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Thread: Thermostats

  1. #1
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    Thermostats

    Hey guys, today I was talking with the guy I am working with about thermostats. In school, I learned that the thermostat gets its 24 volts from the r terminal. While we were discussing this, he told me that the thermostat is actually powered by the c terminal. I was taught that on a call for cooling or heat, r sends power to w for heat and to r g and y for cooling. I guess he has kind of confused me. Can you guys fill me in in regards to what ACTUALLY is going on with the Tstat now that I have heard something completely different. Thanks.

  2. #2
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    I've been doing new construction installs btw.

  3. #3
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    There are lots of different thermostats, but "r" usually stands for power and "c" is usually the common.

  4. #4
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    Yep, that's what I thought.

  5. #5

    t stat voltage

    Quote Originally Posted by Jason2013 View Post
    Hey guys, today I was talking with the guy I am working with about thermostats. In school, I learned that the thermostat gets its 24 volts from the r terminal. While we were discussing this, he told me that the thermostat is actually powered by the c terminal. I was taught that on a call for cooling or heat, r sends power to w for heat and to r g and y for cooling. I guess he has kind of confused me. Can you guys fill me in in regards to what ACTUALLY is going on with the Tstat now that I have heard something completely different. Thanks.
    I think there may be a little confusion on his explanation. While the R terminal is carrying the 24v signal from the furnace to the thermostat and relaying that signal back to suit the call of the thermostat, the thermostat itself can be powered by completing a C common connection back to the furnace board. Ultimately eliminating the need for the double a's. Leaving the batteries in will allow the T-stat to hold its current program when you pull the door switch and break current to the furnace, or it loses power for whatever reason.

  6. #6
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    In its simplest form a thermostat is merely a switch that makes a path between "r"(hot side transformer) and the different circuits which contain a load. i.e. "y" condenser contactor, "g" blower relay, etc.

    Todays "modern" thermostats and some older ones have "loads" in them that require "c"(common side of transformer)

    Then there is "power stealing" which is another subject.

    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    Wayne Pendergast, efficientcomfort.net

  7. #7
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    That makes sense. Thanks for the explanation. Power stealing? Tell me more!


    Sent from my iPhone, Jason

  8. #8
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    Power stealing, a term used to describe how a thermostat operates its' loads without having a "c" (dedicated path to the common side of the transformer). It would utilize an unused circuit..say "w" while in the cooling mode.

    This process sometimes required the addition of a supplied resistor wired parallel with "w" & "c" at the furnace. Because this is out of the norm it can be a source unneeded frustration and should be avoided.

    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    Wayne Pendergast, efficientcomfort.net

  9. #9
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    May 2013
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    Oklahoma
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    Quote Originally Posted by penderway View Post
    Power stealing, a term used to describe how a thermostat operates its' loads without having a "c" (dedicated path to the common side of the transformer). It would utilize an unused circuit..say "w" while in the cooling mode.

    This process sometimes required the addition of a supplied resistor wired parallel with "w" & "c" at the furnace. Because this is out of the norm it can be a source unneeded frustration and should be avoided.
    The electric company in our state has a savings program and installs tstats that tell their customer how much their being charged every hour. They work exactly like this.

  10. #10
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    Jun 2001
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    Moore, Oklahoma, United States
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    Most digital thermostats are battery powered and do not use have C connection. Older thermostats were mechanical switches and don't need a source of power. A few of the digital thermostats use a C connection for power, but they aren't common and for the most part are only avaible online or though supply houses. Most of the stuff you find at the big box stores is powered by AA's.

  11. #11
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    Dec 2011
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    Twin Cities Minnesota
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    Have you seen the damage frozen,split waterpipes can cause,while a customers in Florida for the winter. All because of 2 "dead" AA batterys! change em annually.

  12. #12
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    Quote Originally Posted by Cosmicmuffin View Post
    Have you seen the damage frozen,split waterpipes can cause,while a customers in Florida for the winter. All because of 2 "dead" AA batterys! change em annually.
    Or better yet use a quality thermostat that uses a C terminal for power.

  13. #13
    Join Date
    Jun 2010
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    27
    Quote Originally Posted by 54regcab View Post
    Most digital thermostats are battery powered and do not use have C connection. Older thermostats were mechanical switches and don't need a source of power. A few of the digital thermostats use a C connection for power, but they aren't common and for the most part are only avaible online or though supply houses. Most of the stuff you find at the big box stores is powered by AA's.
    Wifi Thermostats need to have C connected.

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