There’s an entertaining illustration of heater failure in a twin (presumably Janitrol) in one of those ‘Dangerous Flights’ TV shows. Crossing NE Russia in long legs, and scrunching up the maps for insulation.
@Peter Timothy must be referring to the “cycling switch” which cuts the ignition if the temperature reaches 250F or so (there is a screw to tune the temperature).
For me the lack of cabin heat is the limiting factor. Our cabin heater puts out a lukewarm jet that defrosts the pilot’s right knee and that’s about it – motorcycle jacket and gloves is the order of the day. It does make tuning the radio harder though.
Many diesel cars have fuel burning heaters, because sometimes there is not enough energy to heat the cabin just from the coolant. The good thing is that it can be used to pre heat the cabin and the engine, because it’s in the water circuit.
There are preheater mods for many cars, including ones where you can turn it on via SMS. I knew a pilot in Germany who had a Volvo 4×4 which had that. Even my VW Scirocco has a mod available – for a few k IIRC – and these are reportedly popular in countries where most people live in blocks of flats so the cars are parked outside.
But I would imagine these are properly engineered – not like the junk we have to put up with in GA. OTOH, it if catches fire you just stop and get out. And one sees many burnt out cars by the side of the road, with people standing outside wondering what the hell happened.
There are specific maintenance issues which can make a plane cold. On the TB20, there is a temperature adjustment which is a mixer to introduce outside air. If this is not adjusted just right, and there is even a tiny gap when that flap is supposed to be closed, you freeze your balls off
Also, on most planes, the hoses are perished, with holes in them, etc. They can cost a fair bit of money to replace…
Once I flew in an RV which had so many gaps (you could see the ground below through them) that you would need the exhaust of an IO540 to keep it warm 
I suspect this (a reasonable build quality and a certain amount of interior insulation and trim) is one of the things you pay for. I bet a lot of the UL-category airframes are freezing in the winter, with no insulation at all.
Peter wrote:
you freeze your balls off
Other genders may vary
We flew up in the Aztec many years ago, to central Quebec, to pick up a 172 my colleague had purchased. It was -42C that night, on the way up. Though the Janitrol kept working, it could not keep up. We were cold! The next morning, I volunteered (really demanded) to fly the 172 home. I knew I could get better cabin heat. It was -39C when I took off, though it had spent the night in the hangar, so it was not frozen solid. I flew for three hours into warmer air, and it was not so bad. Interestingly, in unrelated research, I did find that the spec on the O rings that Cessna uses in the fuel system say -20C is the coldest (so I avoid sampling fuel colder than that).
The other Aztec I used to fly in northern Ontario had two Janitrols, just in case one quit. They were reputed as being not entirely reliable, though I only have on recollection of popping the breaker.
The coldest I’ve flown my 150 was -28C, no problem….
I think flying on winter nights is not actually possible in a lot of GA planes. The greenhouse effect is huge and even in overcast daylight you get a lot of it.
Have those with the Janitrol heaters ever made allowance for their fuel burn ?
Peter wrote:
Have those with the Janitrol heaters ever made allowance for their fuel burn ?
I’ve seen some flight manuals mention it, but generally it’s within the leaning variability. Some flight manuals make no mention of heater fuel consumption.
