Carrier Wireless Thermostat Manual

• Carrier Wireless Thermostat Manual

Back in the olden days, thermostats were simple on/off devices that didn’t need their own continuous power supply. Modern thermostats with Wi-Fi and backlit display, by contrast, need a steady supply of juice. The C wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat.

Technically speaking, power flows from the R (red) wire, but not continuously (not on its own, anyway). To make it continuous requires a common wire to complete the circuit.

When the circuit is complete, 24V energy will flow continuously. If you’re considering purchasing a smart thermostat, you’re probably thinking of doing the installation yourself. After all, if you’re able to change a light switch or receptacle, you’re skilled enough to install a smart thermostat – assuming your system already has a C wire.

If your system has a C-wire, it might be in use or just tucked away behind your current thermostat. If your system doesn’t have a C-wire, you’ll need to run a new cable from your furnace to your thermostat to install most of the modern smart thermostat models.

Closeup of 18/5 wire. Pull back the plastic sheath to reveal the individual wires. These are what gets wrapped around your thermostat’s terminals. Identifying Your Thermostat’s Wires There are two ways to find out what wires your HVAC system has. Method 1: Look at the wires behind your thermostat Detach your thermostat from the wall and look at the wires connecting to it. If you’ve got a wire connected to the terminal labeled “C”, you’re (probably) good to go with a smart thermostat installation. If you don’t see a C-wire here at your thermostat, don’t assume you don’t have one just yet.

It might be stuffed into the wall, which is what some installers do when the C-wire is present but not needed. You can also look inside your furnace (see method 2). If you find a C-wire in your furnace, then the other end is probably stuffed into the wall behind your thermostat. Method 2: Look at the wires inside your furnace Disable power to your furnace and pop the cover off. (The ease of doing it this way will vary by furnace and installation.) Look for a row of screws labeled R, C, W, W2, G, Y/Y2, like so.

Carrier Wireless Thermostat Manual

Looking inside my 2010 Rheem gas furnace As you can see in the photo above, this furnace does not have a C-wire hooked up. To make this HVAC system compatible with a smart thermostat, it would need either:.

a new bunch of wires run between the furnace and the thermostat, or. the, or. a smart thermostat like the which comes with a Power Extender Kit for systems without a C-wire If the thought of poking around in your furnace or thermostat makes you uneasy, consider hiring a professional for the installation of your smart thermostat. There are no standards for wire color! Any wire can be used for any purpose. A previous owner or handyman may have been “creative”, so what you find behind your thermostat might vary from what you read about online or here. These photos are from my own home, but every home is a unique snowflake.

Here are common uses for wire colors:. Blue or Black – C – Common wire, may be unused by your existing thermostat.

Enables continuous power flow from the Red wire. Red – R – 24VAC power from the furnace’s transformer. Red – Rc – 24VAC (dedicated to heat call). Red – Rc – 24VAC (dedicated to cooling call). Green – G – Fan.

White – W – Heat. Yellow – Y – Air conditioner Do I Really Need a C Wire? Yes, you should install a C-wire if you are upgrading your thermostat. Nearly all modern thermostats, smart or not, require a power source, and that doesn’t look likely to change any time soon. Nest Owners Not Exempt The popular Nest thermostat claims to work without a C-wire, there are some caveats. Without a C-wire, the Nest gets its power from your heating or cooling system assuming it’s running. When it’s not running, the Nest still needs to get power.

The Nest will “pulse” the heat wire, turning on the furnace to pull a bit of power to keep itself going. In some systems, this is unnoticeable, but in others, the furnace responds as if being told to turn on and then immediately off. Nest’s manufacturer updated its literature to warn that the Nest may be incompatible with some single cycle, no-C-wire systems, but the reality remains that outdated wiring is going to become more and more of a pain in the butt to deal with. Future Proofing Most states have outlawed the old fashioned “mercury blob” thermostat.

