I have an old Lennox (gas heat only) thermostat. There are 3 wires coming from the wall. Black, yellow and a light blue color (could be a faded green or an old dirty white). The thermostat wouldn't click on, so I opened it and saw that the black and blue color. I think they are supposed to be connected to the same screw. Any ideas?

Lennox T99
catalog# A100A109 (?)
Serial# J9C


The colors are mostly irrelevant - while there is a "standard", really what is important is that when changing the thermostat, you write down what terminals the wires are connected to and that you connect them to the same terminals on a new thermostat.

The "click" of your thermostat is a relay - it is possible that your thermostat is no longer functioning properly.

It is unlikely that two wires would be connected to the same terminal at the thermostat. I would suggest purchasing a new thermostat and ruling out whether the thermostat itself is the issue. If a new thermostat also does not function, it is likely that there is an issue with the controller board in your furnace, or an issue with the wires running between the two.

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  • I have a new problem. The thermostat works. It was matter of reconnecting one of the wires. The other wire doesnt connect to anything. Anyway, someone came over and tested the line. they had to "jump" it to get it to work. The faulty part was narrowed down to a Thermodisc. Now I am in search of this part. Its from a 1960's Lennox gas heater. On the copper disc backing it reads CHACE 2400 LO. On the metal housing 10-59 is etched. Its got two screws on the front side that wires connect to. Everything I googled so far looks nothing like this... :( – suzan Mar 24 '13 at 11:06
  • Contact Lennox? – Steven Mar 24 '13 at 13:23
  • I did this morning after running around to several plumbing supply shops. They haven't a clue what Im talking about. I might have to take pictures to put online so maybe someone knows what it is. I can't even name the exact part. I fear that I will have to replace my heater for a $10 part! Thanks for all your advice, much appreciated. :) – suzan Mar 25 '13 at 15:06

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