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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
Satisfied Customers: 797
Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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I just replaced my thermostat with a honeywell wifi one and

Customer Question

Hi I just replaced my thermostat with a honeywell wifi one and now the ac condensor will not turn on
Submitted: 1 year ago.
Category: HVAC
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Brian HVAC Guy replied 1 year ago.

Hi I'm Brian, thanks for the request! I'll have to gather a little info from you to help you solve your challenge. Can you tell me what the brand and model number of the old thermostat was and what color thermostat wires were wired to what terminal letters please? Also, the model number of the Honeywell Wifi thermostat will be helpful too.

Thanks,

Brian

Expert:  Brian HVAC Guy replied 1 year ago.

I'm sorry, but tonight, I don't have the ability to fulfill a phone call request. Your request will go out to the entire expert pool shortly so one of the other experts may fulfill it.

Customer: replied 1 year ago.
HiThe brand of the old thermostat was Hunter and I replaced it with the Honeywell Wifi rth8580wf.Color R to RH and RC (rh and rc are jumped together)
Color W to W
Color G to G
Color Y to YThe new thermostat requires a C wire which I do not have. According to their instructions I can use the G wire as the C wire as long as I insert the G wire in the C wire slot on the thermostat and also switch it at the unit in addition to jumping the Y wire with the G wire at the unit (which I also did) after doing all that I get power to the wifi thermostat but the condensor will not kick in. I also check the fuse but it's not blown.
Expert:  Brian HVAC Guy replied 1 year ago.

Ok, thanks for that info, it's very helpful!

Yes, you can wire things that way and it should work ok but bear in mind your fan switch (auto or on) on the new thermostat won't do anything anymore with that configuration. Also, if by chance you have a Trane air handler with electric heat, using the green wire for common and jumping Y to G in the air handler won't work. It should work ok for most any other brand of electric heat air handler and it will work for all furnaces with ac coils attached.

With all that said, you do indeed use the green wire and wire it to C at the thermostat and 24 volt common at the air handler (could be C or 24 Com or something similar). You have to make sure whatever (or wherever) the green wire from the thermostat was wired to in the indoor unit, that it isn't connected to that anymore. The yellow wire from Y at the thermostat will connect both to wherever it was connected originally and wherever the green wire was connected as well (either the G terminal or a wire depending on indoor unit make and model. This may require a small jumper wire.

I suspect, something is amiss in the wiring at the indoor unit. What is the brand and model of the indoor unit?

Thanks,

-Brian

Customer: replied 1 year ago.
the indoor unit is a trane xe80
Expert:  Brian HVAC Guy replied 1 year ago.

Ok so it's a Trane gas furnace. It should have a furnace control board with a low voltage wiring strip. You will wire thermostat Y to BOTH furnace control board Y and G and make sure the connections to the outdoor unit that were on the furnace control board Y and B/C* remain where they were connected at Y and B/C. Then use the wire that was used for G on the old thermostat to wire between C on the thermostat and B/C on the furnace control board.

*some of the Trane furnace control boards 24 volt common may only be designated C while others could be designated B/C.

This should work for your system.

The default settings in the thermostat installer setup should be fine where they are out of the box.

Let me know how that goes and we can trouble shoot from there if things still aren't working.

Thanks,

-Brian

Customer: replied 1 year ago.
Sorry I didin't quite understand. Do you want me to switch any wires at the outdoor condensor unit or leave everything as is? Because I haven't touched the wires at the outdoor condensor.With regards ***** ***** furnace control board, per the honeywells instructions, I did connect the Y wire to both Y and G via a jumper wire.
Expert:  Brian HVAC Guy replied 1 year ago.

No, no, I wasn't talking about the wires inside the actual outdoor unit, just the two low voltage wires that come from the outdoor unit and further, I just wanted to make sure those two wires stayed where they were at in the furnace! It sounds like you did the connections correctly. Just to confirm:

On the furnace control board, on terminal Y, you should have one wire that goes to the outdoor unit (was always there), one wire that goes to thermostat Y and one little jumper wire that goes from Y to G on the furnace board.

