T- Stat Caught Fire

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From the smoke stream going up the wall, I'd be opening up the ceiling on the floor below. It looks like the melted t-stat is just a symptom of the real issue.

Maybe neutral current on the tstat loop:-?
 
There are two points of origin (based on the photo only).

The burn pattern indicates the t-stat melted dripped onto the baseboard and reignited a new fire on the baseboard.

It's is possible a fire started below that, However, you would expect to see greater overall damage, unless the fire was contained to the wiring alone. This is highly unlikely between a first and second floor.

If, I were the FD, the wall would have been opened up just a bit to take a peek, just above the baseboard. Of course, this is arm chair arson investigating.
 
nhfire77 said:
...If, I were the FD, the wall would have been opened up just a bit to take a peek, just above the baseboard. Of course, this is arm chair arson investigating.
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With the use of themal imaging cameras, the FDs do not do that near as much any more.
 
Again, I was arm chairing here. Some Captains are more aggressive than others. But you already know that ;).
 
nhfire77 said:
Again, I was arm chairing here. Some Captains are more aggressive than others. But you already know that ;).
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I agree with the arm chair investigating, but back to what I said in post 19, if the T-stat melted and fell down outside of the wall then the base board would have been damaged, with as much smoke damage to the drywall it seems the smoke came from behind the baseboard, now it could have melted down inside of the wall? but without farther opening of the wall at that point its anybodies guess.
 
hurk27 said:
I agree with the arm chair investigating, but back to what I said in post 19, if the T-stat melted and fell down outside of the wall then the base board would have been damaged, with as much smoke damage to the drywall it seems the smoke came from behind the baseboard, now it could have melted down inside of the wall? but without farther opening of the wall at that point its anybodies guess.
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I don't think so. First the amount of smoke that can be produced from a small piece of burning plastic is huge. I saw an MCC room about 20' x 30' with a 10' ceiling filled completely with some of the darkest black smoke I have ever sceen, down to a level about 4' above the floor. The plastic that was burning was part of a 1500 hp DC drive and was smaller than the thermostat in this thead.
As far as the damage to the baseboard, plastic often just smolders giving off a lot of smoke, but not a lot of heat. It takes a significant amount of heat to start the baseboard on fire.
I am sure the firefighers looked close enough to see that the smoke did not originate behind the baseboard. Something that can be done very easily if you are on sceen, not so easy from a picture.
 
Just to show it is a low voltage t-stat. Hard to see but the wire is in the middle with what is left of the stat.

ry%3D480
 
don_resqcapt19 said:
I don't think so. First the amount of smoke that can be produced from a small piece of burning plastic is huge. I saw an MCC room about 20' x 30' with a 10' ceiling filled completely with some of the darkest black smoke I have ever scene, down to a level about 4' above the floor. The plastic that was burning was part of a 1500 hp DC drive and was smaller than the thermostat in this thread.
As far as the damage to the baseboard, plastic often just smolders giving off a lot of smoke, but not a lot of heat. It takes a significant amount of heat to start the baseboard on fire.
I am sure the firefighters looked close enough to see that the smoke did not originate behind the baseboard. Something that can be done very easily if you are on scene, not so easy from a picture.
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I guess I was wrong as the above second photo above shows no damage to the interior of the wall, or the thermostat cable that was in the wall, the baseboard threw me, as I would have expected some kind of smoke damage to its surface like the painted wall behind it, there is a small point of blackness on the baseboard which I took as maybe where a nail might have been.

As for why it happen the only guess I have then is the small resistor wire for the anticipater for the heat mode if in fact it was hit by more voltage then the 24 volts it was supposed to have, which makes me wonder if the system was a dual supply system, where you would have separate class 2 transformers, one for heat mode and one for cooling? I have also seen more then one transformer used on zoned heating which could explain the reason this T-stat in in a bedroom? 2 24 volt transformers can equal 48 volts?
 
Wayne, I just called my friend and he said one T-stat. Apparently the T-stat was in the hall that was open to the floor below so they moved it into the bedroom. There are 2 furnaces one for the first floor and one for the second floor.
 
No details but, on this morning's radio news, there was a quick blurb about a fire call to a local farmhouse due to "smoke coming from a thermostat" .
 
I just spoke with the homeowner.

Apparently the problem was at the compressor. According to the HVAC guys the compressor went bad and the high voltage wire came in contact with the low voltage wire. At least this is there diagnosis- I have not seen it so that's all I got. Now how did the low voltage cable come into contact with the HV cable is another question and why 46V and not 120v or more.

Maybe someone can answer this-- if one leg of the 240V circuit touched one wire on the thermostat would it be possible that there was 120 to ground but 46V across the T-stat conductors?
 
If a 120v wire touched the common conductor in the condenser, I think it would be 120v back to the furnace, and through to the t-stat. But if it was the switched wire it goes through the circuit board and is like dividing by zero to me. I don't know electronics, but I've seen strange voltages going through circuit boards when things go wrong.
 
According to the HVAC guys the compressor went bad and the high voltage wire came in contact with the low voltage wire.

I think you can probably take that explanation with a grain of salt also. We also don't even know for certain if that 46 volt "reading" is accurate.

...if one leg of the 240V circuit touched one wire on the thermostat would it be possible that there was 120 to ground...

Neither side of a CL2 circuit is supposed to be grounded.

-Hal
 
Dennis Alwon said:
Maybe someone can answer this-- if one leg of the 240V circuit touched one wire on the thermostat would it be possible that there was 120 to ground but 46V across the T-stat conductors?
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Although one leg hit the hot, the other leg is going to its normal place, and therefore the voltage between the legs will depend on what is between the second leg and either ground, neutral, or indeed the other hot of the 240V circuit. You'd need to sit down with a full chematic to work it out in detail.

For example, the control circuit transformer could be acting as a buck transformer.
 
This probably way off but is it possible two 24v transformers were either in series or parallel. I forget if series doubles the voltage or amps.
 
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