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Thread: 24v thermostat connection question

  1. #1

    Default 24v thermostat connection question

    Hi guys-

    I want to use a 24v AC thermostat to control my goodman GM1090-3 furnace and blower.

    The thermostat is a carrier programmable model and I would like to run a day/nite program to cut on heat costs.

    I am confused where to connect the red and black thermostat wires on the furnace.

    I see a control board with a terminal marked "24v hum", and also that board is powered by a 115 to 24v AC transformer. Can I just tap into the 24v side of the transformer? That sounds logical to me but I like to be sure.

    thanks- this forum has been great to me so far.
    Last edited by frostnip; 01-08-2009 at 01:05 PM.

  2. #2

    Default

    here is the manual for the thermostat:
    http://www.docs.hvacpartners.com/idc.../33cs-15si.pdf

    I am pretty sure the existing thermostat is only using 2 or 3 wires. I think there may already be 24v ac present there but I am not sure.

  3. #3

    Default

    upon further investigation I have found out that 24v HUM is for a humidifier.

    I am thinking I will try to hook the thermostat up with out adding any extra power, and see if it works.

    If not, then I will tap the transformer's 24v AC side to supply power to the thermostat.

    I think I may be over analyzing this install.

  4. #4

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    Also, a side question I have.

    I am connecting a heat exchanger from my wood stove to my existing furnace ducting. Air from the returns will be pushed across the exchanger and then to the hot air vents.

    How can I make the furnace run just the blower to maintain a temp with the thermostat? I know in AC/cool mode it is possible, but what about another heat source?

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    A thermostat is essentially a temperature controlled switch. Putting more than one in parallel should allow any one to turn things on. Similar logic for the manual fan switch on a furnace...instead of using the manual fan control, you could move that wire (and extend the a/c common) to another thermostat and have it act as the manual fan on (instead of turning your furnace heat on).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    In the Trades maintenanceguy's Avatar
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    There are two types of 24V digital thermostats. Actually there are hundreds of different types but all of these will be either 1) power robbing or 2) not power robbing.

    Power robbing thermostats take 24V from the transformer in the heating equipment. Non power robbing thermostats get power from their own batteries. Even power robbing stats can have batteries so it's hard to tell from just looking.

    I know this doesn't answer your question but might help you figure out what you need to figure out to get this to work.

  7. #7

    Default

    excellent info gentlemen, thank you for posting.

    What I have in place is an old mechanical/ mercury thermostat. It does not require any external power to my knowledge.

    The replacement is a "power robbing" as maintenanceguy calls that type, made by carrier as linked above.

    I am thinking what I may do is replace the old mechanical thermostat with a battery operated programmable type (to control gas furnace), and use the power-robber carrier thermostat just to operate my blower for heat exchanger use.

    If I understand what jadnasua says I can somehow connect the 2 together to do exactly what I need to do. I will need to run some wiring for the 24v unless I still am not getting the picture.

    I will no doubt have more questions though- the object here is get up and running by next week and not fry any components.
    There is MUCH work to do.

  8. #8
    Plumber jimbo's Avatar
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    The issue your maintenance guy is referring to is this: Your old thermostat is just a switch. The new one has "features"....digital display, etc. That display needs power. There are three ways that may work, and you MUST determine from the instructions what YOUR new thermostat requires.

    1> Some t-stats REQUIRE the batteries to operate at all. The battery is powering the t-stat
    2> Some use the battery only for program backup. But they do "rob" some power and will operate just fine with only as few as two wires from the furnace.
    3> And some need to have power from the furnace, and REQUIRE a common lead from the furnace power supply. You may or may not have that available in your existing wire setup.

  9. #9

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    hey guys I just wanted to follow up and post my success on the first attempt!

    Had I taken a second to look at the wiring the old thermostat connected to at the furnace, I would have noticed that all the wires I needed to connect the 24v thermostat were right there and clearly labeled, no need to tap any transformers or anything.

    So my old thermostat had just two wires, and I needed 4 minimum for the new one. I got some 4 conductor wire from the store and used the old wire to pull it through the wall and floor. worked great.

    So for those who may find this post in the future the goodman furnaces are already set up to run a "power robber" thermostat.

    I absolutely love this new thermostat, it has a nice blue backlight so I don't need to turn on the light to see the temp at night.

    The best feature to me though is that it is controlling both of my heat sources. When in regular heat mode it runs the furnace as normal, using LP gas to heat the house. When I select "electric heat" mode it just runs the blower until the house comes up to temp, then switches it off.

    When in electric mode the air is circulated though a heat exchanger connected to my wood stove, utilizing heat that otherwise would go up my chimney.

    I am not certain if I can program it to use the LP furnace as a back up to "electric mode" but I am hoping that I can. If so it will fully automate my system, except I still have to stoke the wood stove

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