(Simple - I think) question on replacing thermostat for baseboard electric heat

Discussion in 'HVAC Heating & Cooling' started by watson524, Dec 8, 2010.

  1. watson524

    watson524 New Member

    Messages:
    95
    Location:
    Pennsylvania
    Hi all,

    I'm hoping this is a simple one. My mom has an early 1970s mobile home she rents out. The living room has 2 baseboards for electric heat controlled by a single thermostat. The tenant said that sometimes it doesn't shut off, or let's it get way warmer than it should before it shuts off so I want to replace the t-stat. The old one is a federal something or other brand. Square with the dial. It's on a double 30amp breaker (so 240v). I switched off the breaker and took the t-stat off the wall to have a look. In looking at new ones (I'm hoping they're about the same size and I can just use the same junction box that this is all mounted in and not have to cut paneling etc) for sale at Home Depot for example, I'm confused.

    I assume I need a "4 wire" because of the 240v but not sure how it would wire up. In the current world, I have a line in and a line out. One black to line, one to load (makes sense). Then the two whites are pigtailed with a single white to the t-stat and then the 2 grounds are pigtailed to a copper that goes to the ground screw on the t-stat.

    That being said, what kind do I need to replace and how does it get wired up? I'm looking at some Honeywells and seeing 2 black and 2 reds on the diagrams.

    thanks!
  2. watson524

    watson524 New Member

    Messages:
    95
    Location:
    Pennsylvania
    The more I think about this, perhaps I am over complicating things in my head.

    I assume that the new t-stat, to be UL listed, MUST have a ground screw on it somewhere so the two bare copper wires from the wall would just pigtail to that screw like it does in today's world.

    Then black line and load side from wall to black line and load side on new t stat and then I THINK red line and load side from t stat to white line and load side from wall. In this case, white = hot, not neutral as normal because it's a 220v circuit.

    What's throwing me is how in today's setup, the two whites from the wall are pigtailed to one white from the t-stat.

    If someone could confirm, that'd be great.

    P.S. I'm a big fan of testing volts with my meter even after turning off the breaker so I have that covered :)
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    I don't want to comment on what is going on with all your whites, because I can't see it. The preferred method for a 240V circuit is to use a two pole thermostat, which switches BOTH hots. By switching only one, you do of course cause the heater to be on or off. But there is always a live wire inide the unit. That is just less than desirable from a safety standpoint.
  4. watson524

    watson524 New Member

    Messages:
    95
    Location:
    Pennsylvania
    I'm thinking that what's there now might not be relevant at this point. I'm definitely doing a two pole t-state and as such, I think I'll take blacks from wall to t-stat and whites from wall to red on t-stats and then either tape the whites with black tape or get a sharpie and make them black.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,820
    Location:
    New England
    That's probably right, but the only way to tell for sure is with a meter and some knowledge.
  6. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    "For residential wiring, some basic rules given in the NEC are:
    Phase wire in a circuit may be black, red, orange (high leg delta) insulated wire, sometimes other colors, but never green, gray, or white (whether these are solid colors or stripes). Specific exceptions apply, such as a cable running to a switch and back (known as a traveler) where the white wire will be the hot wire feeding that switch. Another is for a cable used to feed an outlet for 250VAC 15 or 20 amp appliances that do not need a neutral, there the white is hot (but should be identified as being hot, usually with black tape inside junction boxes).
    The neutral wire is identified by gray or white insulated wire, perhaps with stripes."

    This is from Wikipedia; I cordially invite anyone objecting to this interpretation of the NEC to edit the Wikipedia article. I'm just the messenger.


    To prevent spectacular outcomes, confirm with your meter that the two conductors going to your heater read a few ohms between them. Ohms = (240^2)/watts so for a 4500 W heater it'd be about 13 ohms.
    Last edited: Dec 9, 2010
  7. watson524

    watson524 New Member

    Messages:
    95
    Location:
    Pennsylvania
    I'm all set. Wired up the new Cadet 2 pole thermo tonite and all is working fine. Black to black twice and white from the wall to red on the t-stat twice. No ground screw on the t stat so those are just wire nutted together. All is in working order. Heat comes on and turns off as it should.

    thanks all!

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