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Old 06-15-2015, 02:25 PM   #1
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Thermostat install without C wire


I'm trying to install a Honeywell RTH6500WF thermostat. I have no C wire. I thought I would be OK after watching the Honeywell video on alternate installation. The wiring coming out of the wall had an extra, unused wire. I checked the terminal block in the furnace and saw that same wire there, still unused. Looking good, I thought. Then I saw the furnace had no terminal marked C. All of the terminals already had wires attached. I called Honeywell, sat on hold for 20 minutes, and got told to call the furnace manufacturer, Lennox, to see if they label the C or common wire something else. I called Lennox, sat on hold for 20 minutes, and got told that they won't discuss such matters with anyone but an HVAC repair person. The next, logical thing for me to do was to come here and ask for help.

The furnace is a Lennox Pulse made in the 1980's. I think the model might be G14 series? The terminals are labeled R, W, Y, G, T. I'll try to attach a photo below.

As usual, any suggestions will be appreciated.
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Old 06-15-2015, 02:35 PM   #2
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Thermostat install without C wire


Should have bought a new furnace instead of a thermostat

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Old 06-15-2015, 02:35 PM   #3
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Thermostat install without C wire


T is C on that furnace. T = transformer, common side. That furnace should have the heat exchanger pressure tested by a Lennox Pro. Most of those furnaces have been replaced due to cracked heat exchangers and some of the parts are obsolete.
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Old 06-15-2015, 02:37 PM   #4
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Thermostat install without C wire


Quote:
Originally Posted by Technow View Post
Should have bought a new furnace instead of a thermostat
It was a VERY good albeit noisy furnace. I replaced Mom`s because some parts are obsolete.
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Old 06-15-2015, 02:56 PM   #5
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Thermostat install without C wire


So, if I attach that unused wire to the T terminal on the furnace and attach the other end to the C terminal on the new thermostat, I should be good to go?

A new furnace sounds great! This one still works, however, and I don't have the extra money to replace a working appliance.
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Old 06-15-2015, 03:26 PM   #6
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Thermostat install without C wire


Yep it will work. Doubt that furnace will last long and when it breaks down you may not get some critical parts for it or any experienced techs. Most have retired as that furnace came out in 1980 and is VERY specialised in how it works.

People have got CO poisioning from them when the heat exchanger cracks. Lennox rexcommends they be pressure tested every 5 years. They can have complete failures where the exhaust pipe breaks off the top of the heat exchanger. Not a furnace I would want anymore and I sold hordes of them. Got rid of Mom's for those reasons.
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Old 06-15-2015, 04:23 PM   #7
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Thermostat install without C wire


I have been told that were awesome units when they worked well. Bit noisy though. Apparently though many techs didn't know how to work on them and many failed prematurely. I've only ever worked on one. The company that went in after us replaced a perfectly working unit because they didn't know how to work on it. Good luck. Hopefully nothing goes wrong with yours. You probably won't find much help if something does.

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Old 06-15-2015, 04:27 PM   #8
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Thermostat install without C wire


If they are prone to leaks , install a carbon monoxide detector near the furnace .

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Old 06-15-2015, 06:59 PM   #9
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Thermostat install without C wire


Success! I got the thermostat installed and up and running.

Good advice on the carbon monoxide detector. I have one on the second floor, but I'll get one for the basement as well.

Thanks for all of the help.

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