Even the most no-frills thermostats on the market today still need a power source. Adding a C-wire, either through new wiring or an adapter, ensures compatibility with new thermostat technology. Don’t do the “fan wire” trick It is true that you can re-purpose the fan wire as a power wire, but then you (and future homeowners who live in your home) won’t be able to manually turn the fan on. If you’re going to spend a couple hundred bucks on a thermostat, spend a little more for an adapter or a technician to do the job right. What to do if you lack a C-wire. Hire a pro to run new wires between your furnace and your thermostat (or do it yourself). Grab a, which adds a 5th wire to your 4-wire setup.

Get a smart thermostat that’s designed to work with systems lacking a C-wire, like the (it has an adapter for C-wireless systems) or the (in most systems, it doesn’t need a C-wire at all). That is a great question! I don’t think it’s very common. But HOW uncommon is an excellent question (one I was all too happy to spend 30 minutes researching) I can’t find a single survey of wiring configurations in U.S. Homes (letalone worldwide), but we can probably make some educated guesses based on what we know about thermostats and housing in the U.S. Programmable thermostats exist in less than 40% of U.S.

Homes, that much is well known. Homes without would have no reason to run a C-wire, so we’re already down to 40% of the U.S.’s housing stock having any plausible reason to run this wire. (This diagram summarizes a thermostat survey’s findings: ). Now, we can also take into account that “setback thermostats” (or just “digital thermostats”) weren’t really a thing until California decided new construction should have them in the 1970s, amid the oil crisis. Still, it took the rest of the country another two decades to catch up. Programmable thermostats weren’t the law of the land until 1995. So then I wonder, how many homes are built after 1995?

Probably not many, indeed, it looks like about 33% of the homes still standing in the U.S. Today were built in the 1980s, 1990s, and early 2000s. More specifically, 16% were built in the 1990s and later (“later” being capped at 2005, the study I’m referencing is getting old: ) So 16% of homes built after the federal mandate to use a programmable, and only 40% have a programmable (and even programmables don’t require C-wires). That’s not a whole lot.

Anecdotally, my 1977 home on the west coast has a programmable thermostat but no C-wire. My sister’s 1983 home in the Midwest is also C-wire-less.

Nest forums and hvac forums seem to be full of people reporting a lack of a C-wire in their home. With ALL THAT in mind, I’m guessing the C-wire is in less than 20% of U.S. Heck, even 20% sounds optimistic. I don’t know if I’d quote me on this, this is just speculation, Googling, and some loose estimates.

I’ll continue to think about your question, though, and I’ll come back and give an update if I find an answer! Hi M.B., I’m actually a DIY consumer. I do a lot of my home improvement because I enjoy researching, understanding and saving money. I installed several Nest and Honeywell RTH9580.

I also have an ecobee (in San Diego, SDG&E installs them for free). I totally agree with your reviews of the 3.

I have not tried the Lyric yet. When I first started this, air-conditioning was a little foreign to me. In my condo, the AC is one of those that is in the ceiling and there is no panel with “C” clearly marked. I had to research and figure it out. I found a lot of info out on the internet lacking and confusing to DIYs, but some sites like yours are very useful. Your Rh and W terminals control your heat. The C and Rc terminals are what powers the thermostat.

The remaining terminals should (I am not an a/c technician so I am using deductive reasoning here) control the a/c. I would start by removing the Rh and W wires and see if the a/c works.

Be sure all remaining connections are tight. As long as you connected the correct wires to the corresponding terminals it should work. Is there a heat / cool switch option on the thermostat that you are forgetting to switch over for the a/c to turn on?

Hi, I am having a similar issue. I only have 2 wires R and W. I plugged these wires into the R and W terminals.

I used an external transformer and ran those wires to C and RC. My furnace has a heat and cool switch. The heater works fine. When I switch the furnace switch to cool it runs continuously Even if the thermostat is on cool, there is no response. The old thermostat looks like it probably either completes a circuit or grounds it. Two pieces of metal touch, and it stops. I have tried to plug the W wire into every available terminal to see if it would work.

Nothing worked. Now I’m confused.

Clearly, the “C” wire issue is not my issue. I don’t understand why the new thermostat will not complete or grounds out the circuit like the old one. Should I remove the switch from the furnace?Thanks. Bill lee is incorrect.

