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I need to replace the wire from my thermostat from the oil burner in the basement to the second floor. I suspect a mouse chewed the wire or some other reason for a break in the wall along the way. When I search online for thermostat 2 wire, I get what looks like brown lamp cord except its solid core. My current wire is twisted black/white and multi wire copper(similar to old school Christmas wire that was red and green twist) not solid core. I suspect the solid core won't give me the flexibility to fish it up through the wall. But I don't find thermostat wire that is not solid core. Please advise.

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    At the absolute least, run 18/3 cable. Ask anybody trying to install a smart thermostat, they desperately need that "C" wire and they'll thank you for providing an extra wire. If you ever foresee an A/C unit, run /5 (traditional RWGY wiring + C wire). Extra wires are dirt cheap at install time, and costly to retrofit. Really, HVAC tech is changing pretty fast, and extended range heat pumps are becoming the next big thing. – Harper - Reinstate Monica Mar 30 at 18:46
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    You can't push wire up through a wall, ESPECIALLY if it's flexible. Wire is pulled. As for rodent damage, consider pushing some conduit, then pulling wire inside it. – Ecnerwal Mar 30 at 18:52
  • @Ecnerwal Couldn't agree with more more than that. I've had several experiences with rodent damage. If I ever build another house, all the wiring will be in conduit, code or not. \ – George Anderson Mar 31 at 2:49
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You'll be fine with solid wire. Tstat wire is generally 18 gauge which is pretty small. Also, since you'll be going thru all the work to fish the cable (which can be a PITA), I'd go with 18/7 cable. That way, if you or the next homeowner wants to install a heat pump or A/C the cable will be in place to support that.

Edit: Thanks for the contribution from manassehkatz-Moving 2 Codidact, I want to add to the answer: Smart tstats also need constant power which requires an additional wire in the cable. So if the OP or some future owner wanted to incorporate that as well, the cable would be in place. I deleted the ref to 18/5.

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    @manassehkatz-Moving2Codidact Great addition! With everybody going to smart tstats, it makes a lot of sense to get those extra wires run in the cable to support those as well,THANKS! – George Anderson Mar 30 at 17:46
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    Solid core 18 awg is very flexible and easy to pull. The difference between it and stranded is stranded can handle more flexing your thermostat and control boards really are not moving enough to be a problem. – Ed Beal Mar 30 at 17:53

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