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  1. #1
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    Confused The importance of thermostat wire gauge?

    Hi everyone,

    I'm looking into installing a Honeywell VisionPro IAQ thermostat and its control unit (part no. YTH9421C1002) in an existing home. The wiring installation instructions for the thermostat say to use 18 guage wire. However, there's only 20 gauge wire in the wall from the furnace to the existing thermostat. It's not trivial to change.

    The three connections between the thermostat and the control unit are data, power, and common; the connections to the actual furnace and humidifier are directly to the control unit, which will live next to the furnace. So I can trivially use 18 gauge wire for those.

    The question I have is, just how important is it that I use 18 gauge wire between the thermostat itself and the control unit? Or, what disadvantages or ill effects should I expect to see if I use the existing 20 gauge wire in the wall?

    This thermostat has every feature I need, but I want to be reasonably confident that it'll work with my existing wiring before proceeding. I tried asking the same question to the folks at Honeywell, who simply told me "this is a contractor installed thermostat; we recommend asking a contractor if 20 gauge wire can be used."

    Thanks for any insight you can provide...

  2. #2
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    quote: Honeywell
    "this is a contractor installed thermostat; we recommend asking a contractor if 20 gauge wire can be used."
    Guess Honeywell would know then huh. hey just twist a couple a them wires together 2- 20awg may make an 18awg.... just messin wif ya dude ..

    I had a 56 Nomad once.........

    and ahh you repeated yourself repeated yourself repeated yourself.........

  3. #3
    Join Date
    Nov 2006
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    Whenever someone asks me this, I ask them to put themselves in the position of sitting at a table with their insurance company's lawyer, a manufacturing company lawyer, and their own lawyer.

    Someone has looked at the after incident report (because their job is to avoid a payout) and noticed "the instructions clearly called for" something it was the homeowner's responsibility to check/install/adjust/change or get a professional to do what he could not do.

    How would you imagine that meeting goes from there?

    So, rather than discussing what wire gauge can actually be used, the "big picture" idea is to consider what happens if less than what is recommended is used, and something bad happens?
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

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  4. #4
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    Feb 2004
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    451
    20 guage wire makes agreat pull wire for 18 guage
    Before you criticize someone, walk a mile in their shoes...that way you are a mile from them and have their shoes

  5. #5
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    To Roscoe's point:

    Guess Honeywell would know then huh
    Based on them telling me to ask a contractor, then no, I guess they wouldn't know.

    Anyway, I realized that I left out at least one potentially important piece of information-- the distance of the wire run. The length of the existing wire is 40-50 feet.

    Also, I found two relatively recent threads, originated by ramlam, asking about wiring Honeywell's indoor temperature sensor to this very same thermostat using 22 gauge phone wire that happened to be in the wall already. Those threads are at http://www.hvac-talk.com/vbb/showthread.php?t=127964 and http://www.hvac-talk.com/vbb/showthread.php?t=127617. The consensus there seemed to be that it was OK, and the poster reported that the installation worked out fine.

    I realize I'm asking about a different component entirely... which is why I'm asking the question. And the answer is relevant regardless of whether I decide to do it myself, or pay a contractor to do it-- yes, I can ask a local contractor, but I'd like to be able to cross-reference his opinion with those of anyone here who has, and is willing to share, relevant knowledge.

    Thanks...

  6. #6
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    Quote Originally Posted by timebuilder View Post
    Whenever someone asks me this, I ask them to put themselves in the position of sitting at a table with their insurance company's lawye, a manufacturing company lawyer, and their own lawyer.

    Someone has looked at the after incident report (because their job is to avoid a payout) and noticed "the instructions clearly called for" somethin it was the homeowner's responsibility to check/install/adjust/change or get a professional to do what he cougd not do.

    How would you imagine that meeting goes from there?

    So, rather than discussing what wire gauge can actually be used, the "big picture" idea is to consider what happens if less than what is recommended is used, and something bad happens?
    bwaa hahaha

  7. #7
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    20 guage wire makes agreat pull wire for 18 guage
    Unfortunately not when the 20 gauge wire is stapled.

    Believe me, that was the first thing I checked. But it's immobile. And worse, the wire doesn't run across the (unfinished) basement ceiling and up to the thermostat, but, rather, it runs up inside the back wall, through the ceiling between the first and second floor, and then down into the wall to the thermostat. So I can't un-staple it without opening at least the wall and possibly the ceiling.

    Hence my question... I do appreciate the suggestion, though.

  8. #8
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    Quote Originally Posted by nomad1066 View Post
    To Roscoe's point:



    Based on them telling me to ask a contractor, then no, I guess they wouldn't know.

    Anyway, I realized that I left out at least one potentially important piece of information-- the distance of the wire run. The length of the existing wire is 40-50 feet.

    Also, I found two relatively recent threads, originated by ramlam, asking about wiring Honeywell's indoor temperature sensor to this very same thermostat using 22 gauge phone wire that happened to be in the wall already. Those threads are at http://www.hvac-talk.com/vbb/showthread.php?t=127964 and http://www.hvac-talk.com/vbb/showthread.php?t=127617. The consensus there seemed to be that it was OK, and the poster reported that the installation worked out fine.

    I realize I'm asking about a different component entirely... which is why I'm asking the question. And the answer is relevant regardless of whether I decide to do it myself, or pay a contractor to do it-- yes, I can ask a local contractor, but I'd like to be able to cross-reference his opinion with those of anyone here who has, and is willing to share, relevant knowledge.

    Thanks...
    I would not underestimate the importance of doing what it says in the instructions. It could be the difference between something coming out in your favor, and something not doing so.

    And, I would not say that Honeywell is directing you to ask a contractor because "they don't know." In fact, someone at Honeywell DOES know, and has been advised to not say, but it has instead been decided that the instructions should suggest the size of wire that is unlikely to become problematic in the future.

    That's the guide that you should use: what the manufacturer is willing to put on paper.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

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  9. #9
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    That is about the simpilest part of this biz we're in. Ain't no trade secret, but I had to develope my own system for running wire long distances.

    But 40-50 feet 20awg....check with a local pro. This is not DIY.

  10. #10
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    Nahhhhh sh*t for brains they Told you
    at Honeywell, who simply told me "this is a contractor installed thermostat; we recommend asking a contractor if 20 gauge wire can be used."
    Basically they said this is not something mechanically minded midgets should not be tackling But Hey you bought it,...........they don't care whether smoke gets out or not you own it.

    They were trying to be nice, I don't care if you fry it, or burn your house down.

    Be nice or beat it...........a.........h....o.......l.....e......

  11. #11
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    Based on your post

    this is a diy project. This site does not offer support for diy projects. Honeywell gave you the best advice, call a contractor.
    >>>Thread Closed<<<

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