Peter wrote:
Is there a good reason why twins don’t use engine heat for the cabin?
They use e.g. DA42 uses left engine for windshield heating and right engine to heat the cabin. However, due to using heat exchanges, loses in transport and poor insulation, it’s not very efficient.
Is there a good reason why twins don’t use engine heat for the cabin?
You would only need to run a pipe of 2-3m, say 5cm diameter, from each engine, which is trivial, and far less troublesome than the Janitrol heaters.
When I was flying the F33A I noticed by accident that running the engine at 2500rpm instead of the usual 2300rpm did really help with the heating.
In fact our engine is running really cool, so although we use 2500rpm usually at this altitude (especially with headwinds to be as fast as possible), the effect of heating is near to zero. But warm clothes like ski gear helps a lot. 
When I was flying the F33A I noticed by accident that running the engine at 2500rpm instead of the usual 2300rpm did really help with the heating. But still after a ski trip I would just keep the ski gear on in the plane for the winter night flight home.
When flying back from Brittany to Stuttgart last Wednesday we had -18 degrees Celsius at FL 110. At this outside temperature no chance for the cabin to heat up. This we could feel especially when coming into the clouds without the sun shining on our faces anymore. But as we have been on the market in the morning, for the transport of the fish we bought, it was excellent, no ice needed. 
-18
-8
-0
It is a preheat system for the cabin, something you install and turn on with a SMS or some other remote means, at the same time when you start preheating the firewall forward side of things.
I think Tanis do ground-ops pre heaters, not meant for use in flight, no?
I’ve just come across this Tanis cabin heater. It’s amazingly cheap… is this if any relevance?
They say it is for a Cirrus but the voltage says 115V or 230V.
I would not in a pressurized plane, but if it is your only solution due to lack of alternator power and you have the option of throwing it out in case of emergency, I think I would , no?
Frankly I rate the Janitrol solution worse in terms of safety.
Latest news on testing the cockpit heating mats:
Both connected in parallel and under carpet.
Initial current draw @ 28 volts: 8.5 A , tapers within seconds to below 8 A . After 2 minutes 3.75 A total and carpet temp around 45 C. Warm but harmless. It’s a keeper !
It might, but do you want to carry a big lithium battery?
