Nest Thermostat - Cold weather behavior

[phonedave]

I have (well had) a Nest thermostat on one of my zones and yesterday I woke up to the thermostat sending me an email saying it had lost power, was running on battery, and if it was not resolved shortly it would turn off and "my house would loose heat". Considering it was 2 degrees F outside at the time, I was not a happy camper.

For those of you not in the NE, my HVAC is likely very strange to you. I have hydronic gas fired heat. Meaning I have hot water baseboards that are supplied by a natural gas fired boiler. I have two zones on this heating system (Upstairs and downstairs). I also have central air. It is a completely separate system from the heating system. It is only one zone (but it supplies both floors)

The upstairs thermostat controls both AC and Heat. The downstairs is heat only. Which sets up the lovely situation of if somebody sets the upstairs to cool, and forgets to turn off the downstairs, the heat and AC get to fight it out on the first floor - but that is not what this is about.

On of the selling points of the Nest thermostat is that it does not need a common wire to run. Smart / WiFi enabled thermostats need a pretty constant source of power, and the common wire acts as a ground where they can drawn 24V from the system transformer on a constant basis. I did not look into how the Nest achieves this magic when I installed it. I just knew it did not need one, and I did not have one.

Apparently the Nest scavenges power from the heating and cooling loops. When the system is not running, it will draw enough power to charge its battery, but not enough to trigger the relay and turn on the heat or AC. Normally this works fine. However it times of extreme cold this is apparently what happens. The thermostat tries to draw from the attached AC air handler when it is in heat mode. However some AC systems (such as mine) have safety interlocks where if the pressure in the system drops too low (either from a leak, or from low temps) it cuts all power to the control board to prevent somebody from starting the system and damaging it. This cuts the power to the Nest.

When the Nest throws the error that it has no power, the temp fix is to pull the thermostat, remove the AC wiring, and then tell the thermostat that there is no AC. It will then begin to power itself from the boiler circuit.

My questions is, if the Nest is smart enough to throw the specific error, and refer me to a web page for that specific error with the directions to provide the temp fix, why can't it just do that all on it's own? Via it's software it should be able to ignore Y, G, and Rc and use W and Rh until power comes back to Y and Rc (when the temps in my attic rise high enough to build the refrigerant pressure back up.

Either way, the Nest is gone for now. I stuck the old battery operated non-WiFi thermostat back on the wall.

 

[reggiehammond]

Yesterday morning, awoke to half our house very cold. Very weird and it was 23ºF overnight so I went searching for a door blown open, or window open, etc. Could not find anything and was very puzzled as our heat pump maintains 68º overnight by schedule, 70º during day, but house temp was 58ºF and thermostat said it was heating with 2 hours until temp. Looking at app, it shows an adjustment to setpoint to 50º after we were all asleep, which is unlikely.

Just a data point - I can only assume this was a software/server glitch on the thermostat itself. It's an older generation - but another Nest in the house, newer gen, had no issue.
Nest app show

 

[phonedave]

Yesterday morning, awoke to half our house very cold. Very weird and it was 23ºF overnight so I went searching for a door blown open, or window open, etc. Could not find anything and was very puzzled as our heat pump maintains 68º overnight by schedule, 70º during day, but house temp was 58ºF and thermostat said it was heating with 2 hours until temp. Looking at app, it shows an adjustment to setpoint to 50º after we were all asleep, which is unlikely.

Just a data point - I can only assume this was a software/server glitch on the thermostat itself. It's an older generation - but another Nest in the house, newer gen, had no issue.
Nest app show

This is the third issue I had with Nest. Note I did not say THIS Nest. As one of the issues I did have was the WiFi board in the Nest totally dying, requiring a new unit under the warranty (which was like pulling teeth to get. That is why the Nest is now back in it's box and staying there. I am looking at installing Ecobee units - in the spring where if I hose something up, I really don't need an emergency fix on the heat or AC

My Nest used to do the same thing as your though, with AC. When it is in Eco Mode, it changes the temp as it sees fit to save you energy. Mine however would do things like make the AC COLDER on its own, in order to "Save me energy". How the heck is making my AC run MORE saving me money.

 

[reggiehammond]

I've got almost all of the "smarts" of our Nest thermostats turned off. With a heat pump, I don't save any energy by letting temps fluctuate much.
Makes this weird issue even more unexplainable.

 

[Turbo1Ton]

I recently replaced a Nest stat at my rental house because it was causing the fan motor to drop out randomly. I chased the issue for quite a while swapping the board in the unit, and trying a few other things. Never occurred to me that the stat might have been the issue because it was turning it on and off as necessary, and the compressor wasn't dropping out. Finally a friend's dad who teaches HVAC suggested jumping the contacts to remove the stat from the circuit and viola, I found the problem. He personally had issues with a Nest and low power and said he has heard of issues like this with the Nest not being able to have enough power. Ran fine since 2016 when I installed it, but just randomly 8 years later, all of a sudden started having issues.

