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IoT: Connecting C Wire For Heat Only Boiler System

Published December 13, 2017

Over the past month or so, I've been adopting a new hobby around IoT, also known as Internet of Things.  On the most simplistic level, IoT is about connecting devices of all sorts to the Internet so you can control them remotely or trigger them to do things based on algorithms or data.  For me, it starts with swapping out some of my thermostats to make them Wifi enabled, as well as swap out some of my light switches to work over Wifi. I have not gone as far to convert my oven, fridge, microwave, sinks or showers to be controlled over the internet - at least not yet.  I hope to share my experiences with this process in an IoT series here.

My first challenge was to convert my heat thermostats to Honeywell wifi thermostats, I specifically used the RTH8580WF model since they were on a crazy sale the week before Black Friday.  I have completely separate heat and air conditioning thermostats, many homes in my area do because it is more efficient in cold areas to have it built that way.  So a year ago, I hired an A/C guy to hook up the RTH6580WF models (I tried to do it myself but A/C C wires are way more complex).   But for heat, I wanted to do it myself.  Sadly, there were not a lot of good documentation on how to do it yourself for heat only.  So I figured I'd share my process, so maybe it can help others looking to do this for their heat only thermostats.

I have a friend who is an electrician and he coached me through the process. I am not an electrician and I was actually pretty nervous I'd short the whole system, but I was told I could do this on my own.  I did have my friend electrician come by to show me what to do and he fixed a wire that was broken, but for the most part, I was able to understand what to do and did most of the thermostats myself.  Here is what I did step by step.

Step 1: Check behind the thermostats you have on your wall to see if there are any extra unused wire.  For heat only thermostats, you should probably see at least two wires hooked up to your thermostat, a red and white wire.  There is probably an extra wire behind the thermostat box hidden in the wall, sometimes there can be more than just one wire, but for heat only, you only need three wires to make this work.  

I personally had three different types of thermostats but they all had only two wires that were hooked up.  A red wire was hooked up the R or RH (H is for heat) and a white wire hooked up to the W.  Then I saw a green wire cut off and unused behind the thermostats.

Here is one on the wall:

Here is one where I pulled it off the wall. This one had me scared because they used the green wire instead of the red wire, so I was nervous the red wire was broken.  It was, but my friend was able to tug at a few things and fix it, so we were able to use all three wires.  The colors aren't that important, how they are hooked up on the other end are.  

So you see, there are three wires, one was unused.  So you are ready now to check your boiler room, normally in your basement, go there and find where all these wires go to.

Here is a photo of the wire section of my boiler room, it can be pretty intimidating for someone like me who has no experience with any electrical work or heating work. 

I live in a newish house, so some of these were labeled pretty well.  Some say family room, bedrooms, kitchen, etc. They pretty much matched up.  This is obviously very useful because I didn't have any tools to help me figure out which wires went to which thermostat.  If you don't have that, then you can guess or hook all the C wires up and then wish for the best when you mount the new wifi thermostats.

Step 2: They say turn off the power so you don't short your heating system or shock yourself.  Most likely you won't shock yourself but who knows, you can short the system.  So shut the power to the boiler in your breaker or your emergency switch.  Note, this will turn off the heat in your house - so do it for a short period of time or when no one is home.

Step 3: I had to extend the non-used green wire (in my case it was green except for two zones, the broken red one and another thermostat that had more wires, so I used blue which is the typical C wire color). 

If the person who did my heat previously was nice, the would have wrapped a lot of slack around for the unused wire.  Only one thermostat was done that way, you can see the wire coil on the far left of the image above.  So that was nice, the others, I had to find cut wire by peeling back at the brown thermostat wire tubbing to find that wire and then peel it out. Here you can see it:

Step 4: Peel those back, expose the copper wire and add new wire to them by capping them off.  I didn't have a wire stripper tool, so I used my finger nails, it took a long time.  But eventually, I was able to extend them all and have enough slack to be able to reach them into the power source to heat them up.  So far at this point, nothing is live, so technically you do not have to turn off the power yet. But if you want to be safe, turn it off.

Here is a photo of the wires extended:


I bought extra three wire thermostat wire from the local hardware store so I can extend them. Now I had enough slack to reach them to the power wire bunch.  Here is a more zoomed out look at this:

I had four green C wires now, one red one (the potentially broken one) and a blue one from the nicely coiled wires.  A total of six heat zones.

Step 5: At this point, I tried to hook up one of the new C wires to the bunch of powered cables going back to the transformer. This was the yellow batch of wires on the bottom left of the photo directly below this:

Make sure when you do this, the power is turned off. I then mounted the new thermostat in the family room.  Red wire to RH or R, white wire to W and C wire (in my case here, a green wire that I extended) to the yellow bunch.  I put the thermostat cover on and nothing. It didn't power up. :(

Step 6: I called my friend who came over and said, wait, reverse the red and white wires.  Hook up the red wire to W and white wire to R or RH and leave the green C wire connected to C.  I did that and it worked.  He said sometimes you need to do that and explained why, but I didn't fully understand why - I am not an electrician.

To make my life easier, since he was over at the time, he drew a new green wire from the yellow batch and told me to feed all the green wires to the off white cap at the top of the picture directly above.  This way I can easily just feed all these C wires to one cap. 

Also while he was there, he was able to pull a staple out of the broken red wire (discussed above) and got that one working again. So he wired the red wire properly and I used the green wire for the C in the broken location - so that was no longer broken.  We then put all the C wires; 5 green wires and 1 blue wire, into the off-white cap above and I mounted the thermostats. We then turned the power back on from the breaker and the new wifi thermostats powered up.

I was all set and good to go. I then hooked them up to the wifi and integrated them with Google Home, Apple Homekit and of course the native apps.

Again, I am not an electrician and I was lucky to have a friend help out.  But I do hope this blog post helps someone who wants to do it themselves as I really didn't find any good documentation for a non-electrician online on how to do this. Maybe you shouldn't do this if you aren't an electrician but hey, people like you and me love DIY type of stuff like this.

Next IoT blog post will be about my experience with the Lutron Caseta Wireless switches and my experience with that.

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Barry Schwartz is the CEO of RustyBrick, a New York Web service firm specializing in customized online technology that helps companies decrease costs and increase sales. Barry is also the founder of the Search Engine Roundtable and the News Editor of Search Engine Land. He is well known & respected for his expertise in the search marketing industry. Barry graduated from the City University of New York and lives with his family in the NYC region.

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