My experience with the Nest thermostat

I purchased a Nest 3rd gen thermostat on Cyber Monday through our electrical supplier (Com-Ed) for $100. Had always wanted one but thought the $250 price tag was too steep. When I saw the $100 price I decided it was time. Love the thermostat and the ability to observe & control it remotely. So I've had it installed for a little over 3 weeks now.

Fast forward to this morning... I woke up to a 60 degree (F) house! The Nest was showing an error of E74 which indicates it can't detect the Rh wire (?) or more simply it can't detect power. My furnace system is just a basic gas forced air with A/C. Standard 4 wire thermostat connection- Rh (24VAC), W1 (Heat), Y1 (Cool), and G (manual fan). There is no common wire for the 24VAC so the Nest derives its "phantom" power through the current of the cooling, fan and heating relays.

I got my meter out and did some checking. And of course I simply jumpered the Rh & W1 wires together and the heat came on as expected. The Nest goes into a diagnostic mode when its not working correctly and it was showing an open circuit voltage of about .50 VAC. So immediately I figured there must be something wrong with the thermostat or its base as I measured approx 15VAC between Rh & W1 (heat) and 24VAC between Rh & G (fan) but 0VAC between Rh & Y1 (cool). I put my meter in current mode and the heat takes about 350Ma AC, and the fan the same. So I scratched my head to wonder what could be the problem. I tried switching wire around to see if I could "fool" the thermostat or maybe identify an input that it wasn't sensing for current all to no avail.

So then I got to wondering why the A/C connections (Rh & Y1) weren't showing any voltage/current. I did some resistance measuring and saw that there wasn't any continuity to the outside condenser unit. We recently fell to below 0 degree's overnight in fact I think is was something like -10. So then I think about my experience with refrigeration and figured that the condensing unit outdoors has reached a temperature that if the suction line were to freeze over it would shut down the compressor as a matter of safety. So I thought I'd try a little experiment- I added a 10 watt 60 ohm resistor in place of the control line going to the condenser unit outside to produce a cooling circuit current of about 400 ma AC. The Nest then came back to life.

I wonder how many calls and unsatisfied customers they get because of this issue. The thing of it is the Nest knows which wires are connected and where it gets its power from. You'd think it would have known that yesterday I was getting power from Rh-W1, Rh-Y1, Rh-G but now today I'm only getting it from Rh-W1 & Rh-G and tell me so. The other option is to install a new thermostat cable that also includes the "C" for 24VAC common. Anyway the heat is back on and working....


  • 6 Comments sorted by Date Added Votes
  • I listen to the IoT Podcast with Stacey Higginbotham and Kevin Tofel and the Nest Thermostat issues do pop up now and again.
    They set up a IoT Podcast Listener Hotline where folks can call in with issues or questions and Stacey and Kevin will try to answer it.

    Stacey on IoT is another good resource;

    Scanning the web, there seems to be a ton of Installation issues with the Nest but I would assume you have seen most of this already.
    Love the thermostat and the ability to observe & control it remotely
    Most recently they had a security expert on the show and having a connected thermostat in their house was one thing they would not have.
    Nest/Google keeps track of what the Nest is doing and learns your A/C habits. That data is passed back to Nest to "Help better the product". However, the way I see it, if the device is connected and accessible remotely by you, then it is accessible by anyone or any thing that is inclined to connect to it.

  • I use an alternative to the Nest, and had a similar setup. When my last "sorta" smart thermostat died after about 10 years, I used a spare wire in the bundle to connect to up the C wire. Never had an issue with the system. Figured a dedicated 24v was a good way to go.
    Ordnung ist das halbe Leben
    I gave up on that half long ago.........
  • I'm thinking or putting just a very basic mechanical thermostat in the basement in parallel as a back up to the Nest. Even with all its neat features it's still vulnerable to catastrophic failure.
  • The mechanical thermostat is just a mechanical switch and it doesn't need a common. Pulling a new cable might be a good idea if possible.
  • I recently replaced my thermostat with a Honeywell T5 7-day programmable - I really had no interest in a Smart or WiFi thermostat.
    I just wanted one with an easy to see LCD and one that can switch between heat and air conditioning automatically.

    Before I bought it a little research told me that a five conductor cable would probably be the best option but I think you can get by with four (there is a R-switch ie jumper) built-in.

    It was easy enough to run a new 5 conductor cable so I did it. I really like the thermostat.
  • One thing I have noticed with "smart" devices is that they only work well when the system they are connected to is working perfectly. In other words, when another part of the system fails, the "smart" device fails or gets confused.
    A friend of mine had a Smart TV that would freeze hard after he had it search the network for media servers (of which he has two). Since the media servers were restricted to only allow connections to specific internal IP addresses, the TV was only allowed to identify them as an available media server, but not connect to the media. This made the TV hang to the point where he would have to unplug it to get it back to life. When he remembered the IP restriction setting and added the TV's IP address, the Smart TV connected fine. Since whatever media server he uses recommends the restriction settings, I could see other users having the same problem.
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