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Nest thermostat battery charging problem and solution

Posted on February 7th, by Shaun Merrigan in News, Tech. Comments Off on Nest thermostat battery charging problem and solution

Nest thermostat battery charging problem and solution

My NEST (Generation 1) had been working flawlessly for several months, both summer and winter (heating and cooling) until about January 23rd, 2013 or so. Then it began to struggle with recharging, had network dropouts, disconnects, and began to insert Delay Times (apparently randomly, and up to 2 minutes in duration) into the heating cycle. I recharged the NEST using the USB interface a couple of times, only to find that it would run down within 8 or 10 hours and never recharge. With -25c temperatures for days on end, the last thing you want are strange delays and the resultant furnace cycle down/up and an unreliable thermostat. In fact one day with -40c windchill, the furnace would run for about 10 minutes and then cycle off as NEST inserted a Delay Time for no apparent reason.

I was at a loss as what had lead to this change in behavior and I still have not determined the root cause. Some would point the finger at my router and suggest that it was not compliant to a particular wireless standard, and my NEST was exhausting its battery trying to maintain the wireless connection. Well, this is very high end consumer grade router which had been working flawlessly (with the same firmware) for several months before this problem arose. So my router is clearly not the problem, its firmware and configuration had not changed in months. I did note that NEST pushed out a software update (3.0.5) just before all my problems started, but I cannot say definitively that the software update caused my issues.

To troubleshoot the problem and eliminate network access (and my router) as the cause, I manually disconnected NEST from my network and ran it as a stand alone thermostat. The same behavior resulted: run down battery, no recharging, strange furnace heating behavior. So the router/network access was not the cause. My Nest was wired as follows:

Nest No Common

Note that there is no 24 Vac common connection. A bit of research on web revealed that the NEST actually steals (“vampires” is a better term) power from the furnace controller when wired in this configuration. This also accounts for the wide swings in battery voltage I have observed over the past months. I will state very clearly that the NEST needs a stable battery voltage of at least 3.70 volts in order to operate correctly over the long term. I solved these problems by bringing the 24 Vac common wire from the furnace controller to the NEST, thus:

NEST Common

Now, my particular furnace controller board does not have a 24 Vac, “C” terminal (some do, some don’t). But of course it has a 24 Vac supply with common and hot, so I just tapped into the common wire and used the spare blue wire in my thermostat wiring bundle to run the common wire up to the NEST. The result is that the NEST recharged itself within a couple of hours, and has never dropped below 3.89 volts in several days. More importantly, my furnace has been running properly and the NEST has remained network connected the whole time.

My conclusion is that since wiring the NEST with 24 Vac common solves the charging/low voltage problem completely, and since it relatively easy to implement, why isn’t this the recommended method of installing the NEST. I would certainly have done this months ago had I known.

Still, no regrets. I would recommend the NEST thermostat to other people, with the proviso that the 24 Vac common connection be wired in as part of the installation. All part of being an early adopter.

EDIT: I have posted updates to my experience here: Nest Thermostat Charging Update and additional information on my charging solution: NEST Voltage Measurements (November 2013)


Shaun M


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