T- Stat Caught Fire

[Dennis Alwon]

I got a call last night from a customer because her kids room was full of smoke. The firemen told her to call the electrician. I couldn't go so I got my friend- another EC to go check it out. Apparently the thermostat had 45 volts on it somehow and it melted the entire T-stat. Never seen this before. How does 45 V get into those thermostat wires. The T stat wires were not shorting to any line voltage wires.

To see the picture better hit ctrl + to enlarge or ctrl - to decrease.

 

[Howard Burger]

And this is why we install SD in bedrooms....

And this is why we install SD in bedrooms....

Holy cow! What did the wires on the transformer look like?

 

[Buck Parrish]

That's pretty scary.

The transformer must have screwed up.

 

[Dennis Alwon]

Some one suggested that the control transformer in the unit had an intenal short that changed it's turn ratio. I don't know what could have been done to protect against it.

BTW Buck, this is in the governors club.

 

[Mgraw]

Transformer winding probably shorted.


Edit: Must have posted at the same time.
An inline fuse on the 24v side might help.

 

[Mgraw]

Also a shorted or grounded 24v wire could have caused this. Seen it happen many times in older units. Newer units usually have a fuse on the 24 volt side.

 

[Dennis Alwon]

Edit: Must have posted at the same time.
An inline fuse on the 24v side might help.

How would a fuse help if the voltage just increased.

 

[kwired]

How does just an increase in voltage cause this? A thermostat unless electronic and using both L1 and L2 of the control circuit is simply a switch or multiple switches. There would have to be a very significant increase in current to cause damage like that to what is otherwise a class 2 circuit. The increase in current had to come from an external source or a fault within the supplying transformer.

Any damage to controls at the HVAC equipment? Is cable damaged anyplace else or just in vicinity of thermostat?

If there was a short between L1 and L2 then the class 2 source should only be capable of delivering limited amount of current or have overcurrent protection. Chances are available fault current was higher than what the thermostat could handle without catastrophic failure.

 

[cadpoint]

It’s detective time! What was the thermostat that being used? What was the source of power being used by the thermostat? Was there any service type work done at the house, days before?

This looks like one of those rooms over a garage! With the several cold spells we’ve had lately, this along could stress on any open connections like a low voltage transformer, exposure for hours and then shout off to not so much as a heated space tempered but stabilized to a temperature difference of thirty degrees and even more.

Note that the smoke damage is at the baseboard. Granted smoke will pour out of any orifice with enough pressure but if the toe board is burnt or charred there is a clue.

The heating unit needs to be inspected! The transformer of the that unit or the house hold bell ringing transformer if it is supplying the source of power should also be checked.

If battery is the only source than that model should be invested;thermostat battery failure.

Maybe someone could proof an equation to show how the voltage could get that high off of a 12v or 24v transformer. If not, use this link to input the basic numbers two numbers and keep the work range of 18 AWG in mind when you increase the voltage.

I would certainly use a meggar for this room and the room connected.
I'd have them remove the sheet rock to the toe board. Maybe even the wall below. great application of a snake light!

Maybe new circuit, truely new LV wire, and new thermostat! Guess it all depends on what all the facts are!

Hope I didn’t kill your thread!

 

[Dennis Alwon]

Again I was not there but the thermostat wire looks perfectly fine except for the where it tied into the thermostat. The power source was in the furnace and the unit is less than 5 years old. Typically gas heat or electric furnace setup. Nothing else in the home appears damage.

When the guy I sent out there called me I asked him to check the voltage on the T-stat wires. He said he was getting 45V across some combinations-- I didn't ask which ones- I'll assume the red and some other.

Could the voltage have been greater before the fire- perhaps, I don't know. I do know I have never seen this before.

 

[don_resqcapt19]

The power supply for a furnace control is almost always a Class 2 power supply and the output current is limited, often by a fuse in the transformer windings under the wrapping. I would be looking for some outside source of power.

 

[Mgraw]

Transformers in furnaces are usually 20-40va. They are usually protected by external fuses. The fuses are usually 3-5 amps. Thermostats are usually rated at one amp. I would be looking at relays, contactors, circuit boards, 24v wiring, and anything else on the secondary side. It is also possible the problem could have been in the thermostat itself. There is a thermostat supplied by utilities that has a known fire hazard.

 

[K8MHZ]

Participants in PEPCO?s Energy Wise Rewards program are being asked to remove the batteries fom their thermostats to prevent a potential fire hazard. If the batteries leak, the leakage can damage the printed circuit board in the thermostat, causing the unit to overheat and become a potential fire hazard.

http://rockvillecentral.com/2010/10/pepcos-thermostats-are-a-fire-hazard.html/



http://www.pepco.com/energy/conservation/meiin/thermostat.aspx

Thermostat Fires

Maryville Tennessee Fire Lt. David Williamson reported that a 24-Volt thermostat mounted on a wall in a Maryville Church ignited causing significant smoke damage. No fire spread took place because of limited fuel surrounding the area of the new multi-million dollar structure. Fire was discovered by a police officer checking the building. This is a case of combustible material being used in connection with an electrical device. Regardless of the failure mode the use of combustible plastics in an area of electrical installations can cause serious problems. This is one of several "thermostat fires" that we have confirmed.

http://www.arson-codes.com/webpages/ReportsandArticle/reports/firewarning.asp

Those seem to all be battery related.

My concern would be how the 45 volts came to be. A typical 10 VA transformer shows about 27 volts with no load via DVOM.

 

[nhfire77]

Maybe this is a dumb question because I don't know your buddy:

Was the 45 volts AC or DC?

 

[Dennis Alwon]

Maybe this is a dumb question because I don't know your buddy:

Was the 45 volts AC or DC?

I suspect a/c. How accurate is his meter and what kind he had I don't know.

 

[hurk27]

There area of concern I'm looking at is the area at or below where the smoke spot is on the wall at the base board, fire (heat) always travals up and the thermostat opening would have been a good exit point in the wall if the top plate was sealed, the 45 volts on the conductors could have been just an end result. to me there is too much missing info to make any clear statments as to what happened.

 

[dbuckley]

The power supply for a furnace control is almost always a Class 2 power supply and the output current is limited, often by a fuse in the transformer windings under the wrapping. I would be looking for some outside source of power.

I'm with Don.

This style of transformer can't (he says, sticking his neck out far too far!) produce enough power to cause that kind of event.

Perhaps "shouldn't" would be better than "can't"...

Is there some smoke damage from near floor level too? How is that involved and/or explained...?

 

[Dennis Alwon]

Is there some smoke damage from near floor level too? How is that involved and/or explained...?

That is a good point and I don't have the answer. All I can think of is that the t-stat melted and fell to the floor starting some damage at that point.

 

[hurk27]

That is a good point and I don't have the answer. All I can think of is that the t-stat melted and fell to the floor starting some damage at that point.

Without damaging the base board?

I believe the smoke damage at the floor level, was in between the floor below and looks like it chimneyed up the wall where the t-stat was.

 

[LarryFine]

Are we sure this is a low-voltage T-stat, and not baseboard heat?