How To Add C Wire To Thermostat

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New Wi-Fi thermostats are available that allow you to remotely control the temperature in your home. Unlike other battery powered programmable thermostats, these new thermostats can't run on batteries alone and even if they can they will drain the battery quickly. It's therefor necessary to connect a "C" wire to these thermostats to provide power. This post details how I provided power to my thermostat that didn't have a C wire running to it by using a plug-in 24 volt AC transformer. With a C wire powering your thermostat you won't need to worry about the batteries dying and you can set the light on your thermostat to be always on to act as a nightlight.

The thermostat I was replacing only controls my gas fired, hot water boiler which provides heat only. I didn't have to worry about cooling as my AC unit runs off a separate thermostat. There were three wires coming out of the wall where my old thermostat was mounted. My heating system only requires two wires, R and W, to operate. I thought I could use the third wire to power my thermostat and use that as the C wire. Unfortunately I ran into a couple of problems.

First, there wiring was very old and there appeared to be a break in that third wire. I couldn't get current to flow to it from the basement.

Second, and probably most importantly, I called the manufacturer of my boiler to double check that it would be okay to connect the thermostat to the C terminal on the 24 volt AC transformer in the boiler. These types of thermostats are called "power stealing" thermostats and the manufacturer (Weil McLain) does not recommend using a power stealing thermostat with my boiler. They were worried that the additional load from the thermostat may cause the transformer to prematurely burn out. They recommend using a separate 24 vac transformer to power the thermostat.

The instructions for the Honeywell Wi-Fi Programmable Touchscreen Thermostat (RTH8580WF1007/U) I was installing didn't specify how to hook up an external power supply to the thermostat but calling customer service I was able to find out how to attach an external transformer. If your thermostat is also controlling your cooling system or you have a different type of heating system the configuration might be different. It's a good idea to call the manufacturer of your thermostat and get specific instructions for your model and HVAC unit.

Update: wi-fi on the thermostat sucks. You can only update the thermostat provided the thermostat can connect to the internet. Hurricane Sandy caused me to lose my home internet but I still had power and heat. When I went to bed the following night I was chilled do the bone having been outside most of the day dealing with the aftermath and i wanted to turn up the heat without getting out of bed but i couldn't because my internet was down. I could access all other wired and wireless devices on my network, just no internet access. There is no way to connect to the thermostat directly. It is all done through Honeywell's cloud service which it turns out collects my homes temperature readings and heating system information every 12 seconds. I don't like that and i don't like not being able to access the thermostat from home when my internet is down.

Change Your Thermostat Before It Gets Cold

With any project things can go wrong. You don't want to be in a position where it's the coldest part of winter and you're without heat for whatever reason!

Also check with your HVAC system manufacturer and thermostat manufacturer for specifics about your installation. Check with all applicable local building codes. 

If snaking wire through walls and making electrical connections is not something you're comfortable with call an electrician.

What You'll Need


  • New Thermostat (I used a Honeywell RTH8580WF)
  • 24 Volt AC plug-in transformer (I used an MGT-2440)
  • 18 gauge thermostat wire. Number of conductors should be at least the amount of wires coming up from your HVAC system + 2 for the transformer. I needed a total of 4 conductors so I purchased the  Southwire 50 ft. 18/2-Gauge Thermostat Wire 64162179. You may need more or less depending on how far you have to run the wire.
  • 4 Wire nuts (one for each conductor you'll be using). Small ones, yellow or orange should do it.
  • Fire Block Sealant
  • Electrical tape.


Step 1: Plan Your Wiring

I could have continued to use the 2 thermostat wires that were in my wall plus add 2 more wires that will run outside the wall to a nearby outlet where I would plug in the 24vac transformer. I didn't like this setup because I didn't want the wire to the transformer exposed where the thermostat was mounted. The installation would look much cleaner if all the wiring ran through the wall as it did previously.