Rh stands for Red-Heat and Rc stands for Red-Cool. If you only have a single red wire the jumper between Rh and Rc are required for some heating and cooling hybrid systems depending on your thermostat. Yellow/Y is the AC compressor.

White/W is the Heating element Green/G runs the fan. Blue or Black/C should be a ground and will be used by both Rh and Rc, and apparently your thermostat. Sounds like the transformer is not supplying power to Rc Vamsee Nathan, if you only have red and white wires, your system is only set up for heat. The cool switch on the furnace is manual for a reason, and it is for running the fan when the furnace is off. If you want to run the cooling fan on the furnace using your thermostat you can run a wire to bypass the switch, and connect it to Yellow/Y or Green/G depending on your needs. Great site and good questions! As a Electrician and HVAC/R technician from north of the border I see a lot of US products up here so this site caught my interest.

Thanks for this info, adding my comment because Google brought me here and this might help others. I found a blue wire hanging out behind my old dumb thermostat and I suspected it was the infamous “C” wire, and it turns out it is.

It’s like the thermostat mfg’s are scared to go into any detail and most general info says the common wire supplies the thermostat with power. So I got my multimeter out and touched the red lead to the blue wire and black lead to ground and got 0 vac, so I was really bummed out. Then my Google foo kicked in and brought me here! So I tested again, this time touching the red lead of my multimeter to the red wire and the black lead to the blue wire and what do you know, I got 24vac! Read John’s post again. He said that he, “touched the red lead to the blue wire and black lead to GROUND.” The 24 vac output of the step-down transformer in a furnace is not tied to “ground”, so it is considered to be “floating”. The only way you can measure a 24 vac output is across the two secondary terminals.

This is different from something like a wall outlet, where the neutral and ground are fastened together at the “service equipment”, and both are connected to ground. Household wiring is not floating, but rather, is considered to be earth referenced.

I know this thread is old, but important since people (including me) still visit this site. Many people have that misconception about electricity. Actually the “C” wire is just as important as the “hot” wire. The neutral on your home wiring will kill you faster than the hot wire if you find yourself in the circuit. Only saying this so DIY folks always put safety first. Turn off power before attempting any wiring project including “low voltage”. In the right conditions–heat, humidity, the amount you sweat–6 milliamps will kill you.

In a typical four-wire system the 24vac is supplied by the R wire. Inside the thermostat are three switches/relays. If the thermostat says heat is required, it allows the 24vac to go back on the W wire. If cooling is needed, 24vac is sent back on the Y wire. If the fan should be running, 24vac is sent back on the G wire. If no heat, cooling, or fan are required, there is no power flowing through the thermostat.

If you have a five-wire cable, the C wire (of whatever color) is a usually a common that allows 24vac to be flowing (therefore used) by most smart thermostats. Hey there, Dan. Sorry for my slow reply, your comment got buried as spam and I just found it. 🙁 Have you tried the compatibility checkers on the manufacturer websites? Here’s Ecobee’s: And Nest’s: I tried both with putting in just an R wire (which is what I’m guessing you have) and they both ended with a request for me to email in a photo. I suspect you’ll need to run an additional wire, but I’m not sure. Do you just have a heating system?

Most WiFi thermostats need a C-wire, so at the bare minimum I think you’d need to add a C-wire. If you do resolve this, either through adding more wires or choosing a particular thermostat that works with your 1-wire setup, come back and let us know how it goes! Hello there, Aaron!

That’s an lovely old beast of a furnace you’ve got there! I can’t say I’ve seen many of these – some are as old as the late 1800s and even the youngest ones are nearly 60 years old. I’m sorry, but yours is beyond my expertise when it comes to adding a C-wire.

I have a few things you might try (sorry if you’ve tried these already): 1. Online compatibility checkers: 2. Try calling the manufacturer of whichever WiFi thermostat you’re interested in. They all like to claim widespread compatibility (90-95%!) and they’ve been good at answering my questions in the past. Here’s a thread I found from someone trying to hook up a Nest to a gravity furnace and some of the things he ran into: 4. Try a pro – there’s probably an HVAC company in your area that has seen this before that could at least advise you over the phone (“nope, not compatible” or “yeah, but you need X Y and Z”).

