Thermostat keeps burning out?


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Old 01-19-16, 05:46 PM
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Thermostat keeps burning out?

Good evening,

I'm stumped. I've found a lot of good information here, but now I needed a bit more...


I replaced a thermostat that had burned out, again, the screen went blank. I say again because I was told after the fact it had been replaced a while back. Not recently, but not that long ago either.

I installed a basic programmable thermostat that's supposed to be a good brand. It burned out in 24 hours. I changed it again yesterday evening, with a stronger model, but it burned out again. They're all single pole, so two wires, feeding convection heaters. The first one I put in was rated for 3000w/240v. Yesterday's was 4000w/240v.

This is on a 240v circuit feeding two convection heaters, one that's 1500w and the second one that's 750w, so well under the limit for the two thermostats that I tried. Now, the heaters are in different rooms, so that might be where my problem is. I didn't do the wiring, so I assume it's ok, but here it goes.

The cable comes out of the breaker box, connected to a double pole breaker (two 20 amps "legs" - both black & red are connected each to a leg). Goes to a junction box, where it's split two ways. Here, all the black wires (3) are connected together, and all the red wires are connected together. This is the part I'm unsure of.

Each cable then goes on to the thermostat box in their respective room. In there, there are two pairs of cables, the one coming in and the one going out to the heater. The two red wires are connected together, and one black is connected to one wire of the thermostat, the other black is connected to the other wire of the thermostat. These are the only items on the circuit.

Is the fact that the split is before the thermostat somehow making the current to each leg stronger than it should be? Or could the wire be damaged and causing a short? Sorry, that was long, I just wanted to make sure I provided all the détails.

Thanks!

Marc
 
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Old 01-19-16, 08:53 PM
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Welcome to the forums.

You have a single 20A 240v circuit feeding two thermostats. That's ok.
Each thermostat only controls it's own heater so there shouldn't me an excessive current draw problem.

What are you using for a thermostat..... make and model ?
 
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Old 01-20-16, 06:12 AM
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Thanks for the confirmation... In all the articles I've read, the split was after the thermostat, not before (one thermostat for two or more heaters). I thought it looked fine.

Now, a bit more info (I got it recently, or pieced it together). The thermostat that burns out used to be a 2000w one (Aube, I don't have the model). At that time, the other heater (not controlled by the thermostat that's burning out) was less powerful, I think. It was replaced at some point by my brother in law, and that's probably the 1st time the thermostat burned out. It was replaced with something similar, and eventually burned out too. (The heaters are on different floors - it's an old house - and we were not aware that the heater upstairs was on the same circuit.) Time went by, it was summer so no heating was required...

The one I put in on saturday was a Stelpro 3000w STE302NP+, and it didn't last. Stubborn a bit, I then tried a STE402NP+, which is 4000w. My thought was, maybe somehow the cable is split somewhere and I just don't see it.

3000w: STE302NP+ | Stelpro
4000w: STE402NP+ | Stelpro

I'll try and figure out the settings on the multimeter to test the heater tonight, and see what I can deduct from there.

But any help or suggestion is still appreciated. Thanks!

Marc
 
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Old 01-20-16, 09:03 AM
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I can't see how your wiring has any bearing on this issue,

You said the thermostats are burning up.
Do they just stop working/go blank or do they smell/appear burned ?

If anything opens in the heater circuit the stat will go blank. There aren't thermostats on the heaters....are there ?
 
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Old 01-20-16, 01:31 PM
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They go blank.

Other than that, they looked/look like new, smelled/smell like new... There is no thermostat on the heaters. And the breaker never goes off. The upstairs thermostat is fine, and the heater connected to it works as it should.

I had someone pick up a new Honeywell thermostat yesterday, 5000w, mechanical, but I'm not in any hurry to install it if it will just stop working too.

I will open the junction box tonight too, and check the connections. Could a loose wire be the cause of that? If for some reason the red wire feeding the downstairs heater (the one tied to the thermostats that stop working) got loose and is not connected to the one from the breaker box... then its connection would only work when the upstairs thermostat was on...

