Low Voc when on, furnace rapidly cycling on and off

[RG5]

We've had a Learning Thermostat that was working fine for several years.  But lately sometimes when it tries to turn the furnace on, it goes on and off rapidly and repeatedly, and this can last for minutes.  We are using it to control a gas forced air furnace only, using just 2 wires.  We have had no low battery or other errors reported.  The value of Iin is always 20.  Both Voc and Vin are ~30 when the furnace is off, but both drop to ~7 when it's trying to turn on.  I see that only Vin should drop to that level, so what might be causing this problem and how do we fix it?

This seems similar to what was posted at https://www.googlenestcommunity.com/t5/Nest-Thermostats/Vin-and-Voc-dropping-with-heat/m-p/27965/thr... by  @Zimmyntrn  and   @Jake though no answer was given there.

[EdwardT]

Hi RG5,

Thanks for reaching out. I'm sorry to hear that you're having an issue with your Nest Thermostat. Your power reading looks normal. I'd love to know more about this and I have a few questions:

• How is your Nest Thermostat wired?
• What type of Nest Thermostat do you have?

Looking forward to y our response.

Thanks,

Edward

[RG5]

It's a Nest Learning thermostat with just 2 wires, R & W.  I am about to install a Fast-Stat that will provide a C wire, to see if that will fix the problem.

[EdwardT]

Hi RG5,

Thanks for the response. Based on your description, it looks like it's an issue of how your system reacts with how the Nest Thermostat gets its power. A C wire usually solves this issue so let us know the result once it's installed.

Thanks,

Edward

[EdwardT]

Hi RG5,

It's me again. I wanted to check back in to see if you have other questions and concerns. Feel free to let me know if you do.

Thanks,

Edward

[Markjosephp]

Hello RG5,

It has been a few days since your last reply, and we're going to lock the thread within 24 hours. If you have any questions, please feel free to create a new post.

Regards,

Mark

[RG5]

We haven't seen the issue again since installing the C wire, but we also hadn't seen it in the week or so previous to that installation.  So at this point I'm not sure if it was weather-related, or intermittent for some other reason, and only time will tell if the C wire is a true fix.

[Markjosephp]

Hi there,

I appreciate you posting back and updating us on your efforts to address your concern and find a fix. Installing a C-wire should help, as it gives additional power to your thermostat. To be sure, could you please check your thermostat's power readouts on its "Technical Info" settings?

I'll look forward to your response.

Thanks,

Mark

[RG5]

With heat off it shows: Bat 3.9 V, Voc 38 V, Vin 38 V, Iin 200 mA

With heat on, Voc and Vin go down to 32 V, the others stay the same.

[Markjosephp]

Hello RG5,

Perfect! Thanks for getting the power status. Your thermostat already has the power it needs from the common wire, so you should no longer expect any power loss. Also, it's normal to see the Voc or the Vin read low when the system is on, as it's energizing the wire from your thermostat's base. For more technical information, please check this out and let me know if you have more questions.

Best regards,

Mark

[Markjosephp]

Hey there,

I wanted to see if you still needed help. Please let us know if you have any other questions or concerns.

Thanks,

Mark

[Markjosephp]

Hi RG5,

I'm checking in to make sure that you've seen my response. Please let us know if you have any additional questions or concerns before I lock this thread in 24 hours. 

Thanks,

Mark