I needed 4 conductors. Two from the boiler and 2 from the 24vac transformer. The closest wire I could find was 5 conductor thermostat wire which is what I used. I was going to run the 5 conductor wire from my thermostat down to my basement and then I'd splice 2 more cables in the basement. One set of 2 conductors going to the boiler and two more going to the transformer which will be plugged into a wall. I used the same 5 conductor wire throughout (except for the connection between the boiler and new wire where I used the existing 3 conductor wire).

There is no standard color coding for thermostat wiring. I decided to use the following convention. R - Red, W - White, Rc - Yellow, C - Blue. The yellow and blue wires will go to the transformer and since it's an AC transformer they could be switched.

The 5 conductor wire will be snaked down the wall into my basement. From there I'll separate it into two runs. The Red and White will connect to an existing wire in my basement that goes into my boiler and connects to the R and W terminals on my boiler. The Yellow and Blue wires will run along another wire that goes to the plug-in 24vac transformer that will be plugged into a nearby outlet in the basement.

Step 2: Turn Off Your HVAC System

Before doing anything turn off the power to your HVAC system. This will vary by system but there should be some sort of switch near your system or there might be a breaker specifically labeled in your main circuit breaker panel.

Step 3: Remove Old Thermostat

Your new thermostat will come with instructions and wire labels. Follow the instructions to label your existing wires with the stickers so you know which wire is which. 

After you label your wires but before you completely disconnect them from the old thermostat, wrap a pencil around the existing wires and/or tape the wires to the wall the prevent them from slipping down into the hole.

If you need to drill holes for new wall anchors to screw in your new thermostat, do that now. Luckily I was replacing a different Honeywell Programmable thermostat that used the same hole spacing.

Step 4: Run New Thermostat Wire

This is the tricky part and how you run your wire is going to depend on how your existing wire is run. I was lucky and my thermostat had a straight run down the wall it was installed on down to my basement. By pulling on the wire from upstairs and down in the basement I was able to tell that the wire was not firmly stapled to the wall and I'd be able to pull it right out. The types of fasteners I saw used in the basement were fairly loose fitting. If that's not the case for you you might have a harder time snaking the new wire. You might have to make small holes in your walls here or there. Fishing wire can be tricky and you might want to hire an electrician to do this for you. Remember, it's best not to try and tackle this when it's very cold and you don't want to be without heat for a couple of days if something goes wrong.

Prepare Fish Tape

The first time I ever used the GB Fish Tape it was very difficult to get it out of the reel. You're supposed to pull the steel tape out by hand. It's coated in oil which makes it tricky. I took the tape outdoors, don't do this on carpet or the oil will stain it, and pulled out as much of the tape as I could by hand. Then I stepped on the end and pulled on the reel slowly working all the tape out. As my reach wasn't sufficient I walked down the tape to change where I was putting pressure. Be careful not to severely bend the steel tape when doing this. One I got all the tape out I reeled it in and repeated the processes. Now whenever I use the fish tape it comes out effortlessly.

Attach Wire to Fish Tape

From upstairs, where the old wire was coming out of the hole in the wall I attached the wire to the end of the fish tape. There's a small hole in the end that I passed the wires through and then used electrical tape to secure everything together. Make sure that the tape is put on neatly so that the point of connection isn't significantly thicker than the thickness of the wire. Tug on it a few times to make sure the wire isn't going to come off of the fish tape while you're pulling it through the wall.

Make sure you have enough fish tape unreeled to reach down to the other end of the opening.

Pull Wire Down

Next I went down to my basement where the old thermostat wire was coming out of the floor boards. I carefully pulled down on the old wire until the fish tape came down to a working level. I had to run up once to feed in the fish tape when it seemed to get a big snagged. This operation is easier if you have one person guiding the tape down from above and another pulling from below but is doable with one person.

Attach New Wire To Fish Tape

First I unspooled a sufficient length of wire and straightened it out a bit. I made sure to unspool enough wire that when I pulled the end upstairs the spool will still be on the ground so that the weight of the spool wouldn't pull the wire back down.