Then also on the furnace control board, on the C terminal, you should have two wires, one that was formerly on G but now is connected to C at the thermostat and one wire that goes out to the outdoor unit that should have been connected there all along.

Also at the furnace control board you should have a connection from W at the thermostat to W on the board.

At the thermostat, you have both R terminals jumped together then to the wire that goes to R on the furnace board, then also a connection at Y, W and C that go to the same respective terminals in the furnace.

That is the full low voltage wiring for your system. If it's wired as above, your new thermostat should work with the exception noted previously that the fan switch won't do anything.

Customer: replied 1 year ago.
There is only one Y wire on the furnace control board ( in addition to the jump wire going from Y to G) and I'm assuming it's going to the thermostat since it's in the same bundle of wires that is routed to the thermostat. i don't see a thiord wire on the Y terminal.With regards ***** ***** wiring at the thermostat W is going to W and R to R/RC. The G wire is connected to the C terminal at the furnace and at the thermostat.
Expert:  Brian HVAC Guy replied 1 year ago.

Ok, well that leaves one of two possibilities:

One, the wire that should be going to the outdoor unit came out of the connection at Y on the furnace control board and hid from you so you didn't see it when you were making the changes.

Or two, there was ONLY one wire connected at Y on the furnace control board originally and the wire from the outdoor unit that normally gets connected at Y on the furnace control board is/was connected to the wire that goes to Y on the the thermostat somewhere else (which does occasionally happen in some installations but it's rare and for extenuating reasons).

This is probably the reason your outdoor unit doesn't run. If I were a betting man, I would bet the wire to the outdoor unit that was on Y at the furnace board snuck out while you weren't looking during the wiring change! That would account for the outdoor unit not running.

I would probably need some pictures of the low voltage wiring at the furnace to tell you conclusively what's going on at this point. You can upload them using the 'Attach Files' button near the send button in the reply dialog box.

Customer: replied 1 year ago.
Hi see the attached pic. As you can see there is only one Y wire. I don't see a dangling or missing wire anywhere near the furnace board
Expert:  Brian HVAC Guy replied 1 year ago.

Thanks for that pic! Pics are always REALLY helpful!

Assuming the cable with the wires connected there is the cable going to the thermostat and there were never any other wires connected there, you need to follow that thermostat cable and it will connect to another cable (probably two, one that goes to the thermostat and one that goes to the outdoor unit). If you can take a picture of that connection, I can do a lot of good here.

Also, I see an unused blue wire in that cable. That is normally used for the C connection at both the thermostat and the furnace board when the thermostat requires a C connection. If the cable at the thermostat has that spare blue wire tucked back in the wall, we can wire your system so you have FULL functionality of the fan with the thermostat fan switch! A picture of the connection where the outdoor unit low voltage wires connect to the other thermostat wire(s) will probably tell me all I need to know to help you get this all wired up properly. I would think it should be very close to your furnace. Perhaps in a taped up bundle near the furnace on the outside of the cabinet?

Customer: replied 1 year ago.
Originally I wired the C from control board to the the thermostat but it did not power on. I suspect that the C wire is either shorted out or is cut somehwere between the control board and thermostat. Hence why I had to go the not having a C wire routeAs far as following the thermostat wire I'm assuming that I will have to go to the attic since the thermostat wire bundle leaves the control board and goes directly into a wall.
Expert:  Brian HVAC Guy replied 1 year ago.

Hmm... That's odd. I suspect all the mysteries will be solved once we find the unseen connections. I suppose it's possible that you have one thermostat cable going from the thermostat to the outdoor unit and then that cable in the furnace also goes out to your outdoor unit. If not, then the connection will be hidden somewhere else, perhaps as you think, in the attic. If you disturbed nothing else, the way you have it wired should have worked but I can't know that for sure without seeing how the thermostat control wires go to the outdoor unit. We're gonna have to find the missing connections to figure out exactly what's going on.

