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What if you want to put the thermostat pre-set at 74 degrees in a clear wall mount tamper-proof enclosure and install a 6 hour timer next to it so the knuckleheads don’t leave the a/c on when they leave? Should I just break the G wire to the timer and back or what? Will that affect the delays the thermostat has for pumping out the ventilation system before it shuts off?

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    G wire is for the fan. Y is for cooling. So you want them to be able to turn the A/C on whenever they want but force it to turn off in 6 hours (at most)? – JPhi1618 Nov 13 '19 at 19:33
  • why not just get a programmable thermostat? they are cheap these days... – dandavis Nov 13 '19 at 20:27
  • Hello, and welcome to Home Improvement. What exactly are you trying to accomplish? And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Nov 13 '19 at 23:29
  • Yes -- most commercial-type programmable thermostats can do this... – ThreePhaseEel Nov 14 '19 at 0:16
  • Okay, these are the specifics... The thermostat is on the wall in a sober living drug treatment center and the young adults have no common sense or consideration for anything or anyone but themselves, they turn on the air and leave the entire day with it blasting cold air... I want them to be able to turn it on but I want it to shut off in three or six hours... if I use a programmable they will mess with it and change it then call me and say it doesn’t work – Tom Ziegler Nov 14 '19 at 0:46
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No one would ever do that.

They would get a programmable thermostat. It is trivial to set up programs that do exactly what you want to do. Further, many thermostats have mechanical gating so the up and down (temperature) buttons are readily accessible and changeable right now, but will reset at the next program time. Access to program is gated by a special door. You can then put a cage over top of it so they can access the buttons, but not the door. They typically allow 4 program changes per day.

This is a $50 solution, and you can't homebrew a thing like that for so little.

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I've personally never seen a programmable thermostat that you can turn on whenever you want and have it automatically turn off after a certain amount of time. A programmable that I'm used to seeing would, for instance, always turn the A/C off at 6:00pm or 9:00pm, but not "6 hours from when it was last turned on". If anyone knows of a thermostat that has the specific function ask for, please post a comment.

Using a programmable thermostat would be the best option, but if you really want what you have described (like a hot tub timer at a public pool - turn on any time, always turn off 20 min later), then you can wire the Y and G wires together and pass those through a timer.

The Y wire calls for cooling and G calls for fan. If you only put the Y wire on a timer, the fan will still run all the time. If G is on a timer you could potentially run the compressor with no fan and it would freeze up and maybe cause some damage. The good news is that it's acceptable to just wire Y and G together. You loose the ability to control the fan separately and you will not get any fan "after run".

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I've seen something like you're asking used in hotels before. They would have a regular thermostat that allowed the guest to adjust the temperature to their liking, but there was also a keycard reader tied in. In order for the A/C to operate, the guest would have to insert their room key into this device and leave it in. After removing the key (which tends to be a requirement when you leave a hotel room, as you can't get back in without it), the A/C would shut off.

An example I found by Googling would be here

This is a similar system, but it uses occupancy sensors instead of the keycard.

  • I APPRECIATE YOU ALL... So I checked out “take our tour”, extremely complicated, confusing and cumbersome, not really something I want to get involved with, but I will tell you all, this site is the first time ever I have posted a question and got a reply so quickly, usually I have to check back a decade later to finally get a reply, and I got my answer from whoever told me to break the Y and G wire, that’s the answer I needed, I knew this was the right place. Thanks everybody. – Tom Ziegler Nov 15 '19 at 4:40

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