I stripped off about 1.5" of the brown outer wire wrap exposing the 5 individual colored wires. I passed those wires through the holes in the end of the fish tape. Then I took 3 of the individual strands one by one and wrapped them around the tip of the fish tape. Two in one direction and the other in the other direction. This will help secure the wires on the tape. Finally I securely wrapped the connection using electrical tape and tugged on it a few times to make sure it wouldn't come apart while pulling the wire through the wall.

I had taken a picture before I wrapped the connection with electrical tape to show you how I fastened the wire to the end of the tape. Unfortunately I didn't realize the photo didn't come out when I took it. The description above will hopefully be sufficient. Any way you attach it it's important that it provides some security and doesn't add much to the width.

Pull New Wire Up Through Wall

The first step is to start feeding the wire up from below until the connection between the fish tape and thermostat wire is past the hole. This is one of the points where the wire might snag. It's easier to do this with two people but I managed on my own.

Once the connection was through I double checked that the wire was straight and that there was enough unspooled. 

Then I went upstairs and carefully pulled the fish tape up until the new wire came out of the hole in the wall.  

I made sure I had enough wire to work with and I wrapped some tape around the new wire, while keeping the tape on the spool, to prevent the wire from slipping back down the hole. You could also wrap the wires around something like a pencil.

Step 5: Connect Wires to Thermostat Base Plate

With the new wire fished through the wall it was time to start wiring up the new thermostat. I trimmed the wire to an appropriate length so about 4 inches was sticking out of the wall. I'd deal with the mess of unwrapping the electrical tape off the end and fish tape later. I began by stripping a couple of inches of the brown outer wire sheath to expose the 5 individual color coded wires within. Then I wrapped the Green wire back against the main wire bundle since I wasn't going to be using it then I stripped about 3/8" off the end of the 4 remaining wires as shown.

Remove Jumper

The base plate of the thermostat comes with a metal jumper connected between Rc and R, it looks like a small staple. This jumper needs to be removed to be able to wire the thermostat to an independent power transformer. To remove it, use the small flat-headed precision screwdriver to loosen the two screws for Rc and R in the black terminal block. You don't need to unscrew them all the way, just until the tops of the screws are flush with the top of the black terminal block. Once both screws are loosened just pull the jumper out.

The Rc terminal is used for connecting the cooling relay. If your thermostat controls both your heating and cooling system I'm not sure how you'd go about wiring an external transformer. In that case you'd likely need to wire your thermostat's C wire directly to the transformer on your 24 volt HVAC system. 

Attach Wires

One by one feed the end of each wire into it's appropriate terminal block and screw it down to hold it in place. The convention I'm using is:
  • R - Red (to boiler)
  • W - White (to boiler)
  • Rc - Yellow (to transformer)
  • C - Blue (to transformer)
Once the wires are attached you can secure the base plate to the wall. I used a small level on the side to make sure the base was mounted straight. (The top of the thermostat and base plate are curved.) The picture, didn't come out quite as straight :) but you can see how the 4 wires attach to the thermostat's base plate.

Depending on your HVAC system your wiring might be different.

Step 6: Connect Thermostat To Transformer

Back down in the basement and I ran the new wire to where the connection point to the wiring that goes to the furnace is. I think when they installed my new boiler they spliced in a new thermostat wire into the old one and the connection point is on a wall above the boiler. I ran the new wire to that point, gave my self a little extra to work with in case there where any mistakes and I cut the wire and stapled it securely to the wall. 

I then stripped off about 2" of the brown outer wire sheath. As before I wrapped the green wire back around the main wire since I wasn't going to be using it. Then I stripped off about 3/4" from the ends of the 4 remaining wires.

Next I determined where I was going to plug the 24 vac transformer in and measured out the appropriate length of wire from the spool. I knew I was going to have extra thermostat wire left over and I will be using that instead of getting a different 2 conductor wire since I won't need all 5 for this run.

After trimming the wire to length, I stripped off the ends of the wires and, wrapped the each around one of the 2 screw terminals on the transformer and tightened them down. The position of these two wires isn't important as this is an AC transformer. I picked this particular 24vac transformer (MGT2440) because it had the screw terminals.

