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So I have an existing thermostat cable on my 2nd floor that only has 4 wires, and I need a 5th wire to use as the common wire.

Can I use the existing cable to fish or guide a new 6 wire cable through down to the basement? If so, what kind of tie or connection should I use between the two wires so that it doesn't come loose as I pull the cable through, and to keep it streamlined so that it can slide through holes or whatever it's going through? Is it better to pull the cable from the top floor or from the bottom floor? Is there a chance I'll get stuck or lose the cable in the wall - and thus should I just leave it to the professional?

Any tips in general for this? Is this recommended?

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    There are too many variables to be able to give you a probability of success. Just one staple or wire tie or unusual turn can end this. The test you should try is can you move the existing cable back and forth a few inches? If so your success rate is good. There is an art to tying the two cables and taping them smooth, but before we get there, find out if you can move the cable a few inches in both directions. – Tyson Feb 11 at 13:05
  • If you do wind up pulling new wire, I'd suggest going with an 8-wire cable, rather than 6. Difference in price is low, but if you ever add new equipment or whatnot, you'll be happy for the (additional) spares – mmathis Feb 11 at 13:49
  • Like Tyson said if you can pull the wire without much force you may be able to use the old wire as a pull. With that said make the splice as streamlined as possible, going through holes is usually where I get stuck on wires that are not stapled.+ – Ed Beal Feb 11 at 19:12
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That's a very labor intensive approach considering some of the latest technologies. Have you thought about converting to a Wireless Thermostat? Take a look at the Honeywell YTH6320R1001.

Honeywell

You might also consider a cheaper option with the VentStar Add-A-Wire. This setup allows you to use your existing four wires and still be able to have a common wire. It's an ingenious idea that piggybacks unused wires and converts them into one common wire.

enter image description here

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    While perhaps a viable alternative, this doesn't actually answer the question as asked. Plus, while programmable thermostats use batteries and they last a while, I wonder what the addition of wifi would do to battery life... – mmathis Feb 11 at 13:32
  • The question was asking if using existing cable to fish new cable was recommended. Your edit includes an alternative way to get an extra (common) wire, so I'm removing my downvote. – mmathis Feb 11 at 13:48
  • I recently installed some Ecobee thermostats in my home and they included a C-wire adding device in the package as part of the kit. – JPhi1618 Feb 11 at 20:00

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