The existing thermostat...in an old house... does not have a permanent power source but is battery powered. Can i use a NEST? null

Question

Nochmara · Accepted Answer

Needs a power source for sure.

Vcmc74 · Answer

Your unit has to have power to operate, that power is from the central heat/air unit.

R8ter · Answer

if you pop off the old thermostat and it has a wire running to the Rc or Rh connection then you should be good. 
Check this link out
https://nest.com/support/article/How-to-tell-if-your-heating-and-cooling-system-will-work-with-the-Nest-Thermostat

Offhand · Answer

The short answer is “Yes” but you might want to consider alternatives. 

Systems that lack the ironically named common wire, will support the Nest thermostat, however, it does this by using the power from the heating and cooling system. This rapidly turns the furnace on and off to power the unit. For some systems this causes issues. 

Some people have suggested using the fan wire, but then the fan is no longer available without the heating or cooling on. 

In my case, I looked at the furnace main board (the motherboard that controls the furnace) and saw that I had a common wire connection. I then replaced my 18/3 with an 18/7 (18 gauged wire 7 strands) from the furnace to the thermostat. I could have run an 18/5, but wanted to future proof. 

There are “add-a-wire” kits ($30) available online that may be simpler than running a new multi strand, but my run was simple and I already had the cable. 

Hope that this helps.

Legman · Answer

I’m not sure ,you may have to run a power line to the air handler

“The existing thermostat...in an old house... does not have a permanent power source but is battery powered. Can i use a NEST?”

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