Baseboard Heat Thermostat Problem

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  #1  
Old 10-29-10, 11:27 AM
J
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Baseboard Heat Thermostat Problem

Hello!

First off, thanks for the opportunity to post a question...I am stumped.

My problem...I have baseboard electric heat (240V) as my primary heat source. The other day, I noticed that it was a little warm in my living room so I attempted to lower the setting. Easy enough right? Not so much. Just after I started twisting the knob, there was a small "pop" at the thermostat and the breaker (I forget the amperage) downstairs threw.

Thinking I had a bad thermostat, I went to the home center and purchaced a replacement thermostat (Honeywell CT410B...240V...DP/DT...baseboard heat only). To note, I have already replaced 7 of these in my house because the newer ones look nicer than the 1984 versions. I have had success in all of these. Well, I left the breaker off and did a direct replacement. I have two wires coming into the thermostat box and both are hot (one black @122V and one white @122V...the wire is 14/2). There is only one set of wires exiting the box with a destination of the baseboard heater unit. Easy hook-up, I think--L1/L2 on the thermostat went to black (hot in) and white (hot in). The thermostat out is labeled T1/T2 (load). I hooked those up to the wires exiting the box keeping the colors the same as the input. When I re-energized the circuit, all was fine until I twisted the thermostat knob. Same thing...it threw the breaker and sparked out the side of the thermostat. Any ideas what is going on? The new thermostat is toast (the L1/L2 is fine...240V) but the T1/T2 is not, it only reads 122V.

Oh, forgot to mention...the thermostat is controlling two adjacent heaters. The wires exiting the thermostat box are routed to the first heater. Inside, they are pigtailed to go to the other heater. All wirenuts seemed tight inside both heaters. Also, this setup has never been touched by me in the two years I have owned the house. Did I have a bad thermostat and try to replace it with another bad thermostat? I am thinking on buying another thermostat and trying it again unless you think I am making some kind of mistake.

I appreciate all help...thanks!

John
 
  #2  
Old 10-29-10, 02:06 PM
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More likely as not you have a bad heater, a fault in the cable between the thermostat and the heater or the cable between the first and second heater. You will have to disconnect all the wiring and check each heater and the interconnecting cables individually. If you do not know how to do this please post back for detailed instruction.

BTW, those "wires" in the thermostat box and heaters are more correctly called cables. A cable is an assembly of two or more wires with an overall covering or sheath. In your particular installation the white wires need to be "re-identified" by using a permanent marking pen or plastic tape to re-color the white to some other color except gray or green.
 
  #3  
Old 10-29-10, 03:00 PM
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Furd,

Thanks for the response.

Are you saying that the cables hidden within the walls could be the problem and my heaters are fine? If so, that sucks.

As far as testing the cables...I would have no clue how to do this, other than to check for continuity between each end as well as check for resistance between each end? I'll take any ideas that you have.

As far as making the white wires black in color? I used a sharpie at first, but found a long, thin strip of electrical tape easier. I do realize that it would be easy for someone to rip that off without them even knowing it was hot. Thanks for look out!

Thanks again.

John

P.S. - Forgot to mention that I tried another thermostat today...same thing.
 
  #4  
Old 10-29-10, 05:37 PM
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UPDATE:

I did some testing of the all the cables (every wire, even the grounds). I used a Fluke multi meter and I tested for continuity as well as resistance. I placed my meter leads on each end of every wire...continuity is good and the resistance was .1-.3 on every wire.

The only thing that is left are the two heaters and the breaker...what am I missing?

Thanks!

John
 
  #5  
Old 10-29-10, 06:20 PM
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So far, so good. Did you test the wires of each cable for continuity to the ground conductor? There should be a reading of infinite ohms resistance for this test.

Next check the heaters for continuity from each wire to ground (shell of the heaters) and as the cables there should be infinite resistance. If this is good then test the resistance of each heater. Divide the watt rating (from the heater nameplate) by 240 to get the ampere rating then divide this number (amperage) into the voltage (240) to get an approximate value for the resistance of the heater element. If the measured resistance differs greatly from the calculated resistance then the heater is bad.
 
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Old 10-29-10, 08:01 PM
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Furd,

Thanks again.

As far as your first question, I did it after you recommended it. Here is what I did and what I found: I switched my meter to the ohm measurement and first measured the cable coming in the thermostat box from the breaker. Specifically, I touched one meter lead on the ground wire bundle in the box and touched the other lead on the black wire. My meter read "OL", outside limits, which is infinity I hope? Next, I left one meter lead on the ground wire bundle in the box and touched the other meter lead to the white wire and got the same "OL".

Next, I measured the cable departing the thermostat box (headed to the first heater). I measured the same way as the hot cable: measurement 1, ground wire bundle and a black wire (I got .3 ohms). Measurement 2, ground wire bundle and white wire (19.9 ohms). Is this supposed to be infinity? If not, this may tell you something?

