Nest Thermostat Power Requirements?

[Endlessknight]

Greetings all,

I would like to install a Nest thermostat on an older heating  system that only has two wires, red and white. The furnace itself has no labels but I can put a multimeter on the red wire and find out what the output is. What's the minimum needed, volts/amps, to power a nest? 

[Houptee]

24 V AC is the standard for most HVAC thermostat wiring.

You really should pull a new 5 conductor thermostat wire so you can connect a C common wire.

The Nest needs a common to keep it powered.

If you take a pic of the heating system wires we can see if you have a place to connect a common wire.

Some hot water boilers that use zone valves can cause problems when the Nest draws power from the transformer do you have any zone valves?

[Patrick_Caezza]

Sounds like you might have an old-style milli-amp furnace. This type of furnace can not be controlled by the Nest thermostat without a little bit of rewiring.

You'll need an external 24Vac transformer, the Google Nest Power Connector, and possibly a relay.

This is the basic wiring that will need to be done.

[aatienza]

Hey there,

Checking in — we hope you've got the answer you're looking for. Let us know if you have more questions in mind.
 

I appreciate the help, Houptee and Patrick_Caezza.

Thanks,

Archie

[aatienza]

Hi there,

I just wanted to follow up to see if you still need our help. Please let us know as we would be happy to answer any questions you may have. 
 

Thanks,

Archie

[EdwardT]

Hi Endlessknight,

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

Thanks,

Edward

[Endlessknight]

Hi All, thanks for the responses. They were very help. Been very busy with other things. I still haven't decided what to do 

[EdwardT]

Hi Endlessknight,

Thanks for the update. Let us know if you have other questions and we'd be glad to assist.

Thanks,

Edward

[EmersonB]

Hello Endlessknight,

Checking in — we hope you've got the answer you're looking for. Let us know if you have more questions in mind.

@archie, @Edward, @Patrick_Caezza and Houptee I appreciate your response.

Best,
Emerson

[Jhonleanmel]

Hi everyone,

Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, I would be happy to keep it open. If there's more we can do, just let me know.

Thanks,
Mel