Of course, there are plenty of thermostats that don’t require a C-wire that are compatible with gravity furnaces (which is probably what you’re replacing), but I don’t know how to advise you on installing a C-wire into a system without a control panel to work with. If you have typical wires behind your thermostat, those wires must go somewhere. I just couldn’t tell you where, or how they interface with your furnace. Sorry to be of such limited help. If you do figure it out – or find out that your system is simply incompatible with C-wire needing WiFi thermostats – do come back and give us an update! Thanks for the quick response. I think your suggestions are a good starting points.

I recently got married and we purchased our first house this year. We, eventually, plan on replacing the furnace but it’s not financially feasible right now.

I attempted to hire an hvac company to come clean the furnace and the guy took one look on the furnace and was completely baffled. He had never seen one before and had no idea how to clean it. We have a more knowledgeable tech coming in a couple weeks.

I’ll ask him if a c-wire can be added. I’ll update you with any results. Thanks again.

Sorry about the slow response, sounds like you have a simple heating thermostat with a fan function, for ease of future conversations use the marked letter designations rather then colors as not everyone uses standard colors. I am assuming its a mechanical stat not electronic but both can work similar in operation. The R terminal on the thermostat usually connects to a red wire from the furnace which is the 24 volt power wire from the furnace transformer marked R on the furnace board.

The W terminal (often the white wire) will be the thermostats output for a heating call to the furnace sending the 24 volt power back to the furnace to its W terminal. This allows a automatic control of the furnace to turn the fan on when the heat exchanger is warm enough. The third terminal should be G (often the green wire) which is the Fan output from the thermostat and connects to the G terminal of the furnace to control the fan directly and bypass the furnace automatic fan function. It is very rare to find a three wire thermostat with a C or common connection so i feel confident enough that you have a simple heat only thermostat with fan function. If you are planning on using a electronic thermostat with these functions it will require a battery powered thermostat to allow relays to turn on and off for heat and fan, otherwise it will have to be a energy scavenging thermostat which gets its power by pulsing the heat function on when it needs to regenerate its power, the only other option is to pull more wires from the thermostat to furnace if you want to use a powered thermostat. (meaning it uses R and C to complete a 24 volt power circuit).

Well, I installed a 120v to 24v transformer which has a R and a C terminal an ran an additional thermostat wire from the transformer to Rc and C on the thermostat. I removed the jumper between Rc and Rh and the transformer powers the thermostat without problems. I then wired TT on the aquastat on the boiler to Rh and W on the thermostat. Unfortunately the thermostat is not sending a heat call to the aquastat when the heat is turned up. I assume the thermostat may be bad. Ad the package appeared to be opened already when we first opened it. I advised my friend to return it and we will try a new one tomorrow.

Wish me luck. Thanks to Bill Lee post. I have successfully installed my new Honeywell Wi-Fi Smart Thermostat, RTH9580WF. I replaced an older Honeywell model that only used 2 wires (R & W, no C wire) and was battery operated. I replaced the old wires with new 18 gauge 6 wire (overkill but for the future), using the old wires to pull the new wire through the walls (please don’t try if you are not sure what you are doing). My first attempt I connected the R & W and also connected a third wire to the C pole on the Transformer that is connected to the boiler to the C position on the Thermostat.

The Thermostat powered up but a few days later when the house temperature dropped below the desired house temperature and the demand for heat was initiated the thermostat would recycle. I googled for a solution and came across Bill Lee post.

I then connected a fourth wire to the R side of the transformer and connected that to the Rc on the thermostat. Removed the jumper in the thermostat between the Rh and Rc. Below is the finished wiring: R (from transformer) —– Rc (on Thermostat) C (from transformer) —– C (on Thermostat) R (the old R from previous connection – Thermostat) —- Rh W (the old W from previous connection – Thermostat) – W The thermostat works great now and no longer recycles and the house is maintaining the desire temperature(s). I’m upgrading from a Honeywell Mercury thermostat to a Honeywell wifi 8580. My old thermostat only used 4 wires, the white,green, yellow and red.

