The trap door simply lifts out of the way. When I finish the porch and figure out what sort of flooring will be there I'll sort out an easy way of dealing with the opening. Ideally I want the trap door to be invisible during the summer months.
The heat pump and furnace are on the same thermostat, its a fancy pants honeywell (as fancy as they get without wifi). the thermostat is at maximum capacity with 2 stage cooling (heat pump), two stages of heating (heat pump) and another 2 stages of propane heat. as well as a whole house HRV.
heating cost is hard to nail down when considering wood fire efficiency, actual wood cost, wood species BTU, propane BTU efficiency and averaged cost, and electricity time of use and delivered cost.