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  1. #1
    Join Date
    Mar 2009
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    238

    Default Replace Thermostat Wire from 2 to 3 conductors

    I want to get a specific thermostat to control the boiler for my homes baseboard water heaters. It requires a 3 conductor connection for the C wire which im pretty sure is for power. Is it as simple as just to replace the 2 wire cable with 3 wire thermostat cable and connect the new conductor to the C terminal on my boiler?

  2. #2
    Join Date
    Mar 2009
    Location
    wisconsin
    Posts
    159

    Default Re: Replace Thermostat Wire from 2 to 3 conductors

    Im not a boiler expert, but if i had a diagram to go off of it would be a simple question to answer. Can ya pos it?

  3. #3
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
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    5,836

    Default Re: Replace Thermostat Wire from 2 to 3 conductors

    I answered this in another thread. Yes, just replace the 2 wire cable with a 3 wire cable. A 2 wire supplies power to the switch in the thermostat and a return. With a digital thermostat you need power on common from the transformer to supply power to the electronics and power to the switch and a return. The "C" connection is the common side of the transformer. It is much like adding neutral to a wall switch to use a digital control.

    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  4. #4
    Join Date
    May 2011
    Location
    Northern Virginia
    Posts
    99

    Default Re: Replace Thermostat Wire from 2 to 3 conductors

    Quote Originally Posted by JLMCDANIEL View Post
    Yes, just replace the 2 wire cable with a 3 wire cable. A 2 wire supplies power to the switch in the thermostat and a return. With a digital thermostat you need power on common from the transformer to supply power to the electronics and power to the switch and a return. The "C" connection is the common side of the transformer. It is much like adding neutral to a wall switch to use a digital control.

    Jack
    Jack: this is interesting . . .
    I have a 4-zone hydronic baseboard heat system (which probably everybody knows by now) controlled by old-style 2-wire Honywell-type round thermostats w/ mercury temperature guage (no timer whatsoever).
    I found at Home Depot, their housebrand, electronic setback units for $10 !, compared to $45-$60 for other 7 day multi-period units. The HD units state they are compatible with nearly ALL systems and has built-in Energy Efficiency temperature cycles. I bought 5 (the 5th for AC) and installed 1, basically, following the same 2 wire configuration and it works ! (No way I can rewire a full brick & plaster house with a concrete floor). They are (apparently) separately powered by 2 AAA batteries (vs. from 24v transformer). (At a former home, I had a Lux setback unit which I think used batteries for memory backup & light the display, but I don't know).

    I mentioned all of this just to say I was able to swap out & in easily, . . .

    however, the installer left no cable slack and I need another 2 inches to connect to the top of the new HD thermostat so I have to patch the 2-wire cable, drill another entry hole in the plaster wall, to have the new unit at original sight height and avoiding painting over faded/unfinished spots. (Right now, it hanging by the 2-wire cable).
    Last edited by Brookworld; 11-07-2011 at 08:43 AM.

  5. #5
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    6,736

    Default Re: Replace Thermostat Wire from 2 to 3 conductors

    Brookworld,

    1. what you have is "heat only" mechanical thermostat, costs $10 or less. Brand not important.

    2. if you need 2" more (don't we all), you can splice a short wire to your existing wire.

  6. #6
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
    Posts
    5,836

    Default Re: Replace Thermostat Wire from 2 to 3 conductors

    Quote Originally Posted by Brookworld View Post
    Jack: this is interesting . . .
    I have a 4-zone hydronic baseboard heat system (which probably everybody knows by now) controlled by old-style 2-wire Honywell-type round thermostats w/ mercury temperature guage (no timer whatsoever).
    I found at Home Depot, their housebrand, electronic setback units for $10 !, compared to $45-$60 for other 7 day multi-period units. The HD units state they are compatible with nearly ALL systems and has built-in Energy Efficiency temperature cycles. I bought 5 (the 5th for AC) and installed 1, basically, following the same 2 wire configuration and it works ! (No way I can rewire a full brick & plaster house with a concrete floor). They are (apparently) separately powered by 2 AAA batteries (vs. from 24v transformer). (At a former home, I had a Lux setback unit which I think used batteries for memory backup & light the display, but I don't know).

    I mentioned all of this just to say I was able to swap out & in easily, . . .

    however, the installer left no cable slack and I need another 2 inches to connect to the top of the new HD thermostat so I have to patch the 2-wire cable, drill another entry hole in the plaster wall, to have the new unit at original sight height and avoiding painting over faded/unfinished spots. (Right now, it hanging by the 2-wire cable).
    Yes battery operated units are available, however the OP stated he wanted a particular unit that requires 3 wire. The new wire connected to "C" which is the common side of the 24 volt transformer.

    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  7. #7
    Join Date
    Mar 2009
    Location
    wisconsin
    Posts
    159

    Default Re: Replace Thermostat Wire from 2 to 3 conductors

    2. if you need 2" more (don't we all), you can splice a short wire to your existing wire.


    hahaha

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