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.
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.