I would like my A/C to self turn on at 8pm, that is before I get home and have the ability to cut off power automatically. I have Sonoff basic devices doing this with my lawn lights, pool, etc. I have a 1 Channel Inching /Self-Locking WiFi Wireless Switch 5V 12V that's been sitting here unused:

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I connected it to 1 zone of my sprinkler system and it worked as expected - when the sprinkler controller turned on that zone I could turn on/off the Sonoff WIFI switch which then turned on/off that zone but for so long, then off. I thought it's summer, turn the HVAC selector to AC, find the power line, cut it, insert them in this low voltage Sonoff, and I can turn on/off AC on my phone or just use a set timer. The thermostat is a Honeywell RTH110B here is installation manual. Sonoff device is powered by cellphone charger or 12vc dc connectors.

  • yes, that's correct, and that's the right sonoff too (most switch power and won't work for this application, that one will). You can wire it in series so that both the thermostat and wifi must agree, or in parallel to allow either or to switch it on, which lets you use the thermostat to set fail-safe limits, in case the wifi flakes out. – dandavis Jun 19 '18 at 7:01
  • Ok. I am not an electrical eng. but an enthusiastic DIY'er. I like to gather my step by step info. then proceed that way I don't mess up anything. Please do an ELI5 explanation on how to do each. Thank you. – user9232 Jun 19 '18 at 12:48
  • I swamped today, but we can break the unique problem into 2 common and well-documented steps. Use the left-hand triple connector the exact same as a thermostat, wiring the two thermostat control wires to the middle terminal and the NO terminal. Then watch a video or two on configuring and controlling sonoffs. You might also ask for help on the ardunio stack, since sonoffs are technically esp8266-based MCUs. – dandavis Jun 19 '18 at 14:52
  • Which is the thermostat controlling wires? There's Rh Heating power supply, Rc-Cooling power supply, W-Heating signal, Y-Cooling signal, G-Fan. Here is a pic of the manual. – user9232 Jun 20 '18 at 11:51
  • as described in question, you would want to switch the cooling signal. if you want to control the heat and fan too you need additional relays. – dandavis Jun 20 '18 at 14:02

This is an old question, but there are several solutions here that may help others who stumble upon it.

The easiest and most consumer-friendly way to do this is with a "smart" thermostat, these are designed to be controlled remotely and also can be activated when you're home/away, at a set time of day, minimum/maximum temperature, or when you reach a certain distance from your house (tracked by GPS).

The Sonoff you mentioned is capable of doing this as well, in a tinker/hacker way. I would actually recommend you use a 2-channel Sonoff so one channel can control heating and one channel control cooling. As you're configuring the Sonoff remember that "inching" means "momentary connection" and "locking" means "closed circuit" like a normal switch. I believe thermostats "lock" until the desired temperature is reached. Sonoffs contain relays with several hundred volts and several amps of capacity.

As @dandavis commented previously, I would wire these in parallel with your thermostat wiring so that your thermostat still controls your system also. The answers about what to connect with what should be in your thermostat manual, I haven't read it. If you're more of a "do-er" than a reader, you can use a voltmeter to check the thermostat's continuity during operation and figure out where to bridge your Sonoff that way.


this Channel Inching /Self-Locking WiFi Wireless Switch 5V 12V will not work. It will stay turn off or turn on. it is not a switch

  • Hello, and welcome to Home Improvement. I'm not sure you're right, but thanks for the answer. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jan 10 '20 at 22:21

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