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I wish to change my greenhouse fan from a 110 unit to 220. I have the fan. The price was right. It was free. The 220 is available. Is there a way to use the 110v thermostat to power a switch (or whatever it would be called) to open and close the 220 power? If so what would this "switch" be called?

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  • What country are you in? What electrical code governs your jurisdiction? There are two different types of switches I can think of off the top of my head, possibly more if you are really looking for a relay (switch driven electronically, as opposed to physically).
    – user4302
    Jun 29, 2017 at 18:14

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You're looking for a relay which is designed for use in Electrical Code approved installations.

First, do you want to upgrade to a safer and far more versatile 24V thermostat? The sky is the limit with those, you can get programmable, touch screen, even Internet connected thermostats like the Nest. They can also control heaters and even multi-stage. They also let you place the thermostat where you please, and do the long wire runs with easy-to-install low voltage thermostat cable.

If so, you'd want a 24V transformer to power the thermostat and remotely control the fan relay. A 40VA transformer should be plenty, and these are readily available for about $12, as they are a common heating/cooling part.

Then you can use an air conditioning relay (about $12 also). This one works on 24V or 120V and mounts on a junction box knockout, saving you the trouble of mounting it in a junction box.

Here's a combo transformer and relay which mounts on a junction box cover. This keeps the 24V on the outside and the 120/240 on the inside, as required by Code.

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    Worth noting: many 120V thermostats are just dry contacts (labeled "relay" or "SPST"), so you can actually have it control a 24V relay as well. The ones I found also support 240V which may make the relay unnecessary (if the rating is sufficient), however, personally, I'd still probably do the 24V system for the reasons mentioned (even if you keep the existing thermostat, replacing it with something better later is simple).
    – gregmac
    Jun 29, 2017 at 19:49
  • Thank you. This is a new area for me, but I'll try to sort it all out. Looks like the way to go though.
    – Harold Young
    Jun 30, 2017 at 16:16
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Just buy one of these.

Less than $20 and they work on both 120 and 240 and have an "off" setting.

Good luck!

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You're talking about a line voltage T'stat. So even though the price is right, a 110V T'stat won't work. I guess you could take to a vendor and see if they will give you anything for it in exchange for the right T'stat. Otherwise follow @Harper's advice.

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