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Samsss
@rogers.com

Samsss

Anon

Connecting C-Wire to Honeywell wireless thermostat

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Old Thermostat wiring
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Furnace terminal block
Hi all,

I purchased a Honeywell wireless thermostat (RTH8580WF) that requires a c-wire connection. My old thermostat works with battery and the wiring is shown below (W Y G RH RC from bottom to top).

So not having a C wire, I came across what Honeywell recommends. They said move wire connected from G to C in the thermostat and in the terminal block, as well as inserting a jumper wire between Y and G in terminal block.
»wifithermostat.com/modal ··· V30ttxgc

I have a gas furnace in the basement. I attached a photo of the terminal block that has all kinds of wiring in it. The A/C unit is outside. I called Honeywell support to double check and they said I can't use this "connect G to C" method since I have RH and RC in my old thermostat. So I'm asking the expert support from here to see if I can safely do this.

Also, note that my old thermostat has RH and RC spots which are connected, but has no spot for 'R'. I'm guessing they're just using 'RH' as the 'R' name. And that even though both RH and RC are connected, there is a jumper wire connected between those two. Do I need to remove that (as per the instructional video) in the thermostat?

Seems straight forward to me that I can do this, but the Honeywell support guy was insisting that I can't and that I need to run a C wire from my cooling panel to my thermostat, hence me double checking.

Thank you for your help.

treefrogg43130
Bababooey
Premium Member
join:2000-09-30
Lancaster, OH

treefrogg43130

Premium Member

I have same one. If the wire through your wall already has some wires that are not being used just run a c wire using an un used one.

leibold
MVM
join:2002-07-09
Sunnyvale, CA
Netgear CG3000DCR
ZyXEL P-663HN-51

1 edit

leibold to Samsss

MVM

to Samsss
Your old thermostat has RH and RC terminals bridged and uses two wires (red and black) to the same R terminal at the furnace.

There is absolutely no reason why you can't separate the red and black wires (continue to use red for R and use black for C).

You will need to keep R/RH and RC bridged in the new thermostat.

Important: be sure to make the wiring change with the power to the furnace turned off so that you don't short circuit the transformer in the process.

Edit: make sure that the thermostat cable is continuous from thermostat to furnace (the main concern would be if it first went to the AC but that would be unusual). You want to be sure that there isn't anything else connected to the black wire before changing its purpose.

Samsss
@rogers.com

Samsss

Anon

Thank you for your help. I'll check and see if the cover for those 5 wires behind the thermostat is the same colour as the one going in the furnace when I get home tonight. Is there another way to check and see if the wire is connected somewhere else before connecting to the furnace?

If I just test this wiring scheme to see if it works or not, would it damage anything if the wires are connected to something else before its connected to furnace?

leibold
MVM
join:2002-07-09
Sunnyvale, CA
Netgear CG3000DCR
ZyXEL P-663HN-51

leibold

MVM

said by Samsss :

Is there another way to check and see if the wire is connected somewhere else before connecting to the furnace?

It would be very unusual if the cable between thermostat and furnace connected to anything unrelated to HVAC components (furnace, air handler, AC). Based on the number of cables visible in your picture of the furnace terminal block it appears that this is the location where all the connections are made (and that is very typical).
For additional piece of mind, you can use a multimeter and make the following measurements after disconnecting the black wire on both the furnace and thermostat end:
1.) measure AC voltage from the black wire to R, C and ground. Don't worry if there is a low reading as a small amount of voltage may be induced into the open wire from nearby electrical circuits. The primary purpose is to determine that it is safe to measure resistance which should not be done if there is a voltage present.
2.) measure the resistance between the black wire and R, C and ground. The Ohm-meter should show infinite resistance indicating that there is nothing connected to the wire.
said by Samsss :

If I just test this wiring scheme to see if it works or not, would it damage anything if the wires are connected to something else before its connected to furnace?

That depends on what exactly would be connected but in general the possibility of damage does exist. One hypothetical possibility might be that the cable is not contiguous and is spliced somewhere in the middle (with red and black connected together in the splice as well). That would create a short circuit between R and C which may blow a fuse or damage the 24V transformer in the furnace.

Samsss
@rogers.com

Samsss

Anon

So i found out that it's not connected to anything. I used the extra wire to be the C-wire and everything works quite well now. Thank you very much for your help. The thermostat seems to be pretty good