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Thread: Nest E74 error code.

  1. #1
    Junior Member

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    Nest E74 error code.

    Been trying to get tech help for this but a 2-hour on-hold on Saturday morning convinced me that 24/7 tech support is a false claim by Nest.
    Any insight on this error?
    This error comes up when trying heat. It is intermittent - heater sometimes works.
    The first call to tech support had me disconnect the blue wire to C.
    The heater worked (as it normally does if furnace breaker toggled).
    It then failed with an E74 again.
    My Nest is Gen-3.
    Last edited by ovi; 11-11-2017 at 11:15 AM.

  2. #2
    Administrator

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    Can you reply with a list of all the wires that are connected to your Nest and what type of heat you have (gas heat, electric strip heat, heat pump)?

  3. #3
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    Hi new to the forum. I installed a Nest 3rd gen thermostat less than 30 days ago. I woke up this morning to the E74 error and a cold house. I live in northern Illinois where it has gotten fairly cold here in the last couple days. I have an electronics and experimental background so I dug in to figure out what might be wrong. I have just a simple gas forced air heating and cooling system using just 4 wires to the thermostat- Rh, W1, Y1 and G for heating, cooling and fan control. The display indicated it wasn't getting power through Rh. So I got my Fluke meter out and did some investigating. I measured approx 15VAC open circuit across Rh - W1 and 24VAC across Rh - G and 0VAC across Rh - Y1. I manually jumpered Rh and W1 together and the heat came on as expected. So I'm wondering if there might be something wrong with the thermostat. I noticed on the internet there has been some discussion about this error showing up during winter months which had me wondering why it would make any difference at all what the outside temperature would have to do with the operation of the Nest thermostat. Here's a little background on how the Nest thermostat gets it's power from a 4 wire thermostat setup that does not have a common wire for the 24VAC circuit. It uses what is known as phantom or current power to charge the LiPo battery inside and operate the electronics. It depends on the current draw through the heat and cool relay to power itself. It draws enough current to operate but not enough to trigger the heating or cooling relays in your HVAC. So that being said I figured out that my outdoor A/C condensing unit must have a thermostatic lock-out in it's control circuit to prevent freeze-ups. And since it is below 0 here that outdoor temperature triggered that lock-out so the cooling relay inside the outdoor condensing unit is no longer in the circuit path for cooling hence no phantom current available to the Nest through the Rh - Y1 wires so the Nest throws an error indicating not enough power.




    My solution outside of running a new thermostat wire with 5 wires to add the "C" common wire was to add a resistor across the 2 wires that run to the outdoor condensing unit to add enough current back into the circuit so the Nest would sense it. I added a 60 ohm 10 watt resistor but any resistance between 60 and 120 ohms should work. You don't want to go any lower than 60 ohms or you may risk damage to the Nest (too much current). Not proven but just surmising.

    Nest should update their software to indicate such a problem. The logic would indicate that hmmm... I was working ok before using the 4 wires and getting my power through the various circuits but now I'm missing power through x circuit and it could tell you which one.

    I may consider running a new thermostat cable so that it gets its full 24VAC without having to rely on the phantom power through the other circuits. So anyway I hope this solves someone else's problem and saves them a bunch of time trying to figure out why it doesn't work today but worked fine yesterday and all that changed was the temperature outside has dipped to below 0.

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