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Thermostats won't work

Condo Home
February 6, 2019

When we bought our condo it came with central heat in all rooms except the sunroom. There we had a baseboard heater with a wall mounted wired thermostat. The thermostat never worked automatically. We always had to turn the heat on and off manually. It didn't bother us too much because the room wasn't used very frequently in the winter.


When that heater died we replaced it with a new one and a new thermostat. We had the same issue with the new thermostat. It could manually be turned off and on but it would not turn the heat on or off at the desired temperature. Hoping that was a fluke, we bought yet another thermostat. This one does not work at all. We really need to be able to use the room but it is now closed off due to lack of heat.


Any idea what is going on? I don't know if it's an installation error, poor quality thermostats, bad wiring or something else. There are two wires to hook the thermostat up to and it seems like this should all be pretty easy.


We are trying to use one of these two thermostats. I am not sure what the difference is between these two models.


Thermostat 1

Thermostat 2

Comments (23)

  • mike_kaiser_gw

    I'm a little confused, the heater has a wall thermostat how did you turn it on "manually." If you used a thermostat/control knob on the unit, something is wired incorrectly. You need one or the other, not both.

    Condo Home thanked mike_kaiser_gw
  • Condo Home

    There is no thermostat or control knob on the actual heater. All of the thermostats have had a manual dial like the ones shown in my two links above. The wall thermostat there now does nothing. By "manual", I mean that the previous ones have worked but only as on/off switches. Instead of being able to set it to say, 65 degrees and have the thermostat turn on the heater if it got below 65 and turn the heater off if it got above 65, the thermostat would just turn the heater on and off and not modulate the temperature. I could hear it click when I turned the dial past the "off" part but then it was full blast heat and would not automatically turn off no matter how high the temperature got. So I might set it before bedtime to 60 or 65 degrees and wake up the next morning to find the temperature was over 80 degrees (ETA - And the heat would still be on). I hope this description makes sense. Let me know if it doesn't.

  • DavidR

    Is the thermostat mounted on an outside wall? Typically that will cause a thermostat to call for more heat than it should. If the wall is poorly insulated, it might keep the thermostat so cool that it doesn't respond to room temperature until the room is very warm.

    Condo Home thanked DavidR
  • Condo Home

    No, it's an inside wall.

    ETA -The same thing happens with any temperature, really. Set to 50 degrees or even lower, it just never turns the heater off.

  • Condo Home

    Now that I think about it, we have a similar set up in a rental house and that type of thermostat works perfectly there. You can hear them click on if you set them above the current temperature and they turn the heat off if you set them below the current temp. I am not sure why we have always had problems at our condo. I have read that those cheap thermostats have a fairly high failure rate and I was hoping that was the problem, but I'm now thinking that it's probably something else.

  • DavidR

    As best I can tell, the only difference between thermostat 1 and thermostat 2 in your links is the color.

    Either one should control the heater better than just off or always-on.

    These mechanical thermostats are very simple and trouble-free. They use a bimetal strip or coil linked to the contacts. As the bimetal warms, it curves, because the two metals have different coefficients of expansion. This pushes the contacts apart, turning off the heat. The adjustment dial turns a cam that changes the base tension on the bimetal strip. This causes it to open the contacts at a higher or lower temperature.

    The off position is an extra high spot on the cam. It pushes the contacts far apart.

    I have seen a couple of cases of very old thermostats where the bimetal strip seemed to wear out. Maybe it was metal fatigue. For whatever reason, the bimetal strip failed to move the contacts either far enough apart, or close enough together.

    A more common failure mode for these thermostats is that the contacts get pitted from arcing, and may either fail to open or fail to close.

    The fact that your first replacement thermostat acted just like the old thermostat still suggests to me that something is chilling the thermostat, even if it's not on an outside wall.

    However, it's slightly possible that the first replacement was defective.

    My best guess at the reason the second replacement didn't work at all is that you connected it wrong.

    When you have a situation like yours, with just 2 wires are in the thermostat wall box, and you're installing a double pole thermostat, it's not difficult to use the wrong pair of wires or terminals on the thermostat.

    With a double pole (4-wire) thermostat, you'll have either 4 wires or 4 terminal screws.

    If it's terminal screws, two of them will be marked L1 and L2 or Line 1 and Line 2. These normally go to the incoming 240 volt power.

    Then you'll have two more screws, typically marked H1 and H2, T1 and T2, or Load 1 and Load 2. These go to the heater.

