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I am replacing the thermostats in 3 separate apartments and have run into the same situation for all of them. Each thermostat controls 2 baseboard heaters. The old thermostats had 2 wires coming out of them. The wall box also has 2 wires coming out, except for one apartment which has 3 wires in the wall. The wires in the wall (3 in one apartment, 2 in the others) are hot. So no load wires, all line. The new thermostats (Honeywell linevolt pro TL7235A1003) have 4 wires (2 load, 2 line) and are to be wired to the load and line. How do I do this if there is no load wire? Do I need a single pole thermostat? How does this wire configuration work?

The picture shows the apartment with the 3 wires in the wall and the new thermostat. This is the only configuration that worked to get power to the thermostat, but it won't kick the heaters on. Thank you so much for your help! enter image description here

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  • The heaters are the load. Load wires have to be in there somewhere. Do more investigting.
    – JACK
    May 22 at 18:36
  • that will not work. Having the line and load connected under same nut
    – asinine
    May 22 at 18:44
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    Is this a thermostat which is "smart" or needs 240V power to operate?
    – Harper - Reinstate Monica
    May 22 at 18:52
  • @JACK The wires already connected are both line and load. The problem is that this thermostat needs two separate line and two separate load because it powers itself and sends power to the thermostat.
    – manassehkatz-Moving 2 Codidact
    May 22 at 19:16
  • @Harper-ReinstateMonica It requires 208V - 240V to operate. Not super smart, but lighted display, etc.
    – manassehkatz-Moving 2 Codidact
    May 22 at 19:17

2 Answers 2

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Repeat after me: Neutral in a cable is always white, white is not always neutral.

Color in US cables are limited: mostly black and white, some red thrown in for good luck. The result can be very confusing.

Your old cables used black for one hot and white for the other hot. That is an allowed use of white in a 2-wire (plus ground) cable. The thermostat uses black for one hot and red for the other hot, which is what you would have if you pulled out all your white labels and marked them with red tape.

Note that this is different from red in a typical 120V switch cable. There red is typically used to indicated switched hot/load. But that isn't the case here.

The old thermostats were most likely purely mechanical (e.g., bimetallic) and did not need power to run themselves. Because of that, they could be inserted between one line and one load. Those could be, in your case, two black or two white wires, and I think picking black is the more natural choice - but it makes the white wires look even more like neutral wires because a bunch of white wires connected together in the back of the box is exactly what you would see behind a typical 120V switch.

The thermostat needs power, so it needs to distinguish between line and load. It actually doesn't care which load wire is which, because with 240V they are the same (unlike 120V where neutral is in certain ways different from hot). The inner wires on the thermostat are line and the outer wires are load.

You need to identify line and load (multiple loads in the "3 wire" box) cables and the rest is easy.

  • Turn off the breaker.
  • Disconnect ALL wires in one of these boxes. That includes finding the connected white wires and separating them from each other. Make sure all bare wire ends are safely away from each other and the metal box.
  • Turn on the breaker.
  • Use a non-contact voltage tester (NCVT) to determine which wires are hot. You should find exactly one black and one white in each box that are hot, and they should be in the same cable. If you don't, stop and ask for more help.
  • Turn off the breaker.
  • Put a piece of red tape on the hot white wires.
  • Put a piece of blue tape on each of the black wires that was not hot.
  • Put a piece of blue tape on the outer black wire on the thermostat.
  • Put a piece of yellow tape on each of the white wires that was not hot.
  • Put a piece of yellow tape on the outer red wire on the thermostat.
  • Connect wires by color (natural color if no tape, tape color if there is tape).
  • Turn on the breaker.
  • Test the thermostat.
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    I like your use of tape. :-)
    – JACK
    May 22 at 20:00
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    @JACK Harper taught me.
    – manassehkatz-Moving 2 Codidact
    May 22 at 20:13
  • He taught me to use yellow for travelers..
    – JACK
    May 22 at 20:20
  • @JACK Correct. But the deeper lesson is: Use the colors to define functions of wires so you can make the later connections/reconnections super easy. Which applies to the travelers (since they are fungible, all of them yellow is fine) and applies here with the schema I listed.
    – manassehkatz-Moving 2 Codidact
    May 22 at 20:29
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    Thank you for this breakdown! I pulled the whites out which were nutted together and I figured they were neutrals. One was hot and thus making the other black wire hot. Disconnected those and it gave me 2 loads and 2 lines for the 4 wires from the thermostat. Thanks so much everyone for all your help!
    – Kevin smith
    May 23 at 19:10
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If you need a primer on 240V that isn't dull, try this.

There are two basic ways to wire a line voltage thermostat, similar to 2 ways of wiring a basic switch.

  • Power to the switch/stat, and then, a spur to the light/heater. This provides both supply wires (hot+neutral, or hot+hot in 240V circuits) to the switch/stat location, which smart switches/stats do require, and then sends both wires to the lamp/heater.
  • Power to the lamp/heater, and then, a spur to the switch/stat. This is called a "Switch Loop". The problem (for lights) is that pre-2011 this does not bring a neutral wire to the switch, and most smart switches need neutral so are a no-go. And (for thermosts) the second pole is not there, which means only single-pole thermostats can be supported, which eliminates many smart 'stats.

The third wrinkle is conduit, which I don't think you have, but if you do it's easy to pull the correct wires into the pipe if you need a configuration different than what you have. However this work is not legal for you to do*, but like I say won't take an electrician long.

If you have a single-pole arrangement (only one black and one white wire going to the 'stat) and no wires hidden in the back of the box), you cannot install this thermostat.

If you have additional wires in the back of the box not now connected to your 1-pole thermostat, then manassehkatz's instructions should get you there. Take careful note of which cable each wire comes from and how the black wires are grouped now. I can't tell whether the double black is always-hot power coming in and going back out.... or whether two cables go two different directions to serve two different heaters.

* All work must be done by a licensed electrician, with only three exceptions: first a homeowner-occupant may work on their own home (not allowed if they're about to sell it in most cases). Trivial like-for-like exchanges of switches, receptacles, lamps and thermostats are allowed. And low-voltage (24v doorbells, intercoms, thermostats) is allowed. Pulling wires into conduit exceeds that "trivial" exception.

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  • I based my answer on the assumption that since these circuits are for heaters they are unlikely to be used for lights, receptacles, etc. and therefore the two blacks together are for the pair of heaters (the "3 wire box") and that presumably the other rooms (the "2 wire boxes") have the hots chained/split closer to the heaters.
    – manassehkatz-Moving 2 Codidact
    May 22 at 20:31

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