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Old 12-07-2011, 05:11 PM   #1
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Can you put two thermostats in paralell?


I am thinking of building a computer controlled thermostat using a relay board / temp sensor kit, but I would like to keep my existing thermostat as an override. Basically I would have it set to the lowest it can go, but if my device stops working or I want to override it completely for maintenance or whatever, I still want to be able to use that thermostat. Is it as simple as just connecting all the wires together? So common goes to both tstats, Y goes to both etc... or is there a proper way of doing it? So if any of the thermostat makes a call it will then go. So whether it's just the fan, the heat, AC etc...

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Old 12-07-2011, 05:42 PM   #2
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Can you put two thermostats in paralell?


oh boy......another late night service call in the works

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Old 12-07-2011, 05:55 PM   #3
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Can you put two thermostats in paralell?


Yeah, sounds really funky, Squirrell. You could always get some 6 or 8 pin molex connectors, then put a female on the stat wires coming out of the wall, then put a short pigtail with a male connector on each stat. Then you could swap over from one to the other quickly. Unplug one, plug the other in.
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Old 12-07-2011, 07:18 PM   #4
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Can you put two thermostats in paralell?


Many stats will backfeed to energize Y (compressor) when you send 24VAC to G. You would need to isolate with a relay to prevent this.
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Old 12-07-2011, 09:09 PM   #5
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Can you put two thermostats in paralell?


Ok so short answer, is no I'm taking it. Not a huge deal to swap them in the event it fails, but figured I'd make it easier. I will be testing my custom setup for a while and checking all the log files before I put it live anyway. My biggest concern is the accuracy of the thermistor. If that messes up, the whole thing does. The kit I ordered uses a digital sensor and not a straight thermistor though, so don't think I have to calculate the temp myself.
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Old 12-07-2011, 09:21 PM   #6
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[QUOTE
oh boy......another late night service call in the works
][/QUOTE]
remember we charge time and a half at night.......
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Old 12-07-2011, 09:40 PM   #7
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Can you put two thermostats in paralell?


double time 2x on Xmas holidays plus Carrier charges us and we pass it on to you the $125 to open the warehouse for that circuit board that got fried.
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Old 12-08-2011, 01:49 AM   #8
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my whole point is if you want to play dumb ass 101 do it during normal business hours.....
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Old 12-08-2011, 06:11 AM   #9
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Can you put two thermostats in paralell?


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my whole point is if you want to play dumb ass 101 do it during normal business hours.....
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Old 12-08-2011, 05:34 PM   #10
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Can you put two thermostats in paralell?


There are some higher end thermostats that won't back feed the Y if your device calls for the fan to run. get one of them, and use it. then you will have your automatic back up as you want. And won't have to worry about your device failing while your a way.

Since there are already themostats that will log temps and run times. I'm guessing your doing this for the challenge.
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Old 12-09-2011, 05:05 PM   #11
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Can you put two thermostats in paralell?


Could you explain the backfeed further? I can always do a reading to ensure my thermostat does not do anything funky.

This is the current wiring of my thermostat:

G (green wire to furnace)
RC: Jumper to RH
Y (no wire)
W (white wire to furnace)
RH (red wire to furnace)

So relay between W and RH for heat, G and RH for just the fan? Does that seem right?


I'm doing this for the fun factor but mostly because I want to be able to control the heat via a custom program I will write, so I can access it from work. I will also code in various algorithms so it can figure out when to start the furnace to get to a certain temp at a certain time. There will also be algorthms to ensure when holding a temp, it does so in the most efficient way possible. For example instead of setting the temp to 20, I will set it to 20-22. So the furnace will run till it hits 22, but it wont go on again until it goes below 20. I still have to figure all that part out and decide what's the best way to code it, but the main thing is getting the hardware side to work. At first I will just be writing a basic app.

I will be starting shift work in the next year or so, so I want to be able to program in really complex schedules. I also want to be able to do it from work so if I'm doing overtime I will remote in to cancel a program. None of the thermostats I've seen come remotely close to getting this advanced, and if there are some, they are probably more than the <$100 of parts I bought.

I just want to make sure I connect this right.
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Old 12-09-2011, 05:44 PM   #12
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Can you put two thermostats in paralell?


No wire on Y would eliminate the backfeed issue.
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Old 12-09-2011, 08:17 PM   #13
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Can you put two thermostats in paralell?


Cool I figured. What is Y used for again, is it AC? I may possibly add it in the future, but since AC is not really an emergency to have running, I'd only hook it to my custom solution. If it breaks or otherwise stops working, I'd just crocodile clip it for an hour at a time if it gets that hot. I'll cross that bridge when I get there, though. Next summer I'm redoing the weeping tiles, I doubt I'll have the money for AC. By that time I'm sure my solution will be proven and solid and I wont have to worry about it breaking.
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Old 12-09-2011, 08:26 PM   #14
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Can you put two thermostats in paralell?


Y is for the A/C. many higher end thermostats won't back feed the Y when something external back feeds the G terminal. But most thermostats will.

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