Cheap Heat is a furnace modification that converts it from propane only, to a hybrid that runs on either propane, or electric, changing modes at the touch of a switch. Hot air flows through the furnace ducting regardless of which mode it's in. Unlike using space heaters or heat strips/heat pumps, the hot air also goes into the underbelly. So although running the furnace quite a bit, I'm not refilling propane tanks.
Here's a link.