I've already installed three separate WiFi thermostats (using an external transformer for each) but never one that controlled both cooling and heating. I can see that I have the red wire for the heat hooked up to R terminal and the cooling red wire hooked up to the Rc terminal. The jumper between them is not present. If I were connecting an external transformer I would hook one wire to C and the other to which? Does it matter? In my three prior installations I had only one wire connected to an R terminal so all I did was connect the transformer's 2nd wire to the free R terminal and remove the R/Rc jumper (per Honeywell's tech support).

 

On a thermostat..... the electronics are powered from the Rc terminal. So in your case it will be C and Rc for the external transformer and NO jumper in place.

Have you tried locating a C on your A/C system or are you short on conductors ?

 

Okay, let me take a step back here to make sure the current wiring is okay.

There are only three wires coming from the A/C system. The red wire is connected to Rc, the white to Y, and the Green to G. For the heating system (gas hot-water baseboard) there are two wires: the red wire is connected to R and the white connected to W. Are you saying that with the R/Rc jumper currently removed the heating system is inoperable? Or is it the case that both the heating and cooling capabilities are being powered from the A/C system? I noticed there was a small DIY jumper sitting on top of the house alarm panel keypad which is just beneath the thermostat. Could it be that the installer (in the current non-WiFi thermostat) forgot the jumper wire between R and Rc? Will the heat work in the current configuration? The A/C certainly does. What if the A/C breaker is turned off for the winter? No heat?

Aside from that, if the WiFi transformer is to be connected to C and Rc, do I still leave the current red wire (from the A/C system) in the Rc terminal?

 

What air handler do you have pushing the air for cooling?

Many units ground the 24 volt common. It would then be possible to provide a ground wire to the stat for your 24 volt common terminal.

 

I believe it's a Trane. Only a few years old. But I have no intention of messing around with the wiring in the air handler. I just don't feel comfortable enough to do it.

 

Another option is using the G wire for the stat C terminal and moving the air handler end to C as well.
Connect the stat Y wire to Y and G at the air handler.

There is a utube video to assist you.

I wouldn't try to wire a third transformer to the stat.

You can always call a professional contractor to wire it for you.

 

why do you recommend against an independent transformer from powering the stat? that's what I did in my first three installations (granted they were heating or cooling stats only).

 

There is no place to land the third R.

 

If I end up using the route where I substitute G wire for the C wire (and make the appropriate mods in both air handlers) I assume that the A/C breaker MUST be on all year round in order for the stats to function...is that correct?

 

Yes, the air handler would require power year round.

You could turn off the breaker for the outdoor unit if you are concerned about using power for the compressor crank case heater.

 

opeets did you get your system working? I'm curious because I'm having a problem with my newly installed honeywell wifi thermostats with turning on the A/C. If you get a chance check out my post and let me know if you were having the same problem.

http://www.doityourself.com/forum/bo...l?#post2307241

I still can't figure it out. NJ Trooper has provided a lot of help and has corrected a lot of my problems in the beginning with a zone valve control panel. we are still trying to figure out my thermostat problem.

Please let me know if you were seeing some of the same problems with the wifi thermostat...I believe that conversation starts at post #11.

I'm sorry, I'm definitely not trying to hijack your thread. Maybe we are seeing the same problems.

 

Okay so I finally got back around to taking another stab at this.
I opened the air handler unit (Trane 4TEC3F30B1000AA) and
observed that the wires from the stat are not connected to any
terminal blocks but are instead spliced into wires coming from
a wiring harness.

I also noticed a bunch of unused white wires as well.
Can any of these be used as the C wire?

The one thing I did forget to do was check to see if there was a 4th
wire at both the stat and air handler ends, because in order for this
to work I need that 4th wire right? Or is that not the case?

The red/white wires from the heat connect to R and W respectively
on the stat (with the R and Rc not jumpered) so this implies that
the heat runs on it's own transformer.

Below is a link to a photo gallery that I made of the two air handlers (1st Floor/2nd Floor) side by side and the wiring within one of the air handlers. I also have some pics of the heating system wiring.

Wiring Photos

 

The blue wire at the air handler is 24 volt common...

 

But that doesn't make sense to me....

Please bear with me as I try to explain my way through this. Maybe there is something I am missing....

There are two sets of wires coming into the AH....one from the stat and one (I assume) goes to the condenser unit.

The stat wires are GREEN, RED, and WHITE. The GREEN wire is spliced into the GREEN AHU wire, the RED wire is spliced into the RED AHU wire, and the WHITE wire is spliced into the WHITE coming from the condenser wire set.

The condenser wires are RED and WHITE (GREEN is unused). The RED wire is spliced into the BLUE AHU wire (which you state is COMMON), and the WHITE wire is spliced into the WHITE wire coming from the stat wire.

Is my understanding correct here? If so, why is the condenser's RED wire not connected to the RED AHU wire like the stat's RED wire is?

