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Heating & Air Conditioning Other

C wire needed for new thermostat

4 years ago

Hi guys,

i just purchased an ecobee 3 wifi thermostat. When i went to install it i realized i needed to add a C wire. Ecobee thermostat even comes with an adaptor to help with this issue. My Problem is that my AC unit is located on my roof and not easy for me to get to. So i was looking for any other solutions before i decided to get a profrssional or just scrap the wifi thermostat.

i came across a 24 volt C wire adaptor that plugs into an outlet. Last night i tried to install the Ecobee with the adaptor. I have 3 wires coming from my wall, RC, Y and G. It is an air only system. It looks like the old thermostat had a jumper between rh and rc. I connected one of the wirrs from the adaptor the The C terminal And the other to the RH. The directions for the adaptor appear to show the second wire going to the RC terminal but since the R coming from my wall already occupied that terminal i improvised. I was able to fire up the themostat however when i attempted to lower the temperature to trigger the Air nothing happened. Ive tried googling and reading different diagrams to help me however i am having trouble finding a diagram that has the setup i do (R, Y and G coming from the wall).

At this point im starting to wonder if the adaptor would work with my setup. I figured i would try and get some help/advice before going any further. Any help would be greatly appreciated.

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Comments (3)

mike_home
4 years ago
Did you keep the jumper between Rh and Rc? Did you disconnect the original wire from from the wall and going to the Rc terminal?
Mike Brino
Original Author
4 years ago
the old thermostat had a jumper built in. When i was reading about the ecobee it said to remove the jumper so i did not run a jumper from rh to rc.

As for the second part of the question im not sure i understand. maybe this will help. The R wire coming from my wall i connected to RC on the ecobee. I connected Y to Y and G to G. i then connected one of the wires from the adaptor to the C terminal and the other wire to the RH terminal. And again i did not use a jumper from RH to RC. Hopefully this helps explain it better. Thank you for your help.
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Not sure I followed it either. Depending on the model, the orange/blue connections on the ecobee are usually the reversing valve connection for a heat pump. Are you sure this isn't the case for your installation? In any event. follow from the connectiors at the thermostat/furnace/condenser and don't assume the colors are as they should be.
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The big reason to use solid vs stranded wire is that solid wire is easier to thread into those terminals. With stranded wire, there can be a problem if a loose strand manages to contact an ajacent terminal. More likely because the terminals in a thermostat are so close together. Just had a problem with our thermostat. It started running 2 degrees past set temperature. (I have it set for a 2 degree temp differential because the heater was constantly kicking on, running for just a bit and then kicking off...which was driving me nutz.) After a bit of sleuthing, I found that the caulking backer rod that I had shoved under an exterior door (across the room from the thermostat) had become unwedged and there was a significant draft from underneath the door. (The backer rod was supposed to be a temporary fix that I did a couple of winters ago but forgot all about it...) Once I got that hole plugged up (with a better temporary fix), the thermostat quit heating past set temp. I don't know for sure if the draft was the reason the thermostat got weird or if it was a coincidence.... Whatever, I did buy a new thermostat to replace the old one but haven't installed it since the problem seems to be resolved. Just tossing my recent experience out there as a potential cause because a new therm hasn't fixed your problem. (Yes, I know my thermostat is in a poor location and should be moved away from drafts from opening/ closing exterior doors. Just no good or easy place to relocate it to and it's a low priority problem.)
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Do you have any other controls in your heating system? If not there would be no reason why you could not replace this with a wifi thermostat. You also have an additional wire which will have to be utilized and used as you will need a common. If you buy the ecobee they have a tech support that will walk you through wiring and set up of your ecobee at no charge. As mentioned if you have no other controls associated with the existing thermostat it would be a simple fit. Just an FYI the AC unit bears no relevance to your concern as the wiring diagram states this is a simple on off signal sent from your furnace. I hope this helps.
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Do you have a gas furnace and AC? When you say furnace comes on, are you saying the blower is running, but the outdoor condenser is not? Where did you connect the wire which was originally connected to Y/Y2? Did you set up the thermostat properly?
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mike_home
4 years ago
The Rh is to power the thermostat for heating, and the Rc is the power for cooling. Since this is a cooling only application, you only need a connection to the Rc terminal. Do not connect the old R wire from the wall to either of the adapter wires.

You need to disconnect the old R wire from the Rc terminal, and connect one of the adapter wires. The other adapter wire goes to the C terminal. The old R wire could be connected to Rh without the jumper, or keep it disconnected and cover the end with tape so it cannot short to any other wire. I recommend you keep it disconnected and covered.

If that does not work, then you may have to install the jumper between Rh and Rc. Consult the Ecobee installation manual about the wiring for applications that are cooling only.

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