Next I attached the yellow wire coming from the transformer to the yellow wire running down from the thermostat. I stripped the ends of both about 3/4", used pliers to wrap the wires around each other in a clockwise direction and secured them with an orange wire nut. I repeated the process for the blue wire from the transformer to the thermostat wire.

Before going any further I wanted to make sure the thermostat was getting power so I plugged the transformer in, ran upstairs, pushed the thermostat onto it's base and was happy to see the screen on :)

I had already configured to wireless settings when I was testing the 24vac transformer before I went through all this trouble.

Step 7: Connect Thermostat to Boiler

There was a 3 conductor wire running from the boiler to the old wiring for the thermostat. Luckily this new wiring was color coded so I knew which colors went to the R and W terminals on the boiler. One by one I attached the Red wire from the new wire that I ran from the thermostat to the basement by stripping off 3/4" off the new wire and wrapping it around the wire running to the thermostat R terminal in the boiler. The boiler wire was previously connected to the old wire and already stripped. I capped the connection with a wire nut and repeated the process for the White wire to the wire that runs to the W terminal on the boiler.

I flipped the switch to turn the power to the boiler back on and ran upstairs to make sure everything was working. I turned up the thermostat and checked to make sure the boiler fired up. While downstairs next to the boiler I used my phone to turn the thermostat down and within a few seconds the boiler turned off. :)

Step 8: Wrap Things Up

Now that I know everything is working correctly I wrapped electrical tape around the wire nuts to help keep them secure and I used the Fire Block Caulking in the opening from the basement up to the wall where the new thermostat wire runs. To be on the safe side I double checked that everything was working. Using the app on my phone I can adjust the thermostat from anywhere.

One of the nice things about having the thermostat plugged into the wall is that the light on the thermostat can always stay on and I don't have to worry about batteries draining. The thermostat is next to an opening I tend to bump into when I go down to grab a glass of water in the middle of the night. Hopefully no more stubbed toes and I never have to worry about replacing the batteries.


  1. Can I use the existing wires going to my thermostat but just add the 24 volt transformer and run the C wire from the transformer to the new thermostat? Or do I have to run all new wires. Im planning on buying the Venstar ACC0454 Skyport WiFi Key For T5800. So I will need the C wire. Thanks

  2. Yes you can use the existing wires and run 2 new wires for the transformer. I used all new wiring because I need to snake the other wires through the wall and it was easier to use the old wires to help bring the snake down the wall by pulling on the wire.

  3. How does the thermostat know to pull the power from the RC terminal? Could it not be pulling it from the R terminal and returning it on the C?

    1. I second this question, would really like to make sure it's not pulling additional power from my furnace transformer. Want the power to only come from my second transformer that I wish to connect to C and Rc in my two wire heating system. Additionally polarity shouldn't matter with a transformer like the one in your blogpost right?

    2. R and Rc are both hot terminals on the thermostat they are commonly connected via a jumper wire. The thermostat switches between the AC and heating functions thus completing the circuit. in his case the jumper wier was removed.

    3. It will depend on the thermostat design but most likely the "R" is supplying the thermostat power for the "smarts". Therefore the 2nd transformer is only supply a return for the current sourced from the furnace transformer... NOT supplying it power. You could get the same result by connecting a neutral from a house plug to the "C" on the thermostat and call it a day.

      The best practice is to create an additional leads coming from the return on the secondary side of the furnace transformer and connecting it to the "C" of your thermostat.

  4. Adding to my previous question above. This video actually suggests leaving the wiring "as-is" and placing one of the two wires from the external transformer on the "C" terminal and doubling up the second transformer wire on the R terminal of the thermostat. See: I also found this kit which suggests the use of a relay. See: Although the kit is for the Venstar thermostat, it states that it can be used with other thermostats that require an external transformer. The kit wires like this diagram. See:

    1. bv123,

      I'm not sure I understand your first question. The thermostat is wired a certain way and I haven't seen the schematics to know how the current flows through the circuit board. I called Honeywell and asked them what I needed to do to attach a 24vac transformer to my thermostat and they gave me instructions on how to wire it.

