Thank you, the air conditioning switch on my B-377 is always left on.
I still have inside temps that are so cold they would kill the crew and passengers in real life.
Sad to hear the solution wasn't that simple. Do you do the start-up manually and by the book? Have you verified you have electrical system properly set up? Obviously, if you're able to start up the engines battery switch much be on already (as having main battery switch turned off, you can't even get APU power to bus and to starters) but as you have the engines running are the generators on and functioning? Have you checked "BUS" voltage with the selector and gauge on lower right of FE panel? When running APU, or when flying (engines at higher than idle rpm) you should have voltage above 24 volts. If not, are the generator condition lights on or off (next to generator switches)?
You could do a ground check of the heater to keep things simple (provided that Accusim 1.0 works the same way regarding heating as COTS Accusim):
Checking your BUS voltage between every operation:
- battery on
(should read 22-24 volts for full battery, lower for half laden, and lower for colder climate)
- APU: start/run, then connect to bus
(should read around 26-30 volts depending on electric load)
- air conditioning master
- ground blower
- body heater.
It'll take several minutes to show a noticeable difference but the gauge should start to move slowly. (Note: you cannot apply cooling system with APU only. Air-conditioning takes ridiculous amounts of energy to operate and is (AFAIK) safeguarded to not engage without two(?) main engines running.)
If, no matter how certain you are you've switched everything on, it still doesn't make the temperature rise above ambient, I'd try reinstalling the plane and expansion to see if the bug will go away like that. It's an addon on top of FSX and whatever Microsoft makes doesn't work exactly the same way twice... usually. Exception proves the rule.
As for what the body heater is, I think the "body" basically means "fuselage" (cockpit+cabin+galley+cargo holds+central oil tank) to differentiate it from carb heat, pitot heat, propeller de-ice, wing de-ice, etc. (De-icing aren't necessarily
done by heat and can also be implemented with expandable rubber boots that physically break the ice. But even the de-icers based on heat tend to prefer bleed air rather than electricity. Same goes for cabin pressurization even though exceptions exist).
Ground blower is needed to circulate air in the cabin. Just warming up a heat exchanger doesn't warm up the cabin if you don't push air through the heat exchanger. You need airflow for warming and cooling. When you're flying, you get can get airflow from numerous sources: ram air (air passing alongside the plane, scooped as needed into the airplane) or bleed air (air compressed by the turbos providing heat+pressurization to the cabin (tuned for a more precise temperature by complementing it with body heater)).