Second, I don't quite follow you on the following: "Next check the heaters for continuity from each wire to ground (shell of the heaters) and as the cables there should be infinite resistance. If this is good then test the resistance of each heater." I gave it a stab anyways below...I removed the connections within the heater and tested just the heater itself. One meter lead on a heater wire and the other meter lead on the shell of the heater (ground screw) and got "OL". I did the same thing on the other wire and got "OL" again. Also, the other heater was the same..."OL" on both.

As far as testing the continuity and resistance of the heaters, do I just open both ends of the heater and test from end to end (which I didn't do)? Or do I just open the one end of the heater (where the cable from the thermostat comes in) and put a lead on each of the wires in there and test for resistance? If the latter, that is exactly what I did. Opening one end only on the heater, I put one meter lead on one heater wire and the other meter lead on the other heater wire (good continuity and there was 23.8 ohms resistance). Using your formula on the first heater on the circuit: measured resistance was 23.8 and the calculated was 23.039 (2500W heater). The second heater: I can only measure the resistance, it was 116.8 ohms...I cannot find a watt rating anywhere (24 inch/240V heater). The measures seems WAY high. Could this second heater be the culprit? And, could this cause the ohm reading of the cable going OUT of the thermostat box to be something other than infinity (remember the black wire was .3 and the white wire was 19.9)?

Thanks for your time!

John
 

Last edited by jrharts; 10-29-10 at 08:23 PM.
  #7  
Old 10-29-10, 11:52 PM
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Okay, now you're scaring me. Never use an ohmmeter on a circuit can be energized. When testing cable always have all conductors on both ends of the cable disconnected from everything.

Connecting one lead of the ohmmeter to the cable equipment grounding conductor (the bare wire) touch the other lead to each of the insulated conductors in turn. They should show infinite resistance. If they don't the cable is bad. You must have both ends of the cable disconnected but you only need to do the test from one end. Do this with both the cable from the thermostat and the cable to the second heater making sure that the second heater is disconnected from the interconnecting cable.

To test the heater make sure that both leads for the heater are disconnected and test each lead to the shell of the heater. Both leads should show infinite resistance. If either (or both) leads show any resistance less than 10 megohms to ground then the heater is faulty. Your meter may not even have the ability to measure such a high resistance.

The readings you got for the resistance on each heater are reasonable. The only other thing I can think of is that you might have miswired the thermostat. Did you follow the instructions implicitly?
 
  #8  
Old 10-30-10, 07:45 AM
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Furd,

You: “Okay, now you're scaring me. Never use an ohmmeter on a circuit can be energized. When testing cable always have all conductors on both ends of the cable disconnected from everything.”

Me: Roger that—won’t do it again. Thanks.

You: “Connecting one lead of the ohmmeter to the cable equipment grounding conductor (the bare wire) touch the other lead to each of the insulated conductors in turn. They should show infinite resistance. If they don't the cable is bad. You must have both ends of the cable disconnected but you only need to do the test from one end. Do this with both the cable from the thermostat and the cable to the second heater making sure that the second heater is disconnected from the interconnecting cable.”

Me: Great instructions! I did exactly as described and got “OL” on all cables. All of them were disconnected (thermostat to heater 1 and heater 1 to heater 2).

You: “To test the heater make sure that both leads for the heater are disconnected and test each lead to the shell of the heater. Both leads should show infinite resistance. If either (or both) leads show any resistance less than 10 megohms to ground then the heater is faulty. Your meter may not even have the ability to measure such a high resistance.”

Me: Great instructions again! Oh, my meter is a Fluke 27FM…the documentation says it can go up to 30Mega Ohms. On to the heaters…I have good news, I hope. The first heater measurement went well. All “OL”…all cables/leads were disconnected. First meter lead was touched on the heater shroud (grounding screw area) and the second meter lead was touched on one of the heater leads. Then I moved the meter lead to the other heater lead…again, all “OL”. On to the second heater—just for grins, I opened up the other side of the heater. I noticed that the two wires were bundled together inside with a wire nut. Also, that bundle was connected to the “core” of the heater…to a probe like thing. I noticed that this “probe” is LOOSE! I checked the other heaters in that house that are of the same model and they are all tight and the “probe” is seated tightly. Onto the measurements: at first I got “OL” on all the measurements. Then I gently touched the probe on the other side of the heater, the ohms ranged from just above zero to “OL”. Could this heater be fried? Would this be why my original thermostat shorted out when I attempted to simply adjust the temperature setting?

You: “The readings you got for the resistance on each heater are reasonable. The only other thing I can think of is that you might have miswired the thermostat. Did you follow the instructions implicitly?”

Me: Well, this is one thing I think I actually got right. I have done a few of these in the house already and I am absolutely positive about which cable are which. I know which one in the thermostat box is the input (from the breaker) and which one goes to the first heater. Also, my thermostat has four total wires: L1, L2, T1 and T2. I know the L’s are the line (the power coming in) and the T’s are the to be connected to the cable that controls the heater.
I have pictures if you would like to see any…just tell me what you want to see and I can e-mail them to you.

Thanks again for your help. I am ready to get a new thermostat and heater…just say the word.

John
 
  #9  
Old 10-30-10, 05:20 PM
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Furd, thanks for the help...problem fixed. Bad heater.

John
 
 

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