    If you have a thermostat with 4 wires coming out of it, usually 2 will be red and 2 black. On the thermostats I've installed, the red wires were Line 1 and Line 2, and the black wires were Load 1 and Load 2. However, check your installation manual if you have one of these thermostats. If you get it wrong, you'll trip the breaker and/or destroy the thermostat.

    These are wired the same as above, Line 1 and Line 2 (red) to power, Load 1 and Load 2 (black) to the heater.

    When you have only 2 wires in your wall box, you have to make sure you connect them to the red and black wires, or terminal screws, that go to the same set of internal thermostat contacts! With screws, that means using Line 1 and Load 1.

    With a thermostat that has wires instead of screws, unless you check the thermostat with an ohmmeter or test light, there's no easy way to tell the differences between Line 1 and Line 2, and between Load 1 and Load 2.

    If you don't have test gear, just hook one wire in the box to a black wire, and the other to a red wire. If the heat doesn't turn on, use the other black wire from the thermostat, or the other red wire from the thermostat (not both).

    If that gets your heater working again, then it's time to test whether it's controlling the room temperature properly.

    If it still doesn't seem to be, first make sure that no cold drafts are blowing on the thermostat. If none, unscrew the thermostat from the wall and let it hang in the room air by its wires, 3-4 inches from the wall, and try it again. If it then regulates the temperature properly, that means that the wall it's fastened to is too cold and is confusing the thermostat.

    Sorry this was so long, but I hope it helped.

    Condo Home thanked DavidR
  • Condo Home

    Thanks, DavidR! You've given me a lot of helpful information. I will try to get this sorted out this weekend.

  • DavidR

    Duh. Why didn't I think of this above? It just occurred to me that on a plain mechanical thermostat, you should be able to connect BOTH Line 1 and Line 2 to the power in wire, and then connect both Load 1 and Load 2 to the wall wire that goes to the heater.

  • mike_home

    "There are two wires to hook the thermostat up to and it seems like this should all be pretty easy."

    If you only have two wires, then this must be a 120 V circuit. Here is the wiring diagram for the thermostat you bought:

    If you bought the CT410A you connect the two wires as shown on the left. This thermostat has no on off switch. If you bought the CT410B then you would only use two wire connections (L1 and T1). That connection is controlled by the temperature setting. If you wired it using L2 and T2 then is works only as an on off switch.

    You could use the CT410B with your two wires and wire in L2 and T2 as the on off switch. The connection would be as follows:

    Hot wire -> L2, T2 -> L1, T1 -> heater

    This would allow you to turn off the heater in the summer when you don't need it.

    Before you connect the thermostat, you can connect the two wires to themselves to verify the heater turns on. If it does not then you have a wiring problem. Make sure you are turning off the breaker when you are doing the wiring.

  • DavidR

    "f you only have two wires, then this must be a 120 V circuit."

    Could you explain why? I see no reason it can't be a 240 volt circuit. A 240 volt baseboard heater doesn't need a neutral.

    It's unusual to see a 120 volt baseboard unit except in a small bathroom. Those are often 750 watt heaters.

    Maybe I'm missing something.

  • mike_home

    I agree there is no neutral. However in a residential house the 240 V is created by 120 V which are 180 degrees out of phase. The load (heater) has to be connected between the two 120 V hot wires. That's why the 240 V line thermostats have four wires.

    I suppose someone could have wired one to the 120 V legs to the heater and installed a single pole thermostat on the other 120 V leg. Electrically it would work, but a very bad practice and probably violates the electric code.

    A 120 V electric baseboard heaters are rated up to 1250 W. They are unusual for a wired installation.

  • Condo Home

    Now I'm confused. I believe there are 4 wires to connect the electric baseboard heater and 2 wires to connect the wall thermostat. I looked at Menards.com and I see that they have both a 120V and a 240V heater. I am not sure which one we bought as a replacement, but I should be able to find out tomorrow.

    Is one of these safer than the other considering my wiring? Both say 1500 W.



    120 V Heater


    240 V Heater

  • weedmeister

    I'm not sure what 'safe' means in this context. They are basically both the same.

    For the 240v unit, the thermostat 'breaks' both hot wires when off. For the 120v unit, the thermostat breaks the single hot when off, the neutral is not switched.

    Condo Home thanked weedmeister
  • Condo Home

    I am not sure either, weedmeister. I am just wondering about mike_home's comments regarding safety.

  • mike_home

    You need to buy the heater that is compatible to your circuit. Wiring a 120 V heater to a 240 V circuit will cause it to over heat and cause a fire. Wiring a 240 V heater to a 120 V circuit will produce a quarter of the rated heat.