I never got any kind of low power warning though.

--Jeff

 

[phonedave]

I recently replaced a Nest stat at my rental house because it was causing the fan motor to drop out randomly. I chased the issue for quite a while swapping the board in the unit, and trying a few other things. Never occurred to me that the stat might have been the issue because it was turning it on and off as necessary, and the compressor wasn't dropping out. Finally a friend's dad who teaches HVAC suggested jumping the contacts to remove the stat from the circuit and viola, I found the problem. He personally had issues with a Nest and low power and said he has heard of issues like this with the Nest not being able to have enough power. Ran fine since 2016 when I installed it, but just randomly 8 years later, all of a sudden started having issues.

I never got any kind of low power warning though.

--Jeff

If you have a C (Common) wire, you are not going to have low power issues. If you have a C wire, the Next draws power from either the Rc or Rh (the hot side for the cooling or heating circuit) and uses the C as the return.

If you don't have a C wire, and have a HEAT ONLY SYSTEM, the the Nest uses either the heating system to provide power. It knows when the heat is supposed to be off (because it is the thermostat, so it knows if it is calling for heat). If the heat is supposed to be off, it then draws current through the Rh and W wires. Normally if you connect the Rh and W wires it closes the circuit, closes the relay, and your heat kicks on. However the Nest is "smart" enough that it will draw enough power that is it will not throw the relay. It is enough to charge the battery in the Nest, but not enough to close the relay and trigger the heat.

On a system such as mine, where I have heat and AC, the Nest is "smart" enough to know if it is set to heat or cool. If it is set to heat, it does not try to draw power from the Rh and W (the heating circuit) it instead draws power from the Y and Rc (the cooling circuit) because since the unit is in heat mode, the cooling circuit should always be available to draw power from. It does the opposite when it is in cool mode, drawing power from the heat circuit.

The problem is when it is in heat mode, and the AC unit goes into safety mode because it gets too cold. In that case, the air handler actually disconnects the Y and Rc so that nobody can call for cooling and damage the unit. However this also cuts the power to that circuit.

The solution - According to Nest - it to remove the Yellow and Rc wires from the thermostat. The thermostat will then think it is connected to a heat only system and start to power itself from the heat circuit (there is an other possible issue. Since it only powers itself when the heat is not running, if you run your heat a lot, it may not get to charge enough, and the battery can drop).

My first issue with nest is that they don't explain this to anybody - they just say it does not need a C wire.

My second issue is that this thermostat is loaded with software and sensors. It seems to me that it would be a simple task to set it up so that if it is on a heating and cooling system, and it set to heat, and it has been drawing from the cooling system, and suddenly the cooling system goes off line, to simply switch itself to heat only and draw from the heat. It can send all of the emails, texts, push notifications, and display messages it wants, saying "Something is up with your cooling", but to tell the user to temporarily disconnect wires to "trick" the thermostat is poor design.

 

[DrewLG]

this thermostat is loaded with software and sensors. It seems to me that it would be a simple task to set it up so that if it is on a heating and cooling system, and it set to heat, and it has been drawing from the cooling system, and suddenly the cooling system goes off line, to simply switch itself to heat only and draw from the heat. It can send all of the emails, texts, push notifications, and display messages it wants, saying "Something is up with your cooling", but to tell the user to temporarily disconnect wires to "trick" the thermostat is poor design.

In the thermostat backplate, each wire position has an extra set of contacts that open or close depending on whether a wire is physically inserted at that position. Those contacts are electrically interconnected to perform simple logic that selects and routes the power source. It's a little like an old Bell System crossbar switch, and it happens without any microprocessor intervention or control -- because the power selection has to work even before the microcontroller has power, and even if there's a bug in the software.

When both AC and heat wires are present, the logic chooses to get power from the AC circuit because that one is generally stable and reliable (with rare exceptions like the one you experienced). The heat circuit, on the other hand, is disconnected by many furnace controllers for fairly common faults -- for example, if a dirty filter causes the heat exchanger to get too hot, most if not all gas furnaces will protect themselves by shutting down until it cools.

It's also not uncommon for a heating system's normal operation to require deliberately interrupting power to the thermostat. Taco zone-control valves, for example, are popular in hydronic systems like yours. They open when power's applied to W, and then close -- powered by an internal capacitor -- when power is removed from W.

So the relative unreliability of the heat circuit's power would make automatic switchover -- and monitoring the system for proper operation after the switchover -- complicated.

And it would also be tricky to allow an automatic switchover while ensuring that the circuitry could never create even a transient electrical connection between Rh and Rc, because -- for example -- in older systems with added-on AC, the heat and AC transformers could be wired with voltages 180 degrees out of phase, and you wouldn't want to connect those to each other.

I'm sure there are more reasons, but they probably all come down to the fact that HVAC systems in the field have a lot of quirky variation, mostly undocumented. And the thermostat as designed handles over 99% of them, silently and safely.