My old thermostat didn’t require continuous power so the blue,C wire wasn’t connected. However, their was a blue, C wire in the wall, so I figured was all set. I followed all the instructions and connected all wires correctly.

Once I powered on my furnace (Ruud Silhouette II) my new wifi thermostat doesn’t turn on. Any ideas as to why I’m not having any luck? I have the same setup. I was upgrading from an older Honeywell thermostate to a wi-fi enabled thermostat.

Only needed 4 wires in the old model. And there is a spare (blue) wire not used. I know I need to connect the blue wire to a C terminal at the furnace side. But I am hesitant because there are already 2 other wires (both black) connected on the C terminal at the furnace. One wire leads to the AC unit, which I suspect powers the central AC.

The other black wire I’ve yet to figure out where it leads to. Is it okay to connect a third wire to power the thermostat? Hello, I have a new home with a Payne forced air furnace. I replaced the cheap thermostat with a Honeywell th9320wf5003. All wires were hooked up on the thermostat to their corresponding colors including the Blue Common wire. The thermostat looks like at times it’s not getting enough power because it reboots at random times. At least when I explained it to Honeywell that is their thought.

When I look at how the furnace is wired I have the following wires on the following terminals. G Terminal – Green Wire C Terminal – Blue Wire and Two White Wires W Terminal – White Wire Y Terminal – Yellow Wire and Red Wire R Terminal – Red Wire. Modern thermostats come with the function to switch the fan mode, that is, you can either keep it on ‘on’ or on ‘auto’ apart from other settings. If the HVAC is cooling unevenly, then it would be a good idea to switch the fan to ‘on’. The reason behind this is that on ‘auto’ the fan is working only as long as the main unit is, whereas in the ‘on’ mode, it runs continuously.

When the house is big, all you need is circulation. So if you the fan keeps on running, then it will keep circulating the air to maintain an even temperature. I have a 5 wire system/cable but the 5th wire is wrapped up and not being used on either end (thermostat or blower/furnace. Can I hook up the spare wire to the C on the panel of the furnace/blower and then have the correct C wire? I see speak about 24v transformerif I have a C labeled spot on my furnace will that automatically be supplying 24vor would it need an inline voltage transformer? And finally which wires would I test with a voltage meter to double check I’m getting 24vthe C wire and which? Thanks, great write up.

My house had old wire (No C wire) Which is the common wireor known as ground. I installed a crude programmable battery operated thermostat years ago. Now That theyre wifi they need a constant source of power. I had an electrical outlet just a couple feet from my thermostat.

I used a meter and verified that my unit is commonly grounded with my house (of course it should be) I simply ran a single wire from the earth ground off that plug with a small wire and up the wall to the new thermostat. Keep in mind the red is 24VAC control.

Your new thermostat already has a rectifier built into it. I did the same thing except I completed the circuit using the white (neutral) wire found inside a switch box that is 2 feet from my thermostat. These neutral wires are what return all of the AC circuits in your home – the ground wires are only there in case of a power surge or other emergency. Tying into the ground wire could get you electrified plumbing or other consequences. I have the same question as the other guy – why doesn’t everyone do this? Well it’s kept secret because building codes don’t let you “pull a neutral” unless it’s on the same circuit (and furnaces are required to have a dedicated circuit hence you’re required to only get your neutral there) but when you consider that only a trickle of 24v current is required to power your thermostat you probably won’t kill yourself if you forget to turn off your furnace circuit next time you service your switch box. At the beginning of the summer, I installed a Z-wave thermostat.

I have only 4 wires that were marked R, W, Y, and G. There was no Common wire. The good news is that it worked perfectly all summer. But now it is heating season Evidently I mistakenly connected the W wire to the C, and this allowed power to the thermostat for the display. Again, everything worked. Until it was time for heat, and then the heater would not come on. I moved the wire from the C terminal to W, and now it will heat but the thermostat display does not come on.

I guess I have two questions: 1) why did this work at all? 2) Since it did work, why not just jumper a wire between the W and C terminals?

