Is C wire needed?

[MrTee1063]

Installed my Nest 3rd Gen. thermostat a week ago. It's a two-wire heat-only system. It's working fine but I've read that over time my battery is going to run down because there is no C wire. Can I tell by the battery voltage or the power level in Mw if this is happening?

[Houptee · Accepted solution]

It could cause problems depends on the heating system do you have a boiler with zone valves?

It steals power without the C wire and can make some heating system valves start to chatter when it draws power away.

Best thing to do is always connect a C wire just run a new thermostat cable with 5 conductors minimum so in future you have extra if new system is installed.

[MelbaDT]

Hey folks, 

Thanks for your answer and for your recommendation, Houptee. 

@MrTee1063, it is always best to have a C wire as it will provide constant power to your thermostat. Take a look at this link to learn more about the C wire: https://bit.ly/3nIztHl

Best, 

Melba