      You might want to contact the manufacturer of your thermostat to get answers to your questions and find the appropriate wiring technique for your thermostat and hvac system.

  5. I followed your directions here and my thermostat works. Basically I was provided with 3 solutions by different people knowledgeable in controls. The first being your method. The second method was almost the same as outlined here in your instructions, with the exception of connecting the transformer. Instead of using the RC terminal, I was told to connect one transformer wire to C and to connect the other to the R or RH terminal, doubling it up with the wire already there. The third solution was to run the transformer wires to C and R, then connect one side of a relay to C and the other side to W. The run the two wires coming out of the other side of the relay to R and W going to the furnace. You are lucky that Honeywell gave you help. When I asked them for help, the only response I received was of no help, see below:

    Thank you for contacting Honeywell.

    First I would like to apologize for the delay in responding to your email.

    I can understand the difficulty you are facing in getting your WiFi thermostat to be compatible with your system.

    We are not trained in wiring of anything to do with transformers. The reason for this is if something is miswired, major damage could be done to your system.

    What I recommend is contacting a contractor for installation of the transformer.

    I can offer you a web address where entering your zip code will generate a list of Honeywell recommended contractors.

    The link to that website is:

    Honeywell ECC Customer Care

    1. bv123,

      Thanks for letting me know and I'm glad everything worked out for you.

      I called the 800 number that was in the installation instructions for the thermostat and I spoke to someone that really knew what they were talking about regarding the thermostat. Not just a CS rep that looks up and recites info from some knowledgebase.

      I asked why they didn't include instructions on how to wire the thermostat to an external transformer and it was something along the lines of they preferred to have people call for directions.

      Sucks that you had a different experience with CS but I'm glad I could help.

  6. Tom,

    I bought one of these thermostats and found out I do not have the 24VAC common either. Is everything still working on this system? I have also looking at a 24VAC, 1.66 A transformer as well. I was a little worried with this high of a current when I've seen others that people have used with only 300 mA. My question really is, is this much current safe to use on this thermostat?

    1. Jeremy,

      It might be a good idea to call the 800 number in the manual to see what Honeywell has to say. It would be interesting to know what is the maximum power consumption of the thermostat. If you call them and ask please come back and let me know.

      The transformer I used is rated as 24VAC 40VA. Power = Volts * Amps so Amps = Power/Volts so the transformer I used is a 24VAC 1.67 A transformer. Same rating as the one you mentioned.

      An electronic device draws power from a source that provides voltage. The amperage A (or VA as is commonly used in transformers) is the power capacity of the transformer. It means a device can draw up to that much power from the transformer. A device that draws less power can work with a higher rated transformer, just not the other way around.

      The transformer I connected hasn't had any issues. See the link in the "What you'll need" section to see the transformer I used.

      Hope that helps.

  7. Tom, thanks for the article on how to install a c wire. My wifi thermostat is up and running now!

  8. I found your website perfect for my needs. Thanks for sharing the great ideas. Whole article is too good and well written.

  9. Tom,
    Thank you so much for taking the time to share. I found your article on a last ditch effort before giving up and returning it. I have a boiler system with no C or G connection and could not for the life of me figure out how to correctly install an external 24 VAC source, but your explanation was exactly what I needed and now I have a fully functional wi-fi controlled thermostat thanks to you. I will be at the AHR show in Chicago next week and will be stopping by Honeywells booth to give them a piece of mind on making this information available. Not sure why they can have videos of all the other ways to connect it but not an external power source explanation. One would think it would help to increase their sales..
    Anyway thanks again and keep them coming,

  10. So If I'm reading this correctly, I have a 2 wire thermostat(T87F) on a hot water boiler system, with no A/C. And in order to use a smart thermostat(Honeywell) I just need to connect a external transformer to the thermostat? I can run it from the wall outlet up to the smart thermostat?