What would be the correct way to reconfigure this wiring so that I can get 24V to the wifi stat? My assumption was that I was going to take the GREEN AHU wire and splice it into both the RED AHU wire (effectively making a jumper) and the RED wire of the stat. Subsequent to that I was going to splice the stat's (now-free) GREEN wire to an AHU wire that is 24V common (which I'm confused about now since BLUE is connected to the condenser's RED wire). Inside the wifi stat the GREEN wire would connect to the C terminal.

 

A standard condensor is connected by two wires. The colors are unimportant. The installer will usually use whatever he has as long as it's at least two wire. At the air handler the condensor needs common and 24v from thermostat.

With the thermostat wiring color is important.

Without re-reading this novel -
You need to move your green wire from G to C at the thermostat.
You need to move the green wire from the green splice to the blue/common splice.
You need a jumper at the air handler from the thermostat white wire to the old green fan wire that was originally connected to the thermostat.

All wiring should be done with air handler powered down.

 

If the AHU red wire was connected to the condenser red wire along with the blue (common wire) it would blow a low voltage fuse (if installed) or fry the transformer. The wire from the Y terminal on the thermostat sends 24V to the condenser's contactor. The blue wire is the common wire from the low voltage side of the transformer and completes the circuit.

Exactly as PJMax stated. Here is a diagram to hopefully clarify the wiring.

 

Okay, it's starting to make a little more sense now but I still have some questions.

Thank you by the way for taking the time to make the schematic for me. I understand the connections that need to be made now but I still need for it to make complete sense to me.

With that in mind....

1) Does it matter that the R and Rc in your schematic are jumpered? Because in the thermostat that I am replacing they are not. The RED from the heat is connected to R and the WHITE from the heat is connected to the W. The jumper between R and Rc is NOT present. This is how the contractor that renovated the house wired it up.

2) In the PROPOSED reconfiguration....when the A/C is turned on via the thermostat, the wire connected to the Y terminal on the stat (i.e. the WHITE wire) sends a 24V signal (which was provided to the thermostat via the RED wire from the AHU) BOTH to the condenser AND the AHU FAN (because the two WHITE wires and the GREEN AHU wired are all spliced together now). The condenser completes its circuit with its RED wire (which is connected to the BLUE AHU common wire). Since the GREEN thermostat wire is to be connected to the BLUE AHU common wire now, it will be used only to power the WiFi thermostat and will have no effect on the AHU FAN. Does that sound right?

3) In the ORIGINAL configuration....when the A/C is turned on via the thermostat, the wire connected to the Y terminal on the thermostat (i.e. the WHITE wire) sends a 24V signal (which again was provided to the thermostat via the RED wire from the AHU) to the condenser (because the two WHITE wires are spliced inside the AHU) AND SIMULTANEOUSLY the wire connected to the G terminal on the thermostat (i.e. the GREEN wire) sends a 24V signal (which again was provided to the thermostat via the RED wire from the AHU) to the AHU FAN. The condenser completes its circuit with its RED wire (which is the only thing connected to the BLUE AHU common wire). Am I in the ballpark?

4) In the ORIGINAL configuration....when the FAN ONLY is turned on via the thermostat, ONLY the wire connected to the G terminal (i.e. the GREEN wire) sends a 24V signal (which was provided to the thermostat via the RED wire from the AHU) to the FAN. Again, does this sound correct?

5) Regarding #2....is the C terminal only used to power the thermostat so it can execute WiFi communication? Or does it play a role in feeding the Y terminal (WHITE wire) when the call for A/C is made?

6) Regarding #2, #3 and #4....how does the AHU FAN complete the circuit both in the original configuration (both with A/C on and off) and in the new configuration when the FAN can no longer be controlled manually? I assume it needs to be a complete circuit like the condenser has.

7) Why does the BLUE AHU common wire have to feed the 24V to the C terminal on the thermostat? Can't the RED be used since it is delivering 24V? I'm sure the answer has to do with the fact that it would probably great some sort of short but I'd need to understand why.

Thanks again!

 

Unless you have two separate transformers you need the jumper. There is only one power wire (red from transformer). So whatever terminal that red wire is connected to gets the 24V. The jumper is necessary to give the other terminal 24V. You weren't trying to run a/c and heat at the same time before.

2. Correct
3. Correct
4. Correct
5. The C wire is the low voltage common side from the secondary (low voltage) side of the transformer. As stated before, the common wire goes to one side of the contactor to complete the circuit to pull in the contactor. Since the green wire will be wire-nutted to that blue AHU wire it will be providing constant power to power the new thermostat.
6. There is a blower relay in the air handler that the blower wires are attached. The circuit is completed inside that relay because the blue common wire coming from the transformer is connected to that relay and completes the circuit. If you find the transformer in the air handler and follow the wires from there to the blower relay you will see how it is wired.

7) Why does the BLUE AHU common wire have to feed the 24V to the C terminal on the thermostat? Can't the RED be used since it is delivering 24V? I'm sure the answer has to do with the fact that it would probably great some sort of short but I'd need to understand why.

