Mechanical or electrical device used to control a heating, ventilation, and air conditioning (HVAC) system, or other temperature reliant system.

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Why doesn't the “fan on” mode work after I installed a new thermostat?

We moved into a new house. I installed a new Hunter Thermostat. I have a furnace only and the old thermostat only had 3 wires connected. SO I connected those three wires (G, RH and W/B). The ...
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4answers
4k views

How can I modify a 4 wire thermostat to a new thermostat requiring c wire?

I attempted to replace my thermostat with a fancy new Trane z-wave enabled model. To my dismay my home was only wired with a 4 wire setup with no 5 hidden in the bundle. I followed Insx on the web ...
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1answer
7k views

Wireless Thermostat C-Wire Substitute

I've read How can I add a "C" wire to my thermostat? and checked out the Venstar Add-A-Wire Accessory. I've watched Honeywell's YouTube video on "DIY Wi-Fi Thermostat Substitute G-Wire for C-Wire," ...
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1answer
470 views

Switching from multiple thermostats to a single one

I have a heating system that has a separate "smart" heater in each room, each controlled with a separate thermostat (a simple one, like Honeywell T410A1013). What I would like to do is to convert to a ...
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1answer
414 views

Does a fridge thermostat need a ground connection?

I've recently replaced the thermostat in my fridge with a new unit with a much higher temperature range (this unit specifically). The unit I took out had a ground connector & the original wiring ...
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1answer
60 views

Honeywell RTH7600 does not responsd

Just installed new Honeywell RTH7600. All wires are correct and the unit displays fine. The issue is that heater and AC do not turn on at all. I even put the fan to ON instead of AUTO and that does ...
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1answer
26 views

Issue with Thermostat

I bought and installed a Trueway thermostat before 2 years. It was working well for the past years. But then a twist! the thermostat started to go nuts, the tempeature would race over 60 degree, even ...
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1answer
147 views

Where is my Common Wire? [duplicate]

I purchased a wifi thermostat and cannot located the common wire. My unit is old and is a Coleman 70 with no terminals inside labeling the wires. As you can see in the second image, there is a ...
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1answer
447 views

How to add “C” wire using a/c fan wire?

Honeywell has a video that shows how to use the fan wire ("G") of the cooling system as the "C" wire for the thermostat. It is in the video - "Honeywell DIY wifi...substitute g-wire for c-wire" on ...
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1answer
64 views

Possible to check NEST system compatibility by looking at current thermostat model?

I'd like to replace my Honeywell CT87A thermostat with a Nest thermostat. I see that Nest has a page to check system compatibility. Is it possible to identify if my system is compatible without taking ...
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1answer
25 views

Wall mounted dial thermostat

I recently bought a wall thermostat I broke the plastic front on the box to remove The thermostat as I tried to pull it out of the box the dial came off in my hand and I do not know where it ...
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2answers
65 views

Is it possible to create a better temperature controlling system?

I have a two story 2.5k sq ft house with a varying degree of temperature throughout the house. The main problem is that the air coming out of vents are very weak if almost nil. Only the living room ...
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1answer
1k views

Where to connect a c-wire on a furnace with existing two wire mercury thermostat

My existing thermostat is an old mercury two wire device, heat only. Looking to upgrade to a better model, but I need to run a c-wire. I don't know where to connect the wire at at the furnace, any ...
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1answer
108 views

Should my A/C compressor come on when I turn only the fan on?

I have a bog standard HVAC system with a Nest thermostat. When I tell the theromostat to turn the fan on for 15 minutes the A/C compressor also kicks on. Is this normal?
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1answer
41 views

choosing a programmable thermostat: wiring and power

I want to replace my non-programmable RobertShaw 9500 thermostat with a programmable thermostat. The current wiring uses these terminals: RH, W, Y, G. With that set of wires, can I get a ...
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1answer
678 views

Which terminal should be used for a “C” wire when none are labeled “C”? Older electric furnace/heat pump

I have an all electric air handler/furnace connected to a separate AC unit and it is also a heat pump. I've purchased the latest and greatest Honeywell WiFi "Smart" Thermostat and it needs a C wire. ...
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1answer
200 views

How to connect a DIY thermostat to the HVAC System

I am building a DIY thermostat similar to this blog. I want to connect it to the HVAC system of my home. How do I do that? I found this post which is very informative but doesn't completely answer my ...
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2answers
2k views

Can I install a Honeywell WIFI Thermostat?

