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Thread: 120V Thermostat Change Out

  1. #1

    Default 120V Thermostat Change Out

    Im in the process of installing a new thermostat in my home. When I pulled off the thermostat to see what voltage it was, the meter said it was 120V.

    So I went ahead and ordered a new 120V line voltage thermostat from Honeywell. Model # TL8230A. I wanted a programable thermostat that would function well and look nice on the wall.

    My question is the thermostat on the wall has 2 wires coming from the box. The new thermostat has 4 wires coming from it. Did I buy the wrong one or is there a way to wire this in correctly.

    BTW: This is for a 120V oil fired boiler baseboard heat system.

    Any help would be greatly appricated.

    Thank you
    Last edited by NY98M3; 11-21-2006 at 09:50 AM.

  2. #2
    Renovator Gencon's Avatar
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    Nice stat. It has 4 wires because it is a double pole switch, which means it can control both legs of a 240volt heater.
    You need a 120 volt stat.

  3. #3
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Or just use one of the switch legs.

  4. #4

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    Quote Originally Posted by Mikey
    Or just use one of the switch legs.

    These are the instructions from Honeywell...Can you tell me which ones to hook up to my 2 wire system?

    http://www.thermostatshop.com/manual...n%20Manual.pdf

  5. #5
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Looks like you could use either both black wires or both white wires; the thermostat apparently interrupts both pairs. Most electricians would choose to use the black pairs (since black normally is the "hot" wire), but it really doesn't matter if you're only switching one single-wire circuit. Use one color, and save the other for a spare in case the thermostat's contacts ever fail (not likely).

  6. #6
    In the Trades Bob NH's Avatar
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    That thermostat seems to have some electronic functions, so it needs power to operate those functions. It appears to be more than just a switch.

    It appears to need power AND it needs to be able to switch.

    Also, original poster said he has a 115 Volt system, but has bought a 208/240 Volt thermostat.

    Something seems strange because I posted on this subject a few days ago but don't find it here. Is there another thread on this subject?

  7. #7
    In the Trades Bob NH's Avatar
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    Here is the reply that I posted where you put your question on the ELECTRICAL fourm. That thread seems to have died so am copying my reply to this one.

    This is a case where you must:

    1. Determine what the old thermostat did with the two wires.
    2. Determine what connection and configuration of the new thermostat will give you the necessary function of the old thermostat.

    Answers to these questions are usually found in the material that comes with the old and new thermostats. If you don't have the material for the old thermostat, then you should try to find the model number and search on line for the data.

    With a 2-wire thermostat, the usual configuration is that the thermostat connects the wires when there is a demand for heat.

    The page at this link http://www.thermostatshop.com/manual...n%20Manual.pdf says that the TL8230A http://www.air-n-water.com/product/TL8230A1003.html is for 208 or 230 Volts; but you measured 120 Volts. You might want to find out what your old one is before you get something crossed up. There is a picture at the link which shows that the new thermostat is connected to power at the inner pair of the four wires, and connected to the heater load at the outer pair of the four wires.

    Your TL8230A may be usable with your system connection, but you need to determine what the internal circuit does before you apply power to it.

    I would not want to GUESS at how to hook up an $80 thermostat. If you do, WATCH FOR THE SMOKE!

    There is a little-known fact about many electrical devices. The manufacturer encapsulates a small amount of smoke in the device and that smoke is essential for the device to work. If the smoke ever escapes from the device, then it will not work and it has to be replaced.

  8. #8
    In the Trades Bob NH's Avatar
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    I just found the edit to your original post which says that you are operating an oil fired hot water heating system.

    That makes me suspect that your original thermostat was switching the hot wires to power to the boiler.

    That would give you 115 Volts when you measure the circuit because you are measuring from the hot, through the aquastat, to the neutral or ground.

    When the switch closes to run the boiler, you will have ZERO VOLTS across the wires at the thermostat.

    Your very nice new thermostat needs power at all times so the electronic functions will work. The literature says it is a 208/240 Volt thermostat. It MIGHT work on 115 Volts if you always had 115 Volts, but you would need a neutral for that to have power at all times.

    You should also be aware that the thermostat is rated for resistive loads such as electric baseboard. It will probably run a small motor such as a 1/6 HP motor for a boiler, but is not rated for those loads.

    The available literature for the thermostat, and the information that you have provided, don't give enough information to figure out how to safely and effectively hook up the thermostat. At a minimum, I believe you need at least one more wire to use that thermostat and I can't tell you if that 208/240 Volt thermostat will work on a 120 Volt circuit.
    Last edited by Bob NH; 11-22-2006 at 09:08 AM.

  9. #9
    Plumber jimbo's Avatar
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    Yes, Virginia, it is a small world, and many professionals help out on more than one forum. We do not object to homeowners forum shopping, just please be consistent with your information from one to the next.

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