I use a 175,000 BTU Torpedo Heater now to preheat my Aircraft. I have a dedicated Heater and it works fine but this is better. The Heater is also heating the interior of the Aircraft as well as the Engine and Hangar.
When the Heater is first fired, you can stand in front of the Aircraft and feel the Air Temperature coming out of the Cowl Inlets. The Cowl Flaps are receiving warm Air but the Air coming out of the Cowl inlet Vents is cold.
Later, you can feel the increase in Air temperature coming out of the Cowl inlet Vents indicating the Engine is warming up. You'll like this approach and it's a good reason to buy a Torpedo Heater. You know you've wanted one for some time ...