I have a gas ceiling mounted furnace for my garage.
I want to set my garage at 35-40° F (1-4° C).
Is there a special thermostat to control furnace at that low temp? (I've seen one works as low as 50° F (10° C), but not as low as 35-40° F)
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You may be able to hack something together with a refrigerator thermostat. You'd have to do a little tinkering though, since it would do the opposite of what you want to do. When the temperature rises above a certain point, the fridge thermostat will cue the appliance/heater to turn on. If you can figure out how to reverse the process, this could be a possible solution.
If you don't want to pay $50 for a thermostat that you may or may not be able to make work for your solution, Hack A Day featured how to build one a few years ago. It may also be easier just to edit the code to make the thermostat turn on the heater when the temperature drops below a threshold rather than above it. (That is, if you're familiar with that sort of thing.)