Thermostat home furnace troubleshooting-step step guide, Thermostat troubleshooting. ‘ find, ‘ find! Common thermostat troubleshooting honeywell, white-rodgers, lux, robertshaw. Thermostat Home Furnace Troubleshooting-Step By Step Guide Elk trg2440 24vac, 40 va ac transformer ptc fuse, Fulfillment amazon (fba) service offer sellers lets store products amazon’ fulfillment centers, pack, ship, provide. Elk TRG2440 24VAC, 40 VA AC Transformer with PTC Fuse Thermostat wire:, Saying wire power source, supplies power misleading. Wire simply “common” side transformer 24v output (. Thermostat C Wire: Everything you need to know about the.

Your excellent common wire color table has an error: The Red 24VAC wire for dedicated heat call (Rh) shows an “Rc”, not the “Rh” you intended. Blue or Black – C – Common wire, may be unused by your existing thermostat.

Enables continuous power flow from the Red wire. Red – R – 24VAC power from the furnace’s transformer Red – Rc – 24VAC (dedicated to heat call) —- this should have the “Rh” label Red – Rc – 24VAC (dedicated to cooling call) Green – G – Fan White – W – Heat Yellow – Y – Air conditioner. Best wifi thermostat reviews ultimate resource, Corrected. Glad thermostat wire article helpful. Best Wifi Thermostat Reviews The ultimate resource for Smart thermostat guide – wifi-enabled thermostat reviews, The ecobee3 hands smart thermostat today. Innovative motion-detecting sensors (1 included thermostat unit). Smart Thermostat Guide – Wifi-enabled thermostat reviews Thermostat wire:, Saying wire power source, supplies power misleading.

Wire simply “common” side transformer 24v output (. Thermostat C Wire: Everything you need to know about the. This has to be the best site to understand the “c” wire issue i am trying to figure out, and yet can’t seem to apply it to my situation. I have a oil boiler in basement, thermostat in 1st level and ac central air blower in attic. My thermostat has R & W going to the T & T terminals on the Aquastat boiler. The 3 going to the central air are Y,Rc,G. At the CA blower the Y (goes to a lot of other white wires), the Rc goes to a red wire of a wire coming from the CA unit wire with no other connections to this.

G goes to a gray wire again a solo connection. So my connections seem to be 2 wires at boiler to tell it to come on and off (W&R) 3 wires to Central Air (Y,Rc,G) which seems to be on, off, fan.

I am trying to hook up a honeywell smart wifi thermostat TH9320WF, and i do have the Honeywell wiresaver module too THP9045. Would like to add the info of the bundle of wires coming from the central air if this slightly helps my cause and your knowledge of the subject. On the fan coil (control) I have these connections. R (Rc on therm) G (G on therm) W2 (Y on therm) W3 (Y on therm) E (Y on therm) C (goes to a red wire on Cent Air compressor in yard, the white wire goes to the bundle of other white wires all connected to the Y wire from thermostat) This red wire “C” i just mentioned in the attic on the central air blower is this wire needed to power my HW thermostat? Installation is easy if you already have a C-wire in place. If you don’t, an adapter is included and a few more steps to do.

Either way, the instructions are easy to follow and you don’t need any specialized tools. Unlike its competitor the Nest Learning Thermostat, the ecobee3 doesn’t rely on “power stealing” to make up for a lack of a C-wire (read more about that here on ecobee’s site), they just include a power extender kit to make up for the missing wire. Read more about the C-Wire here. Connecting new HoneyWell thermostat to old Lennox furncase. The old thermostsat had a C wire connected but probably not used – it was battery powered. The C wire is connected to the cabinet (ground) at the furnace.

I am not sure the transformer return is conncted to the cabinet (probably not) – see wiring diag at When I attach new thermostat it does not power up. I think the blue wire in the diagram comes out as the red wire to the thermostat. Should I disconnect the C wire from the cabinet and connect it to the yellow wire on the transformer? Hi all, This site is really helpful (I think) but I want to make sure I am not getting myself into trouble. I have a two wire (R W) right now, but the builders helpfully installed a multi wire cable despite not using most of them.

