It’s not uncommon for a thermostat to be located somewhere that doesn’t correlate well with the area you really want to be temperature controlled.  In fact, it’s probably the rule rather than the exception.  Thermostat placement is often dictated by convenience (aka how hard is it to run wiring from the HVAC unit) rather than the optimal location for measuring the temperature.  And, even then, HVAC’s often cover a wide enough space that there isn’t necessarily one thermostat location that can cover all your use cases.

The Nest Thermostat was released in 2011 … seven years ago.  As soon as that thermostat was released, people asked “Can we get a remote temperature sensor?”  After all, that’s the promise of a smart thermostat … doing things smarter.

Well, seven years later, and we’ve finally got it.  Yes, that IS a hint of sarcasm.  The inability to release such a basic addon speaks to some organizational issues at Nest, but let’s not speculate about that.  The point is that it took way too long.

My problem and hoped for solution

One of my thermostats is located in an area that gets direct sun during the day.  The temperature at that time is obviously wildly incorrect, even with the “sunblock” feature on the thermostats.  The hope is that one of these temperature sensors can be placed in a more general location in the room out of direct sunlight.

The other problem we want to solve is shifting the temperature target to the bedrooms at night.  One of our HVAC’s services both one half of the living room and a bedroom, and the thermostat is actually in the living room.  So the problem and solution here are self evident.  Use the temperature sensor in the bedroom at night time.

A third problem, which is a very minor issue, is that my study is located relatively far from its HVAC unit, and the area gets hot while gaming due to my PC’s power consumption.  Wouldn’t it be nice if I could somehow tell the AC to run if that area gets hot?  We’ll see.

Unboxing and hardware

At 39 dollars, the sensor isn’t too expensive.  At 99 dollars for 3, you even get a bit of a discount in bulk.

The sensor, however, has some inherent flaws out of the gate.  Despite the Zigbee radio built into the 1st and 2nd generation Nest Thermostat (ostensibly to enable devices exactly like this sensor), the Nest Temperature Sensor only works with the Nest Thermostat 3rd Generation and Nest Thermostat E via Bluetooth LE.  So a lot of people will have to throw away perfectly good Nest Thermostats just to use one of these.  In my mind, it feels like punishment for adopting the Nest Thermostat early.  Not so smart after all?

I ended up picking up a 3rd Gen Thermostat off eBay on sale to replace one of my 2nd Gens.

The hardware itself is sufficiently polished.  It’s a simple 2″ diameter white plastic puck with slightly recessed nest logo on it.  On the rear, there’s a peg hole for mounting the puck on a wall, and a tiny removable battery cover for CR2 battery inside, which should last about two years.  Overall, I’d say the hardware itself seems impeccable.  If I had to nitpick, it would be nice if we could plug the sensor into an outlet … never having to think about the battery life of the sensor has some appeal.  But overall, the design seems nearly perfect.

Installation and software

The software experience, unfortunately, is less so.  You add the sensor by scanning a QR code on the sensor or entering some identifying information manually.  Then your thermostat looks for the sensor and you tell it what room the sensor is actually in.

Unfortunately, there is some sequence of events wherein adding the sensor and the app status itself can get out of sync with the server.  The result is an endless sequence of where you change the location of the sensor, the app shows it update, and then the next time you open the app, the sensor is right back to its old location again.  Completely infuriating, and only fixed when I deleted the sensor entirely and fiddled with the thermostat itself.

Apart from that, if you think about it, technically you only need one thermostat to be the bridge that relays the temperature from the sensors to the rest of the thermostats in the house.  Well, that would be nice, but Nest’s engineers were not kind enough to go that extra mile for us.  You actually do need to purchase a compatible thermostat for every HVAC unit you want to control via remote sensor.    Again, this feels like punishment for the early adopter.

But OK, at least that problem can be solved with money.  There isn’t any reason you should have to upgrade every thermostat in the house, but it can be done.  Now we get to the core issues with the software.

The app lets you set what sensor to measure the temperature at during certain times of day.  Great, that makes sense … after all, one of the most common use cases is ensuring the bedroom temperature is properly targeted as you go to bed.  The problem here is that the windows of time for changing sensors are fixed.  Morning is 7AM to 11AM, Midday is 11AM to 4PM, evening is 4PM to 9PM, and Night is 9PM to 7AM.  “OK, what if I don’t go to sleep at 9PM?”, says virtually everyone who buys this sensor.  The answer is “Deal with it.”

The second issue is that there isn’t anything you can do aside from targeting a specific sensor.  For example, what if I want the minimum temperature in a few rooms covered by the HVAC to be 68 degrees during the winter?  Sorry, can’t do that.

The third issue is that the sensors themselves aren’t motion detectors.  Motion detection helps with targeting the temperature to rooms you are actually inside.  Again, fulfilling the smart thermostat promise.  But not here.  Perhaps in yet another iteration?

Theory vs Reality

Now, while I complain above a lot, my actual use cases are slightly better.

1. The direct sunlight issue on one of the thermostats in the morning is definitely improved by having the remote sensor in an area out of the sun.
2. The “Night” window of time for the second bedroom is completely wrong (starting at 9PM is way too early), but our unit is temperate enough that having the window start early is not much of an issue.
3. The “hot study” is not very improved, but that’s mostly a function of the room to cool being too far from the HVAC system to have much impact.  Nothing really to do here … I wasn’t expecting miracles.

Conclusion

The best thing that can be said about the Nest Temperature Sensor is that it mostly works.  The key word is “mostly”.  The hardware itself, aside from compatibility, is pretty good.  But, as an entire package, it does not “just” work, nor does it work smartly or intuitively.  In fact, short of being broken completely, it is the least advanced implementation of a remote temperature sensor that could possibly be conceived.

The fixed time windows are completely unnecessary and restrictive.  The lack of ability to target multiple sensors, either via minimums and maximums or averages, is behind other smart thermostats on the market.  The out of the box experience with location assignment of the sensor was borderline “return it” bad.

Overall, I hope some of the issues will be resolved in software, although it is seven years and we still don’t have a way to lower the temperature or hold it for a set period, so that may be just a tad too optimistic.  With that said, adding the sensors did improve my overall HVAC setup somewhat.  I find this product falls well short of its potential, but still improves any setup where you already have or plan to buy a compatible Nest Thermostat.  If you have to throw out an old Nest Thermostat to upgrade, it is unlikely to be worth your while.