Janitrol Heaters
Adam - they are effective but there is an FAA mandated check of the htg cylinder every 100 hrs. If this is done - then they are very safe The inspection regime is not straightforward
Unfortunately most a/c I see, servicing of the heater seems to have be overlooked and then it's a bit like flying with your own bomb on board! Let's bleed off some avgas into the cabin and ignite it!
Whoosh!
Stik