Usually the thermostat cables from both the outdoor unit and the thermostat meet in the indoor unit (in your case a furnace). As I said earlier, it's rare that they're run some other way but it does happen.

Customer: replied 1 year ago.
Ok I will have to go to the attic tomorrow as it's too late and dark for me to see anything. How does this work, do we pick up the conversation tomorrow?
Expert:  Brian HVAC Guy replied 1 year ago.

That's fine with me. I usually don't pick up my computer until 11 or 12 Eastern time so bear that in mind. Yes, you can either exit Just Answer then log back in and go to this same question and post a response or you can leave the program open if you're using a computer and again post a response later. I'll be notified when you post another response, both here on the Just Answer site and in email. I'll respond as soon as I'm able. Goodnight and I'll talk to you tomorrow when we'll figure this thing out for sure!

Thanks much,

Brian

Customer: replied 1 year ago.
Hi,I was think about this last night and if there was a missing wire or some other issue then the old thermostat would have had the same issue. Considering this point, what would looking at the unseen connections accomplish?
Expert:  Brian HVAC Guy replied 1 year ago.

I had the same thought already and had said as much already but nonetheless, there appears to be a problem with the Y wire at the very least. Either that or the thermostat is bad out of the box or a problem has developed with the outdoor unit. Viewing the unknown connections could allow you to wire the thermostat as it was meant to be with five wires instead of four. That's the main reason I wanted you to look at it. If you are very sure there is ONLY four wires at the thermostat location, that point is moot though.

I think what you should do is check whether the thermostat is the issue by jumping the yellow wire to the red wire at the thermostat and see if the outdoor unit runs then. If it does then the thermostat is bad out of the box. If it still doesn't run, you either have a problem with the yellow wire somewhere or there is an unrelated problem with the outdoor unit.

So do that jump test and let me know what you find. Do you by chance have a volt meter capable of measuring 24 volts ac? We may need that if the the jump test doesn't bring on the outdoor unit to actually find what the problem is. Bear in mind the thermostat has a five minute delay after it brings on the fan and before it brings on the cooling when you first apply power and set it to cool. I forgot to mention that earlier. Hopefully that wasn't the issue all along!

Thanks,

Brian

Customer: replied 1 year ago.
Ok I'll be home in about 3 hours and can jump the Y to R at the thermostat then. I do have a volt meter.The wifi thermostat has a built in safety where it won't turn on for 5 minutes to protect the condenser. It turns on after 5 minutes but it's just the fan. Are you saying that after the fan turns on I have to wait another 5 minutes for the condenser to turn on?
Expert:  Brian HVAC Guy replied 1 year ago.

That's correct, but just after initial power up of the thermostat and if the cooling was called for within the first five minutes of power up. After that, there is no wait on a call for cooling unless it's been less than 5 minutes since the cooling call was satisfied by the thermostat. In other words the cooling won't be allowed to run for 5 minutes after a power is applied to the furnace AND the cooling has a MINIMUM off time of 5 minutes set by the thermostat. During those off times invoked by the thermostat, the furnace fan will still be allowed to run on a cooling call. This is a compressor only protection. Fan motors do not need a minimum off time but since the outdoor fan and compressor run together, the outdoor fan is off when the thermostat is in a delay cycle as well. Hence the furnace fan running without the outdoor unit running. Again, I hope that wasn't the original issue as I assumed you would have read that in the thermostat manual so I didn't mention it earlier and assumed there is a physical problem of some sort.

Let me know what you find and we'll go from there.

Thanks,

Brian

Customer: replied 1 year ago.
Ok I'll let you know thanks.
Expert:  Brian HVAC Guy replied 1 year ago.

Very good, I'll be around. I do break for dinner but beyond that, I'm on til late tonight.