I have an electrical wiring question.
Background: I'm building my house by contracting out the daunting parts. Through a lack of good communication with the plumber, my plumber ran most of my PEX pipes through the crawlspace under my house instead of the walls. I live in central Missouri, so the temperatures in my crawlspace will likely go below freezing during the winter. I've insulated the crawlspace, but I expect to need a small amount of electrical heat in the crawlspace to prevent the PEX pipes from freezing.
So I need a power socket that will reliably turn on once the temperature gets close to freezing, then turn off once the temperature warms up a bit. Let's say roughly 35 to 45 degrees F. I've found multiple cheap devices that will accomplish this, like the "Thermo Cube" and several products on Amazon (search for "temperature controlled outlet"). You plug them into a power socket, then you plug your heater into them, and they turn some relay(s) off/on depending on the temperature, I suppose. This seems like an easy solution, but I don't want to trust a cheap device with my pipes freezing. The reviews say that they fail after some years.
So, I want a more reliable option, and I think I can accomplish this by purchasing two different temperature-controlled switches and wiring them up in parallel for redundancy. Even if one switch fails (by not providing power when it's freezing), the other switch will provide power. Is there a way I can do this safely?
Here's what I'm proposing: I have a professionally installed GFCI outlet box in my crawlspace, which has 2 plugs. I'd like to install one temperature-controlled switch into each outlet, then wire the outputs of these 2 switches together into a single new outlet.
If the 2 source outlets came from different AC sources, or if the phase was mixed up, I know this would be a recipe for disaster. But if I can confirm that the 2 outlets have the same phase (i.e. the hot and neutral wires aren't swapped), and I keep all the electrical work tidy and capped off, it seems like this should be fine. Thoughts?