Originally Posted by alwechs
Newby here. Noticed a fan sound coming from the area of the pump and Heater. Stays on when pump is off. Any idea what this is? Wouldn't a nonstop fan kill the house battery?
More information on your precise year and floor plan might help, but the short answer is you are correct to worry it will drain your battery.
I'd probably start pulling fuses to stop it until you can diagnose the problem further.