Re: Hurri-Hot heater question
If you close the knob by your left foot, it won't screw anything up. If you have a heater pump or boost pump switch, make sure this is off (not that you'd be using it in the summer, but that could possibly be damaged by running with the valve closed). The underhood valves will help too, but it shouldn't be too bad with just closing the inside valve, and you'll have the option to re-open it while driving if things cool down or you need your defrosters.
The way the heaters work is by running hot antifreeze from the engine to heat exchangers (heater cores) in the driver's heater, step heater, and underseat heater(s). The switches turn on electric fans that blow air across the heater cores, spreading the hot air around. If you don't turn on the blowers, but leave the valves open, you still have 200 degree antifreeze being piped through the bus, which is why it feels like your left leg is about to catch fire. The cooling system in the engine is designed to keep circulating with the heater valves closed, so you won't damage anything.