Thank you

Marc
 
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Old 01-20-16, 09:31 PM
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A loose wire would certainly make the display go out as the thermostat and the heater are in a series loop and if the loop opens.... nothing works.
 
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Old 01-21-16, 05:33 AM
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Good morning,

It seems I was wrong. The downstairs thermostat is not burning out, and neither were the previous ones, probably.

For some reason, it only seems to get power when the upstairs one is active/closed. Now, the person using that room doesn't use it much, and it gets heat from the rest of the house, so the thermostat is often at a very low setting.

I turned it (upstairs one) higher yesterday, high enough so that the heater would be activated. Within a few short minutes, the downstairs thermostat had power again.

I checked the connections in the junction box, and both of them are solid.

For some reason, the downstairs thermostat only seems to get power if the top one is running.

I will look at that again on saturday, maybe undo a few more connections and check them, possibly unhook the upstairs cable completely and see how it works out...

Thanks again for your help. If you have any more suggestions, I'd love to see them.

Marc
 
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Old 01-21-16, 09:49 AM
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I would guess the heaters are wired in series and when upstairs heater goes off it opens circuit and shuts every thing down. PJmax may have better ideas.
 
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Old 01-21-16, 01:51 PM
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They're not in series, they're parallel. Unless somehow one of the wires isn't touching the other two enough in the junction box, but I checked them yesterday and they looked fine.

But I agree, it does look, from what's happening, that it's wired in series.
 
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Old 01-21-16, 04:59 PM
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They're supposed to be in parallel.

If they were wired in series..... the upstairs heater wouldn't work unless the downstairs stat was calling for heat.

Name:  dual stats.jpg
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Have you checked for 240v at the junction box ?
 
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Old 01-22-16, 04:59 AM
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Thanks for the diagram. That is exactly what I am aiming for, and what I have. I was busy yesterday evening and today, but I'll test the power at the junction box tomorrow.

Marc
 
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Old 01-24-16, 06:31 AM
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Good morning,

well, again, I don't know anymore, or I don't want to accept the conclusion...

I tested the junction box, and I get 240v there. So the cable between the breaker and the junction box is good. Still, I unplugged the upstairs circuit, and left the bottom one only. It can't be much simpler than that.

The thermostat still didn't work.

I took it off, undid the other connector (red wire), tested them, and I had 240v too (in the thermostat box). So the cable from the junction box to the thermostat is good. But the black wire coming back from the heater still was dead.

So I took down the heater, connected the two wires together (bypassing the heater), and tested the circuit back in the thermostat box. I had 240v again between the two black wires.

So everything now seems to point to the heater. Here is a link to the manual: http://www.qualitycraft.com/sites/de...al_eng_fre.pdf
It's the 1500w one.

Maybe when the upstairs one is running, the power gets stronger, strong enough to go through the heater which now seems to offer more resistance than it should?

So I guess we have to replace the heater then.

Thanks for your help.

Marc
 
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Old 01-24-16, 07:30 AM
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Did you check the heater for resistance with an ohmmeter ?
 
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Old 01-24-16, 02:27 PM
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Yes, but the results were so all over the place that I blamed it on me maybe not using the multi meter properly.

I bought a new heater, tested it, and got exactly what I had calculated the ohms should be, 38.

The ohms on the old heater were anywhere between 1200 to 1800-2000, generally on the higher side.

The new heater works great. Thank you for your help!

Marc
 
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Old 01-24-16, 04:14 PM
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You're welcome.

That sounds like your old heater had a defective thermal safety switch. So it was basically coincidental with the upstairs heater being on as that had nothing to do with a defective heater.
 
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Old 01-26-16, 05:39 AM
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How many sites are there in each thermostat box? Unless I am missing something on of the boxes should have 3 cable,feed in feed out to downstairs stat and the load should go to the baseboard heater.
 
 

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