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I’m trying to pull a new thermostat wire down from a first floor room down into the basement. The exit point in the basement is in the ceiling, between two ceiling boards pretty inaccessible. Would it be stupid to try to hook some fishing wire to the old wire, pull that through, then tie the new wire to the fishing wire and pull that through? Is that unlikely to work? I don’t know about the opening diameter, and I’d be going from a 2-wire setup to a 4 or 5, so I guess it’d be thicker.

  • Curious: did you try this, and did it work for you? Could you post that as an answer? I'm in the same situation! – mattdm Nov 23 '18 at 17:56
  • I did, and it worked. Posted an answer! :) – Skeptic Nov 24 '18 at 0:31
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Your odds of success are about 1% unless the wire in in conduit, and it almost certainly isn't.

Generally, wires in walls are stapled to the framing. Perhaps your thermostat wire isn't, and then you might have a chance, if it's a straight shot. If there are staples or the wire goes around corners, you probably won't be able to pull the old wire out, and if you do manage that, the point where the new wire is connected to the pulling wire will not be happy about going through staples or around corners.

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    I 100% agree but it is worth a try. Having someone push the new wire while the old wire is being pulled will give the greatest chance of success. – Ed Beal Dec 9 '17 at 16:21
  • Worst case, it doesn't work and you do what you have to do in that case (make new holes, open up drywall, remove ceiling boards, etc.) so sure, give it a shot. – Ecnerwal Dec 9 '17 at 16:32
  • Thanks. The wire is actually loose. (I disconnected it at the furnace, and was able to pull it easily at the tstat, just making sure not to pull too much.) So in that case, does this seem feasible? Also, how do cable widths compare now versus back in the day? I have three wires wrapped in a cloth sheath there now. The new 18/5 Southwire I want to get has a 170 mil diameter, which actually seems smaller than the current wire. Does that make sense? That new wire bundles, even with more wires, are thinner than the older ones? I guess that would make it easier. – Skeptic Dec 9 '17 at 19:54
  • Yes, as already stated, might as well try. You need to make a join between the wire you are pulling in and the wire you are pulling out that is not too fat, nicely tapered, and yet strong enough to pull the wire through. – Ecnerwal Dec 9 '17 at 20:01
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Posting by request: I tried this, and it worked with ease. I severed the old cable at the furnace, and joined it firmly to the new cable by twisting the wires and then carefully wrapping the connection with electrical tape. I made sure the tape tapered off smoothly on either side of the join, so as to make the overall effect smooth. I then went to my thermostat, and pulled out the old wire until the join and new wire came through. There was hardly any resistance. I assume if your wire is stapled this wouldn’t work for you, but that wasn’t my case fortunately.

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