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An Uneasy Evening...HVAC & Electrical Guys

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Late this evening before bed, we could smell something burning in the house. We couldn’t tell where the smell was coming from.  We got up in the attic. We started pulling off switch and wall plug cover plates around the house. Sniffing for smoke. We could not positively ID where the smell was coming from. We started shutting off electrical breakers. I started thinking about a night watch. We both would not got to bed at the same time. 

Our home has in floor heating with a boiler. We don’t have a central furnace moving air around. So a central fresh air exchanger is utilized. There are air vents and ducting all over the house. Fresh air is pulled in from outside. Warm air from the house evacuated. The warm air leaving heats up the cool air coming in. We’re not talking a lot of air here. Just a small amount, but steady. 

One part of the house, it we could smell the burning, more concentrated . We could not find out why. I got up and stuck my nose near the fresh exchange air vent. Yes! I could smell the burning smell stronger than ever. It was entering the room via the fresh air vent. 

I bolted to the basement and opened up the fresh air exchange system. I touched the blower fan motor. Holy crap! Smoking hot. Literally. We immediately flipped the breaker at the panel and shut it down.  I measured 350F at the blower motor with my laser thermometer. 

I removed the motor to have a look at it. I‘ll get a new one. On the motor it reads “Thermally Protected”.  I wonder at what temperature it is designed to shut