    I think you have a 120 V circuit, but I can't be sure. Homeowners get creative when they do electric wiring, so anything is possible. You need to figure this out before you do anything. I don't feel good giving you guidance without knowing how this circuit is actually wired.

    Condo Home thanked mike_home
  • Condo Home

    I appreciate your concern, mike_home. I found the box (hadn't recycled it yet) and the heater is 240 V. How do I figure out the voltage of the circuit? Can I go by the number of wires or do I need a meter?


  • mike_home

    Look at the circuit breaker box. You should see mostly narrow width circuit breakers which are for 120 V circuits. A 240 V circuit breaker would be twice the width. Check the circuit breaker that is wired to the thermostat and heater. Make note of the amp rating.

  • DavidR

    Let's clear up a few things.

    Last time I checked, it was still code-acceptable under some circumstances to use a single pole thermostat to control a 240 volt heater, opening only one hot lead. I don't think it's a good idea, but it's sometimes (maybe often) done.

    A 240 volt circuit is not 2-phase power. The neutral you use for 120 volts in a 240 volt supply is simply a center tap in the pole transformer. The two halves are in phase; otherwise you couldn't get 240 volts.

    In the case of a heater, the 240 volt supply could go to the heater with a 2-wire switch loop to the thermostat. I personally wouldn't wire it that way, but I've seen it done.

    A 240 volt heater fed by 120 volts will use half the current and produce 1/4 the normal output, not 1/2. That's Ohm's law. Power (which is what makes the heat) is proportional to the square of the voltage. P=E^2/R, where P is power, E is voltage, and R is resistance. R is the heater's resistance and it's obviously fixed.

    A 120 volt heater given 240 volts will use 2 times the current and try to produce 4 times the heat output. If you're lucky, the heater will just stop working. It may get very hot and/or go bang. Do not do this.

    A quick check for voltage is, as Mike said, to look at the breaker, but be careful. Not all double width breakers are 240 volt.

    Many GE panels can use a mix of 1/2" and 1" single pole breakers, and 1" and 2" double pole breakers. So you could have 1" 120 volt breakers and 1" 240 volt breakers in a GE panel, though I don't know how likely that is.

    Other panels use double width breakers that can be tandems (two 120 volt breakers in one).

    A handle that looks like 2 handles linked together is a pretty certain sign of a 2-pole, 240 volt breaker.

    A sure way to tell is to use a voltmeter on the live circuit (be careful). Last I saw, crummy-but-accurate-enough digital multimeters (DMMs) were under $10 at the cheap tool place (you know the one I mean). They even sometimes give them away with any purchase.

  • mike_home

    My bad about saying that going from 240 V to 120 V produces half the heat output. DavidR is correct that it is a quarter.

    I don't understand how you can produce a potential difference of 240 V across two 120 V potentials that are in phase. If they are both in phase then the voltage difference between the two would be zero.

  • DavidR

    "If they are both in phase then the voltage difference between the two would be zero."

    But you're not taking the difference! With a center tapped transformer winding, which is precisely how a domestic 120/240 volt service is implemented, the two halves are in series and in phase. The voltage across both halves (the full transformer winding) is therefore the sum, not the difference, of the halves' voltages.

    To see this in practice, find a standard power transformer with two totally separate secondary windings of different voltages -- for example, 180 volts and 6.3 volts. Connect the windings in series and measure the voltage. Then reverse the connections to either winding and measure again. One way (in phase, boost) you'll measure 186.3 volts, the other way (out of phase, buck) you'll read 173.7 volts.

  • mike_home

    Here is a video that does a great job of explaining the relationship of 120 V and 240 V service in a residential home:

    Using an Oscilloscope to Understand 120 VAC Split phase Household Power Supplies

    It is a bit long (11:44) but well done. Its great to watch somebody who knows how to use both a voltmeter and an oscilloscope and can explain the differences.

  • weedmeister

    "With a center tapped transformer winding, which is precisely how a domestic 120/240 volt service is implemented, the two halves are in series and in phase."

    No and No. Not in series and not in phase.

    "The voltage across both halves (the full transformer winding) is therefore the sum, not the difference, of the halves' voltages. "

    Not when measured from ground (or neutral).

  • Condo Home

    Sorry I have not been back to update this thread. I was wrong about the number of wires for the thermostat (miscommunication between my husband and myself). I will try to get a picture tomorrow.

    Unfortunately, nothing was marked in our breaker box when we moved in and we haven't rectified that. We just turn off every single breaker when trying to wire anything and rely on daylight and flashlights.

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