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_-Unbeliever-_ t1_iy9mg5r wrote

Without a common wire the nest (and other smart stats) use ugly hacks to drop the voltage between R and one of the other control pins. The side effect is that a lower voltage means more current to maintain the same wattage. This can cause damage to both old school transformers and new digital control systems. The nest will also short cycle your system occasionally to charge the battery if there is no C wire and the system isnt running enough.

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buddhamanjpb t1_iy9rdux wrote

For anyone concerned about this, I would recommend getting the Sensi thermostat. It works great with 2 wire systems and can be controlled through an app on your phone. I've been using two for about 3 years now. Great purchase.

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Jack__Squat t1_iy9t1ei wrote

I second that. The Sensi uses a battery (I think to make up for the lack of a C wire). I have no complaints about it. The battery lasts a long time. You get a warning when it's getting low. And it has all the same features like a smart app, event alerts, schedules, and Alexa integration. It also has a setting to prevent short-cycling.

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buddhamanjpb t1_iy9t5kv wrote

Yup, uses two AA's, they last about a year.

I actually didn't know this about the Nest, glad I never ended up getting one.

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tommygfunke t1_iyas92k wrote

Which model do you have? I was just on their website and both models require a C-wire (or adapter).

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buddhamanjpb t1_iyb17fw wrote

I have the older white model. I forget the model name.

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jdmess401 t1_iy9p5aw wrote

Despite all the “DIY” references in their marketing, Nest recommends installation by a heating technician or an electrician, in part for this reason.

Having installed a Nest myself, I recommend heeding their suggestion. I replaced a troublesome Lux thermostat with a Nest. It turns out my home’s previous owners had mislabeled the wiring, which was causing the Lux to malfunction and meant the Nest didn’t work at first. It took me about an hour to figure out the issue. A professional probably would have had it figured out in no time.

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MavDrake OP t1_iy9oq8s wrote

This is why they make this:

https://rienergymarketplace.com/Nest-Power-Connector/P-NSTPWRADP.html

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degggendorf t1_iy9q2cc wrote

That's what the other person is describing.

If you only have two wires to your thermostat, they are designed to be connected or not connected; that's the only way the system is designed to communicate. The thermostat can't power itself from two disconnected wires, and if it connects them, then it turns the heat on. Enter the Nest Power Connector that only kinda connects the wires...enough to pull a bit of power for the thermostat, but not enough to actually turn on the heat.

Opinions will vary whether that is okay or not, but it is objectively using things in a way they weren't designed, and the majority opinion of HVAC professionals seems to be that it's no bueno.

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MavDrake OP t1_iy9rglr wrote

I realized it after I had posted my response and agree I misunderstood his point.

I had done some review digging its you are correct. Folks with no C wire seem to have partial success with their units however there is a lot of ugly reviews out there.

https://www.amazon.com/Google-Nest-Power-Connector-Accessories/dp/B093TRFQWP?th=1

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degggendorf t1_iy9sh5b wrote

For sure, it seems highly dependent on the exact system whether it likes it or not, in addition to how frequently the heat runs and how much it can charge its battery.

I ended up running new wiring for my two heat-only zones to avoid all the headache. Before that, using a 24VAC wall transformer with wire running up to the thermostat worked well too, but was kinda unsightly.

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zjanderson t1_iy9rlgi wrote

Google sends these out for free. Don’t purchase it.

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MavDrake OP t1_iy9rrcg wrote

Source?

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zjanderson t1_iy9snh9 wrote

One of mine did not have a C wire installed and the app detected low power. I got an email from Google stating I could claim one.

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MavDrake OP t1_iy9t167 wrote

Kinda sounds like the device thought it had a faulty product and google decided to just send you a new unit rather than deal with troubleshooting..

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Gsquzared t1_iya1jw8 wrote

Same. Just commented with my experience after coughing up the $40 for the connect kit.

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noungning t1_iy9vvlx wrote

This model has battery and shouldn't have that issue, but I guess we'll see in a few months.

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Gsquzared t1_iya44ft wrote

It does request a c wire when you hook it up.

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noungning t1_iya4em9 wrote

Well that goes against the info page then. I might just end up reselling these in that case. I don't want to deal with the headache I dealt with the other one I have.

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Gsquzared t1_iyakoxk wrote

Google had some steep discounts on the Nest E thermostats. Those don't require the c wire.

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phill0406 t1_iy9zu5z wrote

My work around was to snake this power adatper through the wall into the basement and plug it in. Has worked for 2 years no issues.

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Gsquzared t1_iya1cnk wrote

You may be able to get the Nest Power Connect for free from Google.

I got my nest from this promotion last week (came pretty quick), only to find out this version needs a common wire. I have two Nest E thermostats for other zones that work fine already so I didn't think much about it. I coughed up another $40 for the Nest Power Connect and a trim kit. I got it all hooked up and it works ok, but I still get some occasional errors about unstable power. Then today I got an email from Google offering a free power connect to help with the power issue. So maybe you can skip a step and get one free from Google.

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sbaz86 t1_iyb1q1y wrote

This person knows exactly what he’s talking about. It’ll damage your HVAC equipment.

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