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Thread: Nest E Thermostat not keeping heat relays commanded on

  1. #1
    Junior Member

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    Nest E Thermostat not keeping heat relays commanded on

    Hi there. I'm trying to use the Nest E Thermostat to add smart temperature control to my pellet stove, and to replace the faulty thermostat I currently have. I'm having two issues:
    Problem 1) The nest E thermostat does command my 24VAC 15mA relay on when I use the test heating feature, but it doesn't keep delivering current to keep the relay activated, it only sends a pulse. I hear the relay activate and my continuity meter beeps, but then the relay deactivates. The same issue exists with the OB for stage 2 heating. I haven't tried the G (fan) relay control yet because I'm not sure how the Nest would use it.

    Does anyone know if I may be exceeding the electric current capability of the Nest because I'm using two 15mA relays?

    It's my impression from online documentation that the Nest shorts W1 to R when it wants to command heat stage 1, and that it shorts OB to R when it wants to command stage 2. Is that correct?

    Is the short only temporary and the devices it is designed to control have impulse type relays? If so I could get impulse relays.

    Any information, ideas, or advice is appreciated.

    Problem 2) The nest doesn't seem to be charging itself off of the R to C 24VAC 40VA transformer I'm using, so it goes dead after a while. I think I can use the micro USB to provide power during operation if this is an insurmountable problem.

    Does anyone know if it can get power from USB while simultaneously operating?

    Or better yet, do you have a recommendation for a wiring change that would let the nest draw power from my 24VAC 40VA transformer and remove the need for USB power?

    Here is a schematic showing what I'm trying to do. I only have the Nest E so only two stage, so I'm ignoring the possibility of Low fan speed and hoping I can configure the nest to command the stove on with its default medium fan speed, and use the second stage to command the high speed fan option on my stove.

    Click image for larger version. 

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  2. #2
    Senior Member

    User Info Menu

    If the Nest is not keeping power from the transformer feed on R and C then you have a bad Nest. Verify power with only R and C connected. Check voltage and current settings in the menu - Settings - Technical Info - Power. Voltages are peak so divide by 1.414 for RMS. (24vac RMS would be Nest's 33.9vac Peak)

    I believe the Nest's output is rated for something like 1 amp so the relays' load should not be an issue.

    When you say OB I assume you mean the *OB connector and have it configured as W2.

  3. #3
    Junior Member

    User Info Menu

    Interesting. Thanks. Maybe my relay is drawing more than it's rated current and I'm supposed to control the current with a resistor. I assumed because it was an AC relay it was an inductive element and the impedance would control the current for me. I connected just R and C like you said (relay not connected anymore) and the battery voltage is climbing, and it seems to be staying charged now. The Voc and Vin stay in step and oddly bounce around between 30V and 37V (I hope there's nothing wrong with my transformer). I have two of these Nests and this one reports 200mA for Iin, the other one was reporting 20mA. I'm also noticing the battery voltage seems to fluctuate with the Voc and Vin, occasionally dropping, but so far seeming proportional to the Voc/Vin, so maybe it's a charging voltage effect. I'll try to throttle my relay current with a resistor and let you know how it goes. Thanks again.

  4. #4
    Junior Member

    User Info Menu

    Sure enough. 24/0.015=1600 ohms, so I through a 1k in there and it holds the relay on now. I'll look for something a little closer so I'm not pushing this relay too hard. Thanks for the intel

  5. #5
    Junior Member

    User Info Menu

    In hindsight it could have been the nest or the relay protecting itself that was causing the relay to open back up.

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