Mike Dolt Enterprises
Mike Dolt Enterprises
User: mark32, Location: Currently in NJ
Asked on: Friday, August 1, 2025 at 8:07 pm EDT
Answered on: Friday, August 1, 2025 at 8:27 pm EDT
Question: 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
Okay, let's break down this HVAC mystery and figure out the best path forward. Your experience highlights the complexities of HVAC wiring, especially when repurposing wires.
Understanding the Problem Cooling Only with Limited Wires
You're dealing with a cooling-only system, which simplifies things, but the lack of a dedicated C wire is the main hurdle.
Repurposing the G Wire
The theory of using the G wire as a C wire is valid in some situations, but it's not universally compatible, as you've discovered. Some systems rely on a separate path for the fan.
Existing Wiring Issues
The fact that the red wire from the thermostat was connected to the C terminal at the air handler is unusual and suggests the original wiring might not have followed standard conventions. This is vital to understand.
SpacePak System
SpacePak systems, while efficient, can have unique wiring configurations compared to traditional forced-air systems.
Analyzing Your Actions1. Initial Wiring
R to R (Thermostat and Air Handler) - Correct. G to C (Thermostat and Air Handler) - This is where things get tricky. Y to Y/? (Thermostat to Condensing Unit) - Should be correct. The red from the thermostat on the C terminal is suspicious. This is likely the source of your issue.2.
Moving the Green Wire
Moving the green wire to W was an incorrect step. W is for heating.3.
Jumping Y to G
Jumping Y to G is a troubleshooting technique, but in your case, it didn't work because the underlying wiring configuration was incorrect. This often works for older thermostats that don't require a C wire, but it isn't a fix all.
Next Steps: Diagnosis and Solution1. Document the ORIGINAL Wiring
Before you do anything else, carefully document how everything was wired before you touched it. Take clear photos of the thermostat wiring and the air handler wiring. This is crucial to get back to a known state if needed.2.
Identify the Actual Common (C)
The most critical step is to find the true common terminal. Using your multimeter, with power ON (be extremely careful), measure the AC voltage between: Red wire (R) and the terminal labeled "C" (or whatever you suspect is the common). You should read approximately 24VAC. Red wire (R) and the chassis of the air handler. You might get a reading here, which could indicate a grounded common (less common in modern systems).
If you do NOT find 24VAC between R and what you think is the C terminal, this is your problem! The wiring is non-standard, and you need to trace the wires back to the transformer. The common side of the transformer must be your C terminal.3. Examine the wiring diagram more closely Review the wiring diagram that is on the inside of the air handler cover. Determine what the Y/? actually is. Is it a Y/G, or Y/something else?4. Correct the Wiring (Most Likely Scenario)
Disconnect the red wire from the C terminal. Connect the C wire from your thermostat to the actual common terminal you identified (probably not the one originally labeled C). Connect the green wire that you moved back to the Y/? terminal. Double-check that R is connected to R and Y is connected to Y.5. Test the System
Turn on the power. Set the thermostat to cool, and lower the temperature. See if the condensing unit starts and the air handler blows cold air.6.
Alternative (If the standard method doesn't work) If the above doesn't work, then test out the G as C workaround as you tried before. Again, connect C from the thermostat to the Y/? terminal. Reconnect the green wire to G on the air handler. Test the system.
Regarding Running a New Cable Last Resort
Running a new 5- or 8-conductor cable is always the best solution if you can do it relatively easily. This eliminates any ambiguity and provides maximum flexibility for future upgrades or changes.
Ease of Installation
Assess how difficult it would be to run a new cable. If it involves extensive drywall cutting and patching, it might be worth trying harder to make the existing wiring work.
Important Safety Notes Power Off
Always turn off the power to the air handler before working on the wiring.
Voltage Testing
Be extremely careful when using a multimeter with the power on. If you're not comfortable with this, call an electrician.
Professional Help
If you're still struggling after trying these steps, it's best to call a qualified HVAC technician. Incorrect wiring can damage the equipment.
In Summary:
The unusual wiring configuration, specifically the red wire connected to the C terminal, is the most likely culprit. Finding the true* common and correcting the wiring accordingly will probably solve the problem. If not, then you can try the G as C workaround again. If you still have trouble, then running a new wire will be your best bet.Good luck, and be careful! Flag for review