So I think running a C wire should be as simple as attaching my Blue at both ends. The contact panel where my R and W terminate has a point marked C; but it in turn connects to my Honeywell S87B in a port marked “Valve”. That S87B does have something marked as 24v which is actually connected to my White wire.

Does this seem right? Is my C the legit place to connect? Something doesn’t sound right to me (as you have described it). The wire connected to R on your thermostat should be the 24 v (and usually Red). W (usually white) activates heat. The C wire is the common which is the return to the opposite side of the transformer from which the 24v wire comes, completing the 24 v power for the thermostat. I don’t know anything about your furnace but I just hooked up 3 wifi thermostats on old furnaces at my church and that is what I learned from my research and experience.

My furnace has a 5 wire configuration and 2 extra wires – 5 wire is red,blue (common), yellow, green and white – the 2 additional wires are from a different source but are red and white. I am installing a Honeywell EIM and it is not powering on. The common and red from the 5 wire source are not showing power on a volt meter. I checked breakers, the furnace shows it is powered. Configured the EIM exactly as the wires were configured in the furnace. Any thoughts?

I’m sure it is simple but it is not coming to me. Thought id share.I have no C wire, and its just a milivolt heat gas fire, which just closes the contact for heat.

I first found a 24volt DC power(110 to 24 VDC, i had one spare but they are cheap on amazon, less that $10 for plug in 1 Amp wall type. Yes DC power works fine just to power the thermostat) and connect to RC to +24V and C to 0V. Removed the link between RC to R (important). Connect the existing milivolt connection from gas fire to R and W and hey presto, works a dream.

With power supply near thermostat and run power to thermostat (hid wires to rear of thermostat through drywall). Use existing 2 wires for heat contact that were already on old Mercury switch. Thanks for all the information on your site, very useful(this is how i worked it out). Using RTH8500 honeywell wifi.

Hey, Thanks a lot for writing this article. I replaced my old thermostat with a Nest 3rd generation. I hooked everything up as instructed but could not get Rh to have a higher voltage. It was reading around 2V for VOC.

I called Nest support, but they were not able to help. I almost had to call an HVAC technician to come out before reading your page. It turns out the old thermostat had all the wires connected, but the furnace only had W and R wires connected. I connected C, G, and Y to the appropriate connectors on the furnace, and voila! And my VOC for Rh went up to around 32V. So thank you so much for this article! This site is incredibly helpful.

Bought Honeywell RTH6580 WiFi thermostat for a place an hour from home. I knew I was in trouble when I opened the thermostat and realized I only had a mechanical switch with just a two-wire thermostat wire, red and black wire (on a heating system only). I closed it back up, and began research. I do not know what I am going to encounter when I look at the furnace circuitry, but I am going to armed with 18-5 wire. I intend to test new thermostat at the furnace with the solutions that you have given me before I try to pull the new wiring. Do not know if I will need a separate transformer, but this site has done a wonderful job for preparing me for what I am up against.

Thank you, thank you. After reading many of the ‘posts’ here, I seem to be one of few looking to go the ‘opposite’ way for wire connection. We own a house in which we have my almost 80 yr old mother-in-law living in by herself, which has a combination heat/ac Carrier system. It has a Carrier digital thermostat that my mother-in-law has continuous problems figuring out how to set it. Through her attempts in ‘playing’ with the settings – especially for heat – we’ve had to call for service twice when the system failed to operate correctly. Without getting into any further detail, we decided to switch the thermostat to an ‘old style’ manual one – one that has no digital operation, and very simple heat-off-cool controls. Along with the temp control.

I purchased a Honeywell CT31A non programmable therm. (very basic) before knowing how many wires the existing therm. Was connected with – which turned out to be 5. The ‘new’ Honeywell would not accept 5 wires so I called the company, and was advised that therm. Was not compatible.

He gave me a model CT51 which he said could be used. I went online when I got home, and ‘googled’ that particular model and in looking at the wiring diagram, it also shows a 4 wire hookup. In reading further, it said NOT to connect the “C” wire (which happens to be the BLUE), and I also learned this is the “common”. Is this something you would agree with, and could I be confident in wiring this in this configuration (less the C wire) with the results being that of working properly.