The red wire provides 24V to power the other terminals when there is a call for cool or heat. It does not provide power to the thermostat itself. That is why you must install batteries in the thermostat unless you run the common wire from the thermostat to power it. The addition of the common wire to the thermostat C terminal allows continuous 24V flow from the R (AHU red wire) thus completing the circuit.

 

Great, sounds like I'm almost there...

I am "assuming" that there is a separate transformer for the heat. Is there a way to test the leads with a voltmeter to confirm? How would you go about doing that? I did notice a transformer (see pictures I previously posted) on the wall adjacent to the heating system.

So the 24V from the red wire in the thermostat is actually pulled in from the transformer in the AHU only when a battery-powered thermostat calls for it? Or is the 24V already there on the line and the thermostat simply closes the appropriate circuit to send current to Y and/or G (assuming the original configuration)? Or do the two previous statements really say the same thing? Sad but I've forgotten most of what I learned in my EE classes 20 years ago.

 

What type of heat do you have?

The transformer in the air handler sends 24V to the R terminal on the thermostat via the red wire. The R terminal on the thermostat transfers 24V through contacts in the thermostat back to the air handler. For example, if cool is called for the R terminal would energize the Y terminal and the G terminal through dry contacts inside the thermostat and send 24V on the Y and G wires to the air handler (original configuration).

 

I have two zone baseboard hot water.

 

I'll have to defer that portion to another person as I do not work with baseboard heaters. Hopefully PJmax, Houston204 or one of the other regulars will stop in later to get you going.

 

I reviewed your prior posts and the photos you posted of your heating system. Based on that you would not use the jumper between RC and C on the new thermostat as your heating system has it's own transformer. The red wire from the heat system transformer would go to R and the white wire from the transformer would go to the W terminal on the new thermostat. I updated the previous diagram to reflect the heat system transformer.

 

Great, thank you so much. That schematic confirms my understanding of the new wiring configuration. I will be hopefully getting around to doing this in the next two weeks. BTW, what is the minimum outdoor temperature that would be considered safe for testing the A/C system? It's already October and the weather here in NJ is starting to cool off rapidly so I may have to plan this job sooner rather than later. Otherwise I'll have to wait until next Spring.

 

Personally, I wouldn't operate it below 60 degrees Fahrenheit. If you are only testing the thermostat to see if the wiring is correct and properly setup it shouldn't take long to figure that out. The condenser and air handler will come on right away if all is well.

 

The days are still warm enough yet here in NJ.

I follow.... you shouldn't run the A/C if the outside air temperature is 20° colder than the inside air temperature.

 

Well, I FINALLY got back around to this project and was successful in powering and configuring the both WiFi theromstats.

Everything went according to plan. There was a bit of hesitation when the A/C did not kick in for the first few minutes after powering up the first A/C unit but I guess that was due to the delay from disconnecting the power from the AHU before moving the connections around.

I also then verified very quickly that the heating system still worked on both thermostats..... so we're good to go.

The Honeywell mobile app works perfectly on all devices I have it installed on.

The only side-effects that I am aware of based on this installation are the inability to manually operate the FAN ONLY mode of the A/C system and the fact that the A/C breakers must be on year-round to keep the WiFi thermostats powered.

Thank you to all that contributed to this discussion and ESPECIALLY to firedawgsatx who went the extra mile and made me a customized schematic that made it so much easier to understand and implement.

 

To operate the fan from the thermostat you'd need to run more wires.
Most thermostats and many condensor units have time delays built in for delayed compressor turn on to protect them from power glitches.

Glad everything worked out ok.

 

MAJOR PROBLEM!!! Everything worked great for a year, heating and cooling. Today I find that even though both thermostats are permanently set to 50F, the temperature both downstairs and upstairs is 70F and 66F respectively and climbing. This can't be a seized zone valve not closing on two independent zones, can it? I don't know what to do. I am a 90 minute drive from the house, it is unoccupied at the moment, and we have a blizzard bearing down on us in less than 12 hours. Any and all advice would be appreciated. Should I use the Honeywell app to turn the SYSTEM from HEAT to OFF? I worry then that I won't be able to turn it back to HEAT before I can make it down to the house later in the week when the roads are passable. What are the odds that both thermostats are exhibiting the same behavior when they are hooked up independently? There must be something that I am missing.

 

It sounds like the problem is just downstairs. The hot air is rising upstairs and causing that heat to rise.

If it's a stuck valve....changing from off to heat won't change anything.

 

So what's the worst case scenario if I can't get to the house in the next 2-3 days?

Can the heating system malfunction from constantly being on?

Will the temperature just max out at some point in the 80s?

The house was just completely renovated 3 years ago so all plumbing is new.

The 1st Floor thermostat is currently at 74F and the 2nd Floor is at 71F so it's been steadily climbing.

The outdoor temp is expected to be at or slightly below freezing for the next two days so at least that will keep the house from getting too hot hopefully.

 

You mean the actual room temperature is 74 and 71..... not the set points.

If it stays cold you will probably be ok. If it gets warm out it will be hot inside.
Shouldn't hurt the boiler.

 

Yes the actual temps were 74F and 71F. The set points are in low 50s. We'll see what happens I guess.