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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.

an electrical wiring diagram showing a thermo-cube and an electronic thermostat controlled switch

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?

  • install an electric baseboard heater in the crawlspace – jsotola Sep 30 at 6:17
  • for the heater itself you can use heater wire used for the drain for the evaporator of walk-in freezers. wrap it around the pex and insulate around it. – ratchet freak Sep 30 at 8:11
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    If you use two thermostats in parallel to protect yourself from one failing, you will only add a bit more time between failures. If one thermostat fails, you won't know about the failure until the second one also fails. That's not much better than having only one to begin with. – kgutwin Sep 30 at 18:07
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    Aren't self-regulating heating cables an option? – fraxinus Sep 30 at 18:43
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    I'm a bit concerned honestly about a plumber who doesn't think about these sorts of issues. Hopefully you checked the rest of their work... – Joe Sep 30 at 22:38
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No, you can't do anything like that at all. That's a suicide cord.

Meaning it has 2 plugs on it, and in certain conditions the prongs of one can be unplugged and live. Nothing should ever have 2 plugs.

Remove the "plug-in thermostats" from the equation - they're cheap anyway.

Use hardwired thermostats -- those can be paralleled in the manner you are thinking of. And much higher quality.

Speaking of cheap, you should not be using cheap heaters that plug in. Again use hardwired heaters that are made for long-term use and UL-listed to be run unattended.

Also, consider simply having 2 heating devices.

If the readily available quality hardwired heaters (e.g. Cadet, $40) are too high power for what you want, remember you can run a 240V heater on 120V and you will get 1/4 the power. For instance if you want 250W, a 1000W Cadet heater will work great.

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    +1 for the 2 devices - heaters are as likely to fail as thermostats. – Chris H Sep 30 at 19:29
  • Where could I find more info about such a hardwired thermostat? Are you talking about the kind you would find inside a house to control a regular HVAC system? – Patrick Oct 1 at 13:24
  • Oh, found it, the Cadet baseboard heaters. And they sell a mechanical thermostat that goes down to 40 degrees F. Looks like I can find these at big-box home improvement stores. Thanks! – Patrick Oct 1 at 13:48
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    @Patrick a variety of people sell “240V” thermostats that fit in a 1-gang box. You can wire them for 120V. – Harper - Reinstate Monica Oct 1 at 15:28
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Unless you go to more expensive industrial products, you'll have trouble finding thermostats that operate reliably at near freezing temperatures. Using a space heater to heat up the entire crawl space is going to be inefficient when all you need is to keep the pipes above freezing.

Heat tape seems like a much better option, it's made for this exact purpose. There are lots of options available if you shop around. Many work without a thermostat, the heating element automatically stays at a temperature just warm enough to keep the pipes from freezing. The heat is applied exactly where it is needed. You run it inside pipe insulation so very little of the heat is wasted heating the crawl space.

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    We had our well-head freeze twice, so we decided to go to city water instead. When we ran the pipe in through the foundation, it came up through the dirt crawl space floor, then we suspended it from the floor joists for the run to the connection point. We taped on a run of heat tape, wrapped it in foam pipe insulation, and haven't had a freeze in 25+ years. I highly recommend it. – FreeMan Sep 30 at 11:51
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    Self-regulating heat tape (no thermostat need) is the stuff you install after your pipes freeze the first time, unless you manage "learning from the mistakes of others" well and do it the first time. – Ecnerwal Sep 30 at 16:04
  • A little off topic for the main question, but definitely helpful for my ultimate problem! I've thought about heat tape. One challenge for me is that there are several different runs of pipes, including at least one T intersection. Running a single strand of heat tape would involve lots of backtracking and/or going across areas without pipe. Is there a way to do this without it looking like a rat's nest? Particularly a way to make intersections in the heat tape. – Patrick Oct 1 at 13:57
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    Yes, there are many products available and lots of them have splice / tee kits and some can be spliced or tapped to branch off with standard connectors etc. – batsplatsterson Oct 1 at 14:17
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It seems to me the biggest problem is if your heater doesn't turn on and you don't know about it, right? If it fails to turn on but you do know about it, you can address the problem.

As such, if it were me I'd consider this a "good" use for home automation. I'd get a plug that could connect to my network (either directly or via a hub) and have that be in charge. You can get a connected thermostat and put that down there also, or simply set up an automation based on the weather report. You can couple that with alerts if it's not on, but the weather is freezing (but most of those will alert you if they fail to respond to an automation cue), and have an alert on your phone whenever the weather is below, say, 20, to remember to check on it.

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  • I would go with multiple redundancy. Wired freeze sensor on home alarm system, wired temp probe to home automation system, and power draw sensors for automation to detect when the heater is running (or when it fails) – Richie Frame Oct 1 at 0:50
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What's available here in the UK may be a bit different to what you can get, but this might guide you a bit.

We have mains-rated thermostats designed to bring on gas-fired heating systems and protect the system and building from frost (called frost stats). That's what you're trying to do. These are strictly for hardwiring, are are only suitable for switching low loads - but that's OK, as you should be using trace heat tape underneath insulation, rather than heating a lot of air to heat a little pipe. Typically they'd turn on at about 10°C (50°F), and you probably don't want to heat your whole crawlspace that much.

Your second system (as Harper suggests) could be space heating with a lower setpoint, so it only comes on if things are very cold indeed. Greenhouse heaters would seem to be suitable - they're more rugged and splashproof than heaters designed for indoor use, and have lower thermostats.

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it may be possible to put insulation in such that the pipe is insulated from the outside/crawlspace but not from the house. That way, the heat from the house will keep the pipes warm. I had to do this in my house. The foam insulation guy sprayed foam onto the underside of the floor and left the water pipe exposed. The pipe that never froze started freezing. I dug out some of the foam insulation to let heat from the house into the space, then enclosed rest of the space with foam board. Hasn't frozen since.

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I don't disagree with the other answers, but if you really want to do something like in your diagram, what would would want is for the two thermostats to switch a low voltage like 12vdc or 24vac to then power a relay that would switch power to the heater. That way, you don't have two 120v switches that are connected together making a dangerous situation. You just have harmless low voltage that turns on a relay and is isolated from the dangerous mains voltage.

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