  11. Your post was the lightning bolt of inspiration I was looking for! Installed my RTH9580WF today with the existing original 2-wire setup from my old therm and snaked the 24VAC transformer wire back up the same route. Hooked the 2 wires from the transformer to C and Rc and voila! Power! What a beautiful thing! Thanks for your detailed instructions my friend, everything works great! Now if I could just get the web interface to actually control the thermostat... But that's for another post... :)

  12. Thank you! I need to run a c-wire and yours is the only explanation I could find for how to *physically* do it. Quick question, though - couldn't I just tape the new wire to the old at the thermostat, then let the old wire pull the new as I pull it out from the basement? What benefit does the fish tape give me?

    1. Your welcome Jeane. You can use the old wire to pull the new wire through. I just had the fish tape and don't get to use it that often so I did. :) Also if it's a difficult pull the fish tape is stronger and won't break were as the wire might. But if the wire pulls freely it's not a big deal.

      What I would recommend is that you also twist some of the wires together in addition to taping for a stronger connection. Fold the wire over another wire of the other cable then twist it back around itself for each. Then tape them up.

  13. i have a running Honeywell RTH9580WF wifi thermostat hooked to a oil furnace. can I add a separate attic a/c unit to this thermostat without bothering anything on the basement furnace? I wouldn't mind if the oil furnace fan turned on too when the unit turns on in the attic. If not that's fine.
    Both units are independent of each other. the a/c unit has its own thermostat unit and only uses the green and yellow and red wires. I can run them down to the first floor near the RTH9580WF but can this be done?

    1. Check the manual that came with the thermostat. It should have information on how to wire a separate AC and heating system. If not contact Honeywell and ask them for assistance. I didn't have to do that so I'm not familiar with the process.

  14. Thanks for this great post. I have an ancient heater and there was no "C" wire at the thermostat. I thought that a transformer might work and this article made it easy. Great job

  15. I was thinking about powering a smart thermostat (ecobee3) using a 24 vac transformer but as I don't have a basement I was going to plug it into an unused outlet in my attic.I would imagine this is ok ? It's temporary until I have central air installed. I now have an oil boiler that has only 2 wires, red and white.

  16. Great instructional information. Question - can I connect three Honeywell wifi thermostats to one transformer? I have three zones. If yes, can I use the single recommended 40va? I'm also planning to install a direct wired transformer and not a plug in. Thank you. Anthony

    1. I have the same question. Did you ever figure out if you needed 3 transformers?

  17. I am about to add an external transformer to my Honeywell WIFI thermostat. Did you pay any attention to the 120vac phase used to power the transformer? Thanks in advance.

  18. I have a two heat zone System and I'm looking to replace my old round honey wells with the rth9580 that need a C wire. I'll go with the external transformer approach here. My question is can I just have one transformer power up the two thermostats . . . In this case by screwing both new C wires into the suggested transformer and sending that power in tow wires off to the two thermostats ?

    1. I'm the guy who posted the question and wanted to answe it in part with my research. The suggested transformer is 24vac and 40 va. So it's 1.667 amps or 49 watts. I have a question In to Honeywell as to how I have power one rth9580 draws. Another post said that Honeywell answered 1.5A which sounds high. One of the reviews on a similar 40va transformer said it runs hot with one other model of wifi thermostat. So I'd be inclined to use two transformers (one per thermostat).

      One other related thought is I could have run power wires to my boiler/furnace which has a com outlet, but that would be power stealing as Tom said above. In this case If I blew that switchboard I'd be out $300 instead of a $15 transformer.

      Yet another thought is I might leave my old Honeywell Mercury thermostat and its wiring in as a backup. I'll skip the description of the wiring of the a/b switch I'll set up by the boiler. The logic is the Mercury ones work well and never had issues, and they are hard to buy. As Tom mentioned, wifi can suck when wifi isn't working.

