  can be of some advantage if your main water heat exchanger
size is marginal on those days of long hot climbouts.  Sort of like driving
your car across the desert with the windows open and the heater going full
blast to help cool the engine. Anybody remember this?
The other advantage of a water heat exchanger is that going high and fast
you will probably want to pull in the cowl flap somewhat to lower
aerodynamic drag.  The cockpit heater then takes on the  additional  role of
helping to cool the engine.

The disadvantages of the water heat exchanger is weight and quality of heat.
The air coming out the water heat exchanger would be lucky to be 180F
degrees. Probably less.  In cars with small cabins, low outside to inside
delta Ts, and higher tolerances for space and weight limits, water heaters
are a good fit.   In arctic conditions a fuel driven space heater  is
sometimes needed.   Of course near 18K ASL  arctic conditions exist and
unless i'm mistaken, homebuilt cabins aren't insulated well.   Could be
though.   Check out Aerogel.     http://tinyurl.com/yff3rjp

Hence with the water to air method,  to get the BTUs needed, mass flow  will
have to be increased.   In either  case you are going to need a fan or a
source of forced air to move that heat into your cockpit or dump it over
board when not needed.  Increasing air flow will dilute the heat more.

Which system you choose depends on the BTU  needed,  the  space and weight
limits available, and if your engine, could benefit from an additional heat
exchanger.   It is obvious of course,  but the air in your cockpit also
needs to change at least eight times per hour to keep you alert. That infers
positive air intake and a  well designed exhaust path and vents.  Calculate
your cabin volume,  and multiple  by eight.  Divide this number by 60
(minutes) to obtain your minimal flow rate in CFM.   Restrictions to flow
will be the duct size, nozzle size and number of outlets, both the in
incoming and (most designers forget) the exiting air.
This will be more of a concern in using a fan to force air thorough and heat
exchanger. In reality it makes more sense to use forced air since the
airplane encounters that in abundance.


In summary:  Forced air, through an exhaust muff is hard to beat.

Doug in Japan