Looking to get this replacement completed before the next need for heat (LOL). Thanks for any feedback you can send my way. With Regards, Tommy E.

I know this is an old post, but thought I’d try to get a little info. I have a Honeywell wifi thermostat set up. I have a generator wired to power several circuits in my house in case of power failure (wired the right way as to avoid arguments). The power circuit powers the furnace but not the condenser due to the generator not being strong enough to power it anyway. My first power failure resulted in everything working fine but the wifi thermostat was not powered. Does this power originate from the condenser power? Very informative post.

I have been investigating this because a painter damaged our thermostat (obsolete, works for now, long story). I wanted to go smart thermostat, and like an idiot, didn’t get the Ecobee unit. Honeywell I have found my carrier system to be even weirder than one might expect.Carrier Infinity 80 2-stage with humidifier, air filtration, AC unit, Infinity Control Thermostat SYSTXCCUID01-B drives these. Carrier uses a proprietary ABCD junction block (Block PL7 in the circuit diagram) for the thermostat. A-Green= Data A to Green wire B-Yellow= Data B to Yellow wire C-White= 24VAC (common) to white wire D-Red= 24VAC (hot) to red wire. In the wall is the fifth, blue wire, unconnected. At the furnace end, also unconnected, I am unsure where to connect it at the furnace control board.

ABCD block at either end has only 4 connection points. Furnace Control Board Wiring diagram doesn’t seem to clearly indicate the attach point at the furnace end. Well qualifier, there is a screw-terminal block (W2, Y1, DHum, G, Com 24V, W/W1, Y/Y2, R) on the board /screw labeled Com 24V. I suspect that is the proper point, but don’t know if the ABCD connection disables the screw-terminal block. It would seem dumb if they put it on there but proprietary ABCD disables it, but you never know. Any recommendations? The experiment starts.

Greetings, I have a Carrier system and I wish to replace the failing thermostat with a smart thermostat. Carrier uses a 4-Wire ABCD connection. A = Green = Data A B = Yellow = Data B C = White = 24Vac (common) D = Red = 24Vac (hot) This is the labeling (and wire color matches, yay) at both thermostat and the furnace. Both ends use a 4 pin connector (thermostat and Furnace Control Board). Lucky me, the Blue C wire is present in the cabling.

Unconnected at both ends, natch. As I understand it, connection at a smart thermostat in this case is simply, blue wire to the C terminal. Furnace end is my question there on the FCB is a full screw-terminal strip (W2, Y1, DHum, G, Com24v, W/W1, Y/Y2, R) none of these presently have connections because, ABCD connection is for the established thermostat cabling. In other words, the FCB has a dedicated terminal, rather than use the screw terminals.

So, finally the question Do I simply take the unconnected Blue C wire, and land it at the Com24V screw terminal? I assume the first step is to check between Com24V and R for voltage.

Sound correct? If that is live, would there any reason to NOT just make the Blue C wire connection there, then connect C wire at the thermostat? (I have the schematic, and I see no other landing point for C.) Logically, this makes sense to me. I just don’t know enough about the FCB to be 100% certain.

My Carrier installer wants nearly $1000 to install the replacement thermostat. It seems with my approach, I should be able to do this. I am trying to hook up a Honeywell RTH 221B thermostat to my furnace. This is new work and there are no previous wires. The furnace panel has the typical Y W R G C terminals. The thermostat has; O/B Y RC R G W terminals. I only need heat, no AC, heat pumps, etc.

I just need the furnace to keep us warm. We are 90 miles south of the Canadian border in MN. My question is: From the transformer terminals W & C, to which terminals do W & C connect at the thermostat? There is no “C” on the thermostat.

By the way; The Honeywell 800 number and the help on the other end barely speak English. It is NOT customer service, but more like a Joke! Smart Thermostat Guide is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com Some of the links on this website are affiliate links. This means if you click on the link and purchase the item, Smart Thermostat Guide will receive an affiliate commission. Disclosed in accordance with the Federal Trade Commission's 16 CFR, Part 255: Guides Concerning the Use of Endorsements and Testimonials in Advertising. Read our full.