    2. Man it's nerdy to answer your own questions . . . But hey, you are the one reading this. I got thru to Honeywell, and unlike my first call I got someone who knows what they are doing (Rafael). Each Rth9580 draws 46 milliamps, or just 1.38 VA at 24vac. The suggested transformer by Tom was 40 VA. So I should be good with three thermostats plugged in. Given my boiler has a com/24vac outlet I could have run a line to it and given how low this is I should have been able to power all three thermostats (refer to his power stealing comment) but hey, for $16 better safe than sorry with blowing out a $399 switch board on the boiler

    3. Good info thanks for coming back to post. When I called Honeywell I got someone that really knew what they were talking about the first try. I guess it's hit or miss.

  19. I plan on installing the same model thermostat on my forced air oil furnace. I have two wires - a black and a white one. Does it matter which one goes to the red and white connectors on the new one? And does it matter which wires from my plug adapter go to the C and Rc connectors?

  20. Great article! In my situation, I do not have a convenient outlet available to plug a transformer into. However, on the wall behind my furnace there is a Honeywell transformer mounted on a metal electric box that is connected to a short length of Romex coming from the box with the main shut-off switch. Honeywell tech support said I should not use this transformer to power the thermostats, as it already powers the contol board and two zone controls. My question is: Can I mount another metal box with a new Honeywell transformer next to the one with the existing transformer, and add the wires from the new transformer to the connection between the existing transformer and the romex from the power switch? I would then use the new transformer to power my 2 new wifi thermostats. I hope I explained this properly, thanks in advance for assistance!

  21. Thanks so much for posting this! Our situations were near identical with the exception of thermostat type (I was installing a Nest). When power sharing (before the C-wire install) the gas valve would short cycle constantly under a call for heating. Before finding this post I was getting ready to try and splice wires into my boiler control wiring; this solution is far more elegant. Next step for me would be to install a box + outlet between my boiler's "off" switch and the built-in boiler control transformer so that everything can be controlled via a single circuit. Thanks again, I don't know how this would've turned out if I hadn't read this post.

  22. Any advice on achieving the same results with a nest thermostat?

  23. What a great article!! Here is my summary of a few scenarios....the issue with the "power stealing" thermostats is only with hot water heat. This is because if you use the C wire from the heating system the voltage to the zone valves drops. This will increase the current to the zone valves possibly causing them or the transformer to fail prematurely. This voltage drop is not an issue with forced hot air or air conditioning systems.

    My understanding is the thermostat needs power for wi-fi. And we all know you need two wires to complete the circuit. Running an independent C wire and its associated "hot" to Rc supplies the thermostat with the required power source. If there was air conditioning and hot water heat, the C would come from the air handler and the hot water zone valves would not be an issue.

    I guess after ranting on my question to you is, did you have to "trick" the thermostat into thinking there was air conditioning in order for the wi-fi to work and program for air conditioning OR did you just program your thermostat for heat only?

    Also, I lived through Sandy also........even though your internet and the wi-fi function was not working.....could you still manually control the thermostat??


  24. so why not use the old wire as a snake to pull the new wire with?

  25. Heads up on the 24vac transformer (MGT2440) link in the article. I bought one and it was DOA (putting out 1-3 volts). Noticed a fair amount of other Amazon purchasers with same experience.

  26. I have a 4 wires cable controlling my ac and I still need to get a 5 wire cable...
    I just bought honeywell rth8580wf

  27. Thank you for this article! To concur with another post, for my system the transformer needed to be connected to the RH and C terminals; didn't work connected to the RC terminal. I have a Control-4 thermostat only handling the heat connected to a gas boiler.

  28. I live in Canada ans can seem to only find a wait pt5000. I is 24V and 750ma will this work with that thermostat. Thanks

  29. Hi Tom,
    Try as I might, virtually every wiring configuration, and using an external 24V power source for a heating only, 2-wire system, I CANNOT get the wi-fi thermostat to power up. I even tried to connect a known 24v source to the terminals on the face play ... nothing. At this point I believe I have a thermostat on the fritz: Honeywell RTH6500WF. Yes, I admit, it is an outdated unit as I procrastinated installing it for 5-years or so.
    Gas boiler system is more than 50-year sold.
    If you have an answer I WILL donate.

