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Under the 2020 NEC, I'm looking to add an extra fan to a freestanding gas fireplace beyond the built in blower. The extra fan is an ordinary wall fan with a 15A/120V plug.

The existing blower uses a 120℉ on and 90℉ off temperature switch connected by magnet to the fireplace.

temperature switch female disconnect terminal

The existing blower uses insulated disconnect terminals for all of its connections. There's no ground on the blower's switch, and there's no ground on the analogous switches that I've been able to source. Here's a quick schematic:

simple schematic

My wall fan's circuit seems easy enough to build if I naively analogize to the existing blower's circuit. I anticipate, however, that I'm entering a minefield and that my bumbling will land far away from code conformance.

  • My first instinct was to use the blower's current draw to trigger a relay feeding the fan. The "current switches" that I found required a minimum current draw of 0.25A (for tools triggering dust collection) which is my blower's exact rating. The dang blower is obnoxiously loud at full throttle, so I rejected these "current switches." If there exists a more sensitive switch (say 0.05A or 0.10A) that's reasonably priced, then coupling the fan and the blower becomes a viable option. Without anticipating a dust collection use case (and an inline blower use case), I probably wouldn't have found any such switches. I would love to learn of any other name besides "current switch" for these trinkets.

  • Assuming that I install a new double gang box with a 15A receptacle in one side for the wall fan, does there exist some combination of cable plus strain relief gland that I can legally use for diverting the receptacle's hot, where the cable would integrate the temperature switch into the circuit? Both of the cable wire ends (no ground) would not technically be exposed, but the insulation of the required female disconnect terminals make me uncomfortable. The uninsulated male disconnect terminals on the switch make me uncomfortable.

  • Assuming that the cable with technically unexposed wires is no good, can I terminate the cable at a metal gang box? The metal box would conduct the fireplace's heat to a switch that is wholly contained within the box. Could I get away with "fastening" this box to the fireplace by magnet? If not, then could mechanical fastening make it code conformant?

  • Any other ideas?

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  • Not sure I understand the safety / enclosure concern with the existing switch. Is it accessible without disassembling the fire place? Is it on the outside?
    – jay613
    Nov 15 at 11:23
  • @jay613, the blower came with the fireplace, so I assume that the setup is listed. The existing switch is on the side of the fireplace and all the touchable electrical is insulated. If it was disassembled a little, however, then I can imagine an uninsulated male terminal being energized as the fireplace is running. I was just surprised that there wasn't some kind of mechanically fastened barrier to prevent access to the switch without using some kind of driver. I'm not really concerned about it, but I suspect that I can't do the same exact thing within the code.
    – popham
    Nov 15 at 11:31
  • What is that additional fan going to to? add ventilation for the combustion?
    – ratchet freak
    Nov 15 at 11:38
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    Pictures of the switch in place might be more helpful to formulating a method of powering the other fan from it, or from a signal by it. The switch in isolation and the verbal description is not doing it for me, at least.
    – Ecnerwal
    Nov 15 at 14:05
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    How about using a roof vent thermostat? You can mount it right next to the fireplace and then tweak the temperature until you get the behavior you want. There are plenty of UL listed ones available with full enclosures.
    – KMJ
    Nov 15 at 16:09

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I found temperature probe switched outlets on Amazon. CPU thermal paste and flue tape look like a good method to attach the probe. I would prefer the simplicity of the 120℉/90℉ temperature switch mentioned in the question, but I don't know if code conformity is even possible with such a switch installed DIY.

There exist high temperature probes for inside ovens and for industrial applications. Instead of getting a very heat tolerant probe, I'll experiment with an infrared thermometer and watch my candidate probe position's temperature as the fireplace heats up.

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  • I would be very wary of the device you linked to - it does not appear to have any listing, testing, or certification marks. There are probably similar devices available from reputable companies that do proper testing and certification, but this isn't a shopping site.
    – Moshe Katz
    Nov 15 at 21:42
  • Thanks, @Moshe Katz. I'd like to think that I would research further before buying the dumb thing, but there's a good chance that you spared me. I'll find a listed one and update the link.
    – popham
    Nov 15 at 21:45
  • @Moshe Katz, I found a listed controller, amazon.com/RANCO-ETC-111000-Digital-Temperature-Control/dp/…. At 400% of the price, I'm going to stick with unlisted. These products tend to be created for home brewing, reptile enclosures, etc. I would never run 15A on the thing like it specifies. Even a 1/3HP fan would only draw 2A, though, where I'm probably drawing 1A. And in a dry environment. I do hate having something like this laying around the house with 15A stamped on it. Maybe I'll cross it out and write "2A" on it.
    – popham
    Nov 15 at 22:36

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