Smart t-stat and three conductor hook up

[mark32]

So, a friend bought a couple of Honeywell smart thermostats and asked me to install them (along with some other work). While scoping things out on my first trip, I noticed one of the t-stats (cooling only) had a three conductor cable going to it. So, I knew I'd have to replace the cable when I returned to do the work. However, I stumbled upon an article saying that the G wire can be repurposed as the C wire, as long as the customer knew they would no longer be able to turn the fan on and off separately, which I made my friend aware of. (Certainly I'd rather make this connection than run a new cable). So I get to his house after work and begin on my tasks (I first had to swap out an attic fan motor, it was a pita). Afterwards, I then killed the power to the airhandler and wired up the new thermostat. (Red to R, green to C, and white to Y) Back at the air handler, I was somewhat puzzled to see the red from the thermostat was on the C terminal. Now, I don't normally find myself poking around airhandlers, but I've installed about a dozen smart t-stats thus far, and unless I'm mistaken, in all of those cases, nothing was existing on the C terminal. Hmm, so, after observing the diagram you see here, I took the green wire off of what is labeled "Y/?" (I know it was Y and slash something, I thought it was a G, but I can't be sure now) and put it on W as it's on the other side of the transformer from R. I turn the power back on to the airhandler and the t-stat lights up and we go through all the setup/settings. I bring the temp down to start the a/c and I begin cleaning up. Almost ten minutes go by and I realize the a/c never kicked on, wth? I open the air handler up and with meter in hand, I see 26v on the white conductor(s) (again, this is coming from the Y terminal on the t-stat, the other white is going out to the condensing unit) so I know the t-stat is calling for cooling. I get on Google and I see that not all hvac equipment will gladly accept this G to C work around. I then see another article stating you can jump the Y terminal to the G terminal to get the fan working. Okay, so I jumped from the terminal with the white conductors and ran over to the Y/? terminal, which is where the original G wire was terminated. I turn the power back on and still nothing. At this point its like 9:30pm and I still need to get home and have dinner and shower, etc. I put it all back the way it was and put the old t-stat back up, turned it on and the fan kicked on. I told my buddy I'll be back another day. So, do you guys think I can get this thing running with three conductors, or do I have to bite the bullet and run a new cable. Btw, the airhandler is made by Space Pak

 

[Joe Villani]

On a conventional thermostat when the thermostat is in the cool mode a connection is made between the Y and the G.

On a call for cooling the R (Power for this conversation) will make to Y (cooling) and G (Fan relay in the air handler) at the thermostat.

The fan will turn on in the air handler and power will be sent out to the condenser contactor.

If you have no G (Fan) at the thermostat you will have to jumper G with Y at the air handler.

Keep in mind on BASIC air handlers the Y terminal ( some dont even have one) at the air handler is just a splice point to get power to the condenser contactor.

You should have no problem getting this to work with three (3) wires.

Find the 24v at the air handler.

Install at the thermostat R and C. Also keep in mind most smart thermostats the Rc is where the thermostat is getting constant power.

Install the Green on Y.

Jumper the Y terminal in the air handler to the G terminal (fan relay) to get the fan relay to make. Also splice the condenser wire to the Y to get the condenser contactor to make.

 

[kwired]

Did it work before you got there?

Is there a time delay before compressor starts and you did not wait long enough?

Many electronic thermostats have such delay built into them and is used either after power failure and/or after a call for compressor has terminated to allow refrigerant pressure to stabilize before starting the compressor again.

If you had mechanical thermostat before sometimes there can be delay in the condenser unit to serve same purpose. There are two types of those. One is simple delay on make and it delays every time you call for run. Other is delay on break, and after a call to run has terminated will not let it run again until the delay time has expired.

 

[infinity]

So, do you guys think I can get this thing running with three conductors, or do I have to bite the bullet and run a new cable.

I'm not sure about the work around. I have 5 Honeywell thermostats and I had to run a new cable to 4 of them for the C wire connection. Luckily one of them had a 5 conductor cable with a spare conductor.

 

[kwired]

So, do you guys think I can get this thing running with three conductors, or do I have to bite the bullet and run a new cable.

Should be able to jumper the Y and G terminals at the air handler if it is cooling only and get by with three conductors. will not have the ability to run fan only.

If it is a difficult run to replace the cable, there are modules out there that can send digital signals over just one pair to get multi terminal functionality or maybe even wireless devices that can be used. The nice thing about a thermostat that uses the system to power it is no need to have to replace batteries or come home to hot or cold house because the battery failed.