    Norm (not lazy, just slow to pull the trigger)

  30. I have an older Rheem heat only furnace in my home. I want to install a Wifi thermostat so I can monitor and control the heat on my smartphone.
    It currently only has two wires connected, red (Rh) and white. The cord that runs to the thermostat has 5 wires so I want to use one of them for the C wire.
    The red (Rh) wire connects to the transformer in my furnace.
    Can I connect one of my spare wires to that transformer for my C wire? And if so, which pole on the transformer should I use, the one the red (Rh) wire is connected to. or the other pole?

    1. The other pole. This worked for me on a 40 year old Lennox system.

      I should clarify some things.

      1. Make sure you're dealing with the secondary power (24vac outgoing) and NOT THE PRIMARY (120v incoming). If your 24vac transformer

      2. If this transformer only has 2 secondary wires, trace back the one that the Red wire is connected to. Use the wire that the Red (R, or Rh) is NOT connected to for Blue (C)

      3. Connect your Blue (C) wire directly to the transformer. There will be other wires coming off the opposite secondary pole. Those are being used for G W Y relays. DO NOT CONNECTED THE BLUE TO ANY OF THE OTHER WIRES, just hook it straight up to the transformer's secondary pole, opposite of the red wire.

      4. ** Check the wattage requirements of the new thermostat and the output wattage of your existing 24vac transformer. Make sure the existing transformer has enough wattage for your new thermostat and the relays it has to control in the HVAC unit. **

      Be safe. Contact a professional if in doubt.

  31. Thank you so much for you article. I connected my RTH9580 wifi thermostat using a second transformer. works perfect!!!

  32. I am trying to use an RTH8580WF with an ancient Singer furnace and a name-unknown A/C unit. There was no C wire coming to the existing thermostat. *Honeywell told me there is no way to use a separate transformer* with this thermostat, so I followed the instructions in one of their "official" training videos on YouTube for re-purposing the G wire as a C wire. This works fine for heating, but when we set the thermostat to cool, it "Wait"s for several (5?) minutes, and the instant the relay clicks the thermostat loses power then immediately powers up again with the "Wait" indicator displayed. The system never does actually start cooling.

    When I called Honeywell again, they said that re-purposing the G wire is intended only as a temporary measure and may not work with all systems; they recommended running a separate C wire or using one of their Wire Saver modules.

    1. meh. Honeywell is useless. They told me the same thing with my 9580. I went to Lowes an got a humidifier 24 volt transformer and wired in as above.. Wait #PT 5000 works flawlessly. They said it wouldn't. 4 months and going strong. Really appreciated the above article.

  33. Had been doing this with my Nest for 2 years. Rh and W to my boiler, and Rc and C to my plug-in 24 VAC wall transformer. Worked fine until a recent Nest update. Now (with no change in my setup or wiring), Next says Rh is not powered. If I remove the Rc and C wires from the Nest, everything is fine. I checked the voltage across Rh <-> W and get 24V as I should. I checked the voltage across Rc <-> C and also get 24V. Yet when I have all 4 wires plugged into the Nest, it says Rh is not powered. I have to assume it was due to a recent software/firmware update.

  34. I have two Lux Geo thermostats for a zoned 2-wire baseboard heater. The thermostat cable had two unused wires so I thought I could do this to keep power to the thermostats.

    My original furnace source was connected to RH and W. I installed a second transformer and wired it to RC and C. The thermostats were only powered when the RC-RH jumper was in place.

    I realized this could deliver 48V to the furnace if it switched on, so knew this was not a solution.

    The Lux technical support claimed their thermostats would not work with two-transformer systems. There is nothing in their technical documentation to back this up.

    So today I connected my external transformer to RH and C, my furnace to RC and W. No jumper, and power to both thermostats. I'm afraid, though, to test the heater.

    Could this harm the system if I test the heater?

  35. Your instructions were spot on and very helpful! Thank you!

  36. Thank you very much for these instructions!! I just replaced the 2-wire thermostat wire for my oil furnace with a five wire + AC transformer, and now my Nest is happy once again.