Of course that doesn't make your experience any better.

There's a relatively new Nest Power Connector that works with thermostats built after September 2015 (3rd and 4th generation Nest Learning Thermostat, Nest Thermostat E, and the terribly-named Nest Thermostat). It costs $25 and creates a phantom C-wire -- either with a clever 4-wire-to-5-wire scheme or by allowing an external transformer to be connected. If it's difficult to run a C-wire in your house, that may be the solution.

Another thing you can do, if you don't mind losing separate control of your fan and AC, is to disconnect your G wire at both ends and reconnect it to the C terminals at both ends, then run a jumper wire from Y to G at the air handler. That will give you a C wire, but the fan and AC will always turn on and off together.

 

[phonedave]

In the thermostat backplate, each wire position has an extra set of contacts that open or close depending on whether a wire is physically inserted at that position. Those contacts are electrically interconnected to perform simple logic that selects and routes the power source. It's a little like an old Bell System crossbar switch, and it happens without any microprocessor intervention or control -- because the power selection has to work even before the microcontroller has power, and even if there's a bug in the software.

When both AC and heat wires are present, the logic chooses to get power from the AC circuit because that one is generally stable and reliable (with rare exceptions like the one you experienced). The heat circuit, on the other hand, is disconnected by many furnace controllers for fairly common faults -- for example, if a dirty filter causes the heat exchanger to get too hot, most if not all gas furnaces will protect themselves by shutting down until it cools.

It's also not uncommon for a heating system's normal operation to require deliberately interrupting power to the thermostat. Taco zone-control valves, for example, are popular in hydronic systems like yours. They open when power's applied to W, and then close -- powered by an internal capacitor -- when power is removed from W.

So the relative unreliability of the heat circuit's power would make automatic switchover -- and monitoring the system for proper operation after the switchover -- complicated.

And it would also be tricky to allow an automatic switchover while ensuring that the circuitry could never create even a transient electrical connection between Rh and Rc, because -- for example -- in older systems with added-on AC, the heat and AC transformers could be wired with voltages 180 degrees out of phase, and you wouldn't want to connect those to each other.

I'm sure there are more reasons, but they probably all come down to the fact that HVAC systems in the field have a lot of quirky variation, mostly undocumented. And the thermostat as designed handles over 99% of them, silently and safely.

Of course that doesn't make your experience any better.

There's a relatively new Nest Power Connector that works with thermostats built after September 2015 (3rd and 4th generation Nest Learning Thermostat, Nest Thermostat E, and the terribly-named Nest Thermostat). It costs $25 and creates a phantom C-wire -- either with a clever 4-wire-to-5-wire scheme or by allowing an external transformer to be connected. If it's difficult to run a C-wire in your house, that may be the solution.

Another thing you can do, if you don't mind losing separate control of your fan and AC, is to disconnect your G wire at both ends and reconnect it to the C terminals at both ends, then run a jumper wire from Y to G at the air handler. That will give you a C wire, but the fan and AC will always turn on and off together.

Your explanation makes sense, but I still think Nest should have a better explanation of what is going on during the error code, or even a better description of how the device is drawing power that somebody could read at installation (or even before buying). I knew that the nest backplanes "did stuff" but did not realize it was mechanical closures.

I could likely fish in a C wire from my AC to the thermostat on the upstairs, but the downstairs is heat only. The thermostat is on the other side of the wall from the boiler, so running wire is not an issue at all, but I don't believe my boiler controller has a C connection.

I know it is my specific case, but I don't have zone control valves, I have a dedicated circ pump for each zone. My old house had one circ pump and 4 zone valves.

 

[DrewLG]

Your explanation makes sense, but I still think Nest should have a better explanation of what is going on during the error code, or even a better description of how the device is drawing power that somebody could read at installation (or even before buying).

Right, I also want that level of information when I'm buying stuff. But from the manufacturer's point of view it's counterproductive: Warnings and disclaimers and complicated instructions not only scare away buyers for whom the thermostat would have worked fine, but they also unnecessarily complicate the buying and installation experience for everyone who reads them.

AND the extra warnings don't even prevent installation failures, because a) a lot of people won't read them, and b) the ones who do might not know that they have an unusual HVAC system, or even whether they have a C wire.

I could likely fish in a C wire from my AC to the thermostat on the upstairs, but the downstairs is heat only. The thermostat is on the other side of the wall from the boiler, so running wire is not an issue at all, but I don't believe my boiler controller has a C connection.

One Nest Power Connector will allow you to connect a 24V transformer to the boiler, and a second one will synthesize a C wire for your other system. The Pro installation guide shows the wiring: https://storage.googleapis.com/support-kms-prod/k5rpNEXV1F0aVYhr8jN1L2GAxhcAPDXhk39r

In the old days you could probably have gotten at least one Power Connector for free by just calling Nest and being shocked at the $25 price, but I'm not sure whether that's still true.