C wire and internal backup batteries

[Sierreone]

Does the nest thermostats have a internal backup battery as well as the AAA external batteries. Also, do I need a C wire

[CoolingWizard · Accepted solution]

Yes thermostat models which are powered by batteries that can be changed out when they are dead, do not have built-in back up batteries. However, if you go ahead and use a C wire, you will cause those batteries to last longer as the main power for the thermostats control and operation will come from the HVAC equipment transformer. 

AC Cooling Wizar

[CoolingWizard · Accepted solution]

Yes thermostat models which are powered by batteries that can be changed out when they are dead, do not have built-in back up batteries. However, if you go ahead and use a C wire, you will cause those batteries to last longer as the main power for the thermostats control and operation will come from the HVAC equipment transformer. 

AC Cooling Wizar

[Sierreone]

My nest thermostat only has 2 AAA batteries for power with no internal backup batteries and and no C wire connected. I’m concern about the batteries failing when I’m not there and the air conditioner not coming on. I have a question regarding the use of a C wire. I live in a condo, my blower unit is location in the ceiling of condo but the rest of unit in on the building roof about 50 feet away. My air conditioning guy said he would have to run a C wire from both piece of equipment at a huge cost.  Is there any other methods I could use. I have read a few methods like adding a 24 volt transformer C wire adapter or using green (fan) wire. Any advice or links to websites with solutions that wouldn’t damage the air conditioner or thermostat would be greatly appreciate.

[CoolingWizard]

@Sierreone , can you get a picture of your thermostat so I can see the wires that are currently coming to it and I’m curious to know if there any conductors that are tucked behind the wall that are still available for use.

AC Cooling Wizard

[Sierreone]

This was the old thermostat 

 and this is the new Nest thermostat 

From what I can see there are no other wires

[CoolingWizard]

The Nest Learning Thermostat has the ability to use a sort of back door to reach the Power Common. However the basic Nest thermostat you purchsed does not have that ability.  You can use a Nest Power Connector.  This requires you update the software on the Nest Thermostat, then installs the Nest Power Connector. 

AC Cooling Wizard

[Sierreone]

Ok, thanks for your feedback. As a final thought, if I open up the panel on my air unit and find there is a C terminal available on the circuit board would this be a 24v C wire connection I could run a single line to my thermostat? 

[CoolingWizard]

@Sierreone , if you follow the thermostat cable where it enters the air handler cabinet, you might find a control board. On the control board you find C terminal. If you haven’t, unused conductor, say the blue one, attached it to the sea terminal and the HVAC equipment, and then attached the blue wire to the sea terminal in the thermostat base. 

AC Cooling Wizard

[Sierreone]

First of all I’d like to thanks you for your time and patience.

I looked at my system and I cannot access the control panel, also notice the wire going to the thermostat is a combination of wires from the unit in the ceiling and unit on roof.  This whole thing started because of a note I noticed when I look at the device in Google home app

 when I installed the nest thermostats I got not errors or messages saying I need a C wire, is this note more of a generic comment. The thermostat has been running fine with no issues for over a month and have not seen any warnings on functionality or batteries. One of my other concerns was a comment I read online (grain of salt, I known) indicating that there may be an issue with the AAA batteries draining faster due to no C-wire, can this happen? In your opinion did I buy the wrong thermostat, should I buy a different model.

By the way, my old thermostat didn’t have a C-wire and worked perfectly for 8 years

[CoolingWizard]

@Sierreone , your old thermostat is most likely a mechanical thermostat and not a digital or micro-switching thermostat.  Can you post a picture of the indoor unit? You live in a condo correct? The wires go from your thermostat to the indoor unit, and then in the indoor unit there’s a cross connection to the outdoor unit. 

The AC Cooling Wizard

[Sierreone]

The old one was digital 

 here’s what I can see of the unit in the ceiling

 

 

 

[Sierreone]

Ok, thanks . I will try and open panel on Monday to see

[CoolingWizard]

@Sierreone , where the larger wires come out and are wire nut connected to the thermostat cable. Inside that cover is where the common wire off of the thermostat is going to be.  

AC Cooling Wizard

[Sierreone]

Hello, sorry for getting back to you so late. I opened up the electrical panel I showed you in the earlier pics, there is no control panel I can hook up a C wire to. As a note the system has been running fine with no issues or batteries errors for a couple of months. 
All my concerns stem from a comment I read online that indicated that in certain cases the Nest Thermostat AAA batteries can drain faster/completely in a short period of time if a C-wire is not connected. Is this true or is this a very rare case caused by other circumstances.

[CoolingWizard]

When you opened up the panel, did you find the step-down transformer? 

AC Cooling Wizard

[Sierreone]

There was no step down transformer that l could see. There was only 4 or 5 electrical components mounted on the backplate in series with at least 4 wire connecting each one to the other

[CoolingWizard]

Can you get the manufacturer and model number of that unit so I can look at the documentation. 

AC Cooling Wizard

[Sierreone]

This is all I can get access to, can’t seem to find original documents.

 

 

[CoolingWizard]

https://youtu.be/9lu1wNcMGAw?si=vysHjYCIjobDPd95

this video shows you the transformer,  locate the blue wire coming out of the transformer. This is your power common. 

AC Cooling Wizard

[Sierreone]

The video indicates high voltage and mentions the 208V blue wire. Is this a step down transformer and the volt coming out of the transformer is 24V (same as thermostat)

[CoolingWizard]

Yes, the input is. 208/240VAC and the output is 24 VAC. 

AC Cooling Wizard

[Sierreone]

Morning, I took a look again at the transformer, see below for what I see.

transformer

wires coming out of housing

 There is another wire terminated but it looks purple and is hard to tell if it’s coming off the top of transformer 

 

[CoolingWizard]

@Sierreone , the brown wire is the 24VAC common. 

AC Cooling Wizard

[Sierreone]

Below is the wiring I see

 Not sure if I mentioned this before but the guy who does my inspection/cleaning of air system told me that the C wire must be run from both units (blower & condenser) based on the on the wiring of system.

[CoolingWizard · Accepted solution]

The man that does your cleaning/inspection is wrong. The common from the transformer does go out to the condensing unit and does go to the indoor blower control. However, you need only connect to the power common at the indoor unit. 
When I get a free moment, I will draw this up for you. 

AC Cooling Wizard

[Sierreone]

Ok, thanks

[Sierreone]

Hello, just thought I touch base and see if you had time to draw up a diagram for my C wire connection to thermostat, thanks

[CoolingWizard]

@Sierreone , I am sorry, been very busy. I will try to get this drawn up for you S soon as I am able. 

AC Cooling Wizard

[Sierreone]

Hello, are you available for a question. I did solve my C wire issue

[CoolingWizard]

Ask me your specific question. 

AC Cooling Wizard 

[Sierreone]

Once a schedule is created for nest thermostat is will function with out wifi, correct 

[Sierreone]

Also to maintain a constant temperature of 79 degrees I created a schedule preset called CoolAway which is set to Cool mode with a temp of 79 start time 8:00 and assigned it to all 7 days, this will maintain a temp of 79 24/7 until I charge schedule, correct