I have done quite a bit of research and have not been able to find an answer I am looking for. I currently have a manual digital thermostat that is not programmable made by Braeburn. My system is a ...
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1answer
212 views

Is my thermostat transformer voltage too low?

I measured the voltage at the thermostat Rc to G and Rc to Y terminals, and they read 16.2V and 16.34V respectively. Shouldn't I be measuring 24V+/-? I purchased a Nest thermostat and it will not ...
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1answer
531 views

Please help establishing a C wire for new tstat

I bought a new WiFi tstat as a replacement. The tstat controls my second floor hot water baseboard heat and my central air conditioning. I've located all the wires....the ones coming up from the ...
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2answers
179 views

Replacing old thermostat with new wi-fi Honeywell

I'm trying to replace my old mercury thermostat with Honeywell Rth8580wf. The cable has 8 wires, only 4 were being used. The wire colors and letters on the thermostat do not match up, so I'm left with ...
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1answer
1k views

Need help running and connecting a 'C' wire to my new thermostat [duplicate]

I have a Laars gas boiler that has 3 heating zones; basement, 1st floor, and 2nd floor. I want to replace the old mercury honeywell tstats on the 1st and 2nd floor with newer honeywell RTH9580 wifi ...
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1answer
46 views

What should I do with an old thermostat? [closed]

I recently had a security system installed which also included a replacement thermostat. Should I just throw this old one in the trash?
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1answer
350 views

Thermostat C-wire replacement using a wall socket?

I have a Filtrete 3M WiFi thermostat. As expected, it takes a C wire, which my apartment doesn't have and the apartment maintenance folks wont' let met wire either. However, there's a wall socket ...
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3answers
2k views

How to wire a relay for an evaporative cooler to a thermostat?

I have an evaporative cooler with 120VAC control that I want to switch on and off using a relay that is activated when the cool call comes from my thermostat. I plan on just using 1 relay to activate ...
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1answer
1k views

How do I turn off the power to my programable thermostat?

I am installing a new programable thermostat. In trying to cut power to the old one I have tried a couple of things but not sure I have done enough to cut all power. One, I shut off the furnace ...
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1answer
292 views

How can I replace a 4-wire manual thermostat with a programmable thermostat?

I"m replacing a 4-wire manual baseboard thermostat with a programmable SBFT2W thermostat. After connecting the wires, there are two lead wires from the old stat that are still unconnected. They are ...
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2answers
27 views

trouble shoot air conditioning thermostat

my compressor outside is not kicking in. Here's the sequence of events: I came home and the thermostat - a Honeywell LCD - was no more numbers and the battery indicator was flashing. replaced the ...
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2answers
46 views

How can I wire my WIFI thermostat to control my millivolt fireplace?

I have a gas fireplace that has an millivolt on/off switch on my wall (and a remote that includes flame on/off, a countdown timer and a thermostatic flame control). I want to be able to program ...
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1answer
8 views

switching out a heat/cool only thermostat to one with “auto” feature.

I have a honeywell deluxe programmable thermostat with a sub-base sub-base:32207415-011/0008 model:T8602D2000 the terminals that are used are R W Y G (RC and O are not used) my question is: ...
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1answer
130 views

Can I use a WiFi thermostat without a C wire?

I have a very old Honeywell programmable thermostat. I am going to replace it with a White-Rodgers UP500W Emerson Sensi WiFi Thermostat. I just want to know if the WiFi unit will work without a C ...
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0answers
23 views

Residential HVAC: one main thermostat, what do the other thermostats in the rooms do?

Are the other thermostats only meant for non-force air heat only? Does it mean only one thermostat in the house needs to have both heating and cooling, and auto or fan-only switches? It sure make ...
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0answers
83 views

AC Compressor outside is not running with new thermostat

my old thermostat stopped working, so I purchased a new one to replace that. The old one was powered by two AA batteries. A green wire connected to G, a white wire connected to W, a blue wire ...
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0answers
40 views

Changing Trane Baystat240a to another thermostat

I want to change my current thermostat to a programmable electric one (no batteries). Could someone verify if it does have a common wire and if all wiring could fit modern thermostats? Looking at the ...
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1answer
375 views

Can I connect two wires to a single screw terminal in my HVAC system?

