I have an idea but I have no way to validate if its even possible because I'm not an engineer and know only enough about heat science to get me into trouble.
My cruiser as a separate liquid powered rear heat unit that branches off the main coolant line. I love this heater but its generally regarded with disdain the community because it can cause problem in unmaintained cooling systems which mine isnt. Let me tell you what my idea is and maybe there are some smart engineering types that could help. It pisses me off that my dad is gone, because:
A. This was his living and his passion (engineer specializing in heating and cooling)
B. This is exactly the kind of project he would have loved. And I could have even got him to talk...maybe.
Anyway. So I've got this small heater that will easily heat the entire car by itself, I'm guessing its in the 3000-5000 BTU class. The only problem with it is that because its engine coolant fed, when the engine is off, the party is off.
The whole reason this came into my head was thinking about ways to stay warm when I go camping with my little girls. My thought was that if i could find an alternative heat source for the water, I could bypass the coolant and run a closed circuit system that would heat the car without having to run to engine.
I'm guessing that the supply temp is ~210 F and the return is likely in the 200's for a delta T of 10, and the flow rate is ~8 gpm from the water pump currently
My goal would be to design an 12v healing element and aux pump to circulate hot water (coolant, really) through this forced air heating unit powered off the battery. The things I would need then would be
1. the smallest heating element I can get away with
2. A voltage regulator to make sure to not flatten the house battery
3. a suitable low power waterpump in the 5-10 GPM range.
The trouble is, I don't know what it would take to heat 5 gallons a minute 10 degrees so Im not sure what heating element would work.
In the end, its looking like in order to heat a car interior from 32 outside to 50 inside I would need something like 1000-2000 btu/h, which works out to 300-600 watts/h, not including high flow high temp pump and when it all comes down to it...I'm probably looking at 50 Ah draw. Can't sustain that on a battery.
Damn. It was a neat idea.