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Thread: Lennox G23 Furnace - C wire intermittent voltage drop kills thermostat

  1. #1
    Join Date
    Apr 2015
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    2

    Default Lennox G23 Furnace - C wire intermittent voltage drop kills thermostat

    I have a Lennox g23q2/3-75-4 furnace and I recently installed a new thermostat wire, one that includes a C wire. I found the previously un-used wire post inside the furnace servicing panel and found 26-27vac on the C and R wires.

    A half-dozen times a day the voltage will drop to 2vac and my smart thermostat will die during this time (not nearly enough power to keep it up).

    Is this a symptom of a bigger problem? I am considering just getting an external transformer for the smart thermostat and bypassing the furnace's built-in power supply, but I don't want to do this if there is a bigger overall problem going on.

  2. #2
    Join Date
    Feb 2012
    Location
    Kent, WA
    Posts
    216

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    It seems like a symptom of a larger problem. The furnace contactors and thermostat need to be powered from the same source, so a separate power supply should not be required and may not work. First question is why did you need the C (assuming Common) wire?

    Is the thermostat the same one you've had and all that was changed was the wire?
    How many wires were run in the thermostat cable, what gauge, and how long is the new cable?
    Did you measure 26 VAC between the red and C wire, or from R to ground and C to ground?

    I'm thinking there is something wrong with the thermostat, or you've nicked the wires and they intermittently short out, or possibly the transformer isn't large enough to run what needs power in the thermostat. But a thermostat should draw almost no power at all.

    When this voltage drop occurs, can you measure the voltage at the furnace transformer to see if it has sunk to 2V or is it still 26 VAC? Can you lay a new thermostat cable along the floor for a few days to see if a new wire fixes the problem?
    Mark
    Kent, WA

  3. #3
    Join Date
    Apr 2015
    Posts
    2

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    Quote Originally Posted by suemarkp View Post
    It seems like a symptom of a larger problem. The furnace contactors and thermostat need to be powered from the same source, so a separate power supply should not be required and may not work. First question is why did you need the C (assuming Common) wire?

    Is the thermostat the same one you've had and all that was changed was the wire?
    How many wires were run in the thermostat cable, what gauge, and how long is the new cable?
    Did you measure 26 VAC between the red and C wire, or from R to ground and C to ground?

    I'm thinking there is something wrong with the thermostat, or you've nicked the wires and they intermittently short out, or possibly the transformer isn't large enough to run what needs power in the thermostat. But a thermostat should draw almost no power at all.

    When this voltage drop occurs, can you measure the voltage at the furnace transformer to see if it has sunk to 2V or is it still 26 VAC? Can you lay a new thermostat cable along the floor for a few days to see if a new wire fixes the problem?
    I installed a new smart thermostat about 9 months ago. Previously there was a manual thermostat on it and only 3 wires were coming from the furnace. The new thermostat manual said i could wire an external power supply to the thermostat if power from the furnace was not available, but when i saw the furnace had a line reading the required 26-27vac I decided to re-run a new cable that had 5 wires. I bought cable that is specifically marketed as thermostat cable and it's about 25ft long. It's a brown cable with five 18 gauge wires. This is what is printed on the cable: E111272 (02-31688) 18 AWG CL2R (UL) or CMR C(UL) US sun res (06/14) RoHS.

    26vac is being read at the thermostat end of the cable on the C and R leads. Honeywell said my model thermostat (RTH9590WF01) requires between 24-30vac on these wires. They asked me to monitor the voltage when my reboots were happening and so I looked a voltmeter up to the thermostat and watched the voltage drop from 26vac to 2vac at the point in time when the thermostat shut off.

    I have a few feet of cable left over that I can use to hook the thermostat up to the furnace. I'll set that up and see if that changes anything.

    Lennox G23 Furnace - C wire intermittent voltage drop kills thermostat-thermostat-wiring-panel-jpgLennox G23 Furnace - C wire intermittent voltage drop kills thermostat-furnace-jpg

  4. #4
    Join Date
    Feb 2012
    Location
    Kent, WA
    Posts
    216

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    OK, what you did sounds fine (good wire size, short run, enough wires). What is the white cable with wires going to the Y and C terminals??? That could be an issue if you have air conditioning or the thermostat decides to go into AC mode.

    Moving the thermostat close to the furnace with its own wire sounds like a good test plan. But I'm still curious about that white cable with the red and white wires, as it looks like that will be turning something on when in cooling mode (and if so, then what's the purpose of the yellow wire)? If that is getting energized during the power drop, it could what what is sucking all the current.
    Mark
    Kent, WA

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