I am installing a new honeywell WiFi thermostat but the old battery operated thermostat had batteries and no "C" wire. I look at both the thermostat and the air handler and found an extra un-used blue ...
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1answer
81 views

Smart Thermostat - No C-Wire - No Controller Board

This one seems to be a little different from previous questions on the subject. Here is the dilemma, I don't see a transformer. I have included a few pics. There are Three wire bundles. For ease ...
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0answers
127 views

Why would my AC turn on and off randomly?

My AC unit turns on and off randomly regardless of whether the thermostat is set on or off. I've even tried removing the batteries. Does anyone know why this might happen?
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1answer
320 views

Trying to figure out 'C' wire

I'm looking to upgrade my programmable thermostat to a wifi thermostat. I purchased a basic Honeywell wifi thermostat but upon installation could not find a 'C' wire. I started to investigate more ...
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0answers
17 views

Can I use the following parts to create a remote duct temperature sensor for my thermostat?

Parts: DS18B20 Temperature Sensor (PDF) or TMP36 Temperature Sensor (PDF) Cat5 cable Would those parts make a viable replacement for a remote duct sensor replacing a Venstar Remote Duct Digital ...
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0answers
83 views

Can you hardwire a residential through wall A/C to a 3H/2C thermostat?

I have a boiler with radiant baseboard throughout my home with four zones of heating and two through wall air conditioners and Aprilaire 350 Humidifier. I am upgrading my thermostats to the Honeywell ...
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0answers
20 views

What type of control systems are available for a home with multiple HVAC systems?

Our new (used) home has two HVAC systems, one for upstairs and one for down. There is a thermostat in the dining room, which is directly affected by sun. And one in our bedroom, that clicks all ...
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0answers
37 views

Thermostat needs to be set very high to work

Our home thermostat is behaving strangely. Let say it shows that the current room temperature is 60F. If we set it to 63F it will start the Fan (ventilation) but will not actually start the heating so ...
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0answers
15 views

Zones and setback thermostats

I've got a two-floor house, forced-hot-water and traditional radiators, with a thermostat and zone for each floor. I'm working from home at the moment, so the ground floor is set up to be "office ...
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0answers
404 views

Honeywell WiFi Smart Thermostat Wiring

The Honeywell WiFi Smart thermostat was operating fine when on AC until the heat went on this fall. Now, each time the fan goes on, the thermostat loses the connection, turns blank, then reboots ...
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1answer
748 views

Where do I connect the C wire in my furnace?

I am upgrading my thermostat to a Nest, and would like to add a common wire. My current furnace controller doesn't have a C terminal, but the current thermostat wire has 2 unused wires (black and ...
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1answer
386 views

My attic unit already has a wire connected to C, can I still connect a wifi thermostat? [duplicate]

I have an extra wire I can use for the C wire on both ends but in my attic the main unit has a wire already attached to the C spot and going away from the unit to something else. Can I attach two ...
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1answer
529 views

Where to add C wire on this air handler?

Thanks for helping me here. I am adding a new wifi thermostat to my system but need to add a C wire directly to the air handler (system is a First Co air handler with hot water boiler for heat and ...
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1answer
720 views

Where is my common wire on the unit?

I have 2 systems in my house, my heat is gas hot water radiator heating and A/C is a separate newer system. I'm trying to figure out if I can run a C wire off air handler from the A/C system to run ...
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1answer
354 views

Why did I have to change the wiring to use a smart WiFi thermostat?

I just installed a Emerson Sensi wifi thermostat.It works, and I ran a separate C wire. I had a 24 volt 5 wire Robertshaw programable thermostat that ran my boiler, air handler and AC. It is all ...
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0answers
51 views

How can I install a Honeywell Linevolt Pro 8000 in a multiple thermostat situation?

My roommate who just moved out installed a Honeywell Linevolt Pro 8000 in his room several months ago (replacing an M602 with no brand name that I can find) and it seems to have worked fine - it had a ...