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I have a C150 in a hangar, but no jet heater. I was planning on just putting a directional space heater (an FH-06B) in the footwell and opening the cabin heat to let the heat through to the engine.
I figured that running that for an hour or two would be better than 20-min with a jet heater and almost as good as a heated hangar.
Testing this out, the rudder pedals get very hot, but not so hot that you can't touch them.
Are there any flaws with this plan?
Almost no heat gets to the engine. What hot air drifts through the heater SCAT hose might warm the exhaust muff a bit. Plus you have that fire hazard heater in the cabin.
Get some of that aluminum flexible ducting used for clothes dryers and form the end to fit over the heater, and tape it on with aluminum speed tape. Set the heater on the ground to the side with the duct stuck in the bottom of the cowl. Put some foam in the cowl inlets. 30 minutes to an hour is all you need.
A lot of people just stick the heater itself under the cowl but it's way safer to duct the air in with the heater outside and works just as well.
Preheating is traditionally all about warming up thick oil and the galleries the oil has to flow through, not so much heating the metal itself for the sake of the metal (the cylinder doesn't much care if it warms to 350F starting out at 20F or 50F as long as oil is getting to the parts).
If the OAT is above 20F and you are using multi-grade oil like Aeroshell 5W50, the oil will flow fine at ambient temperature and there is really no need to preheat if the rise in oil pressure is not significantly delayed from warm weather starting. Not saying not to do it, but if I had no preheater and it was 20 degrees, and I was using multigrade oil, I wouldn't ground myself and would start the engine anyway.
This won't work well as there's no way for the hot air to get from the cabin to the engine directly. The airplane is designed to prevent air getting from the engine bay to the cabin, and vice-versa. Cabin air is heated by exchanging heat with the engine exhaust system, the air is forced through that duct from the outside airflow over the cowling. So there will be nothing to force air back the other way. What heat does go that direction will heat the engine exhaust system a bit, however very little is going to reach the engine.
Heating the cabin will heat the whole airplane a bit as the heat conducts through the metal, but it's very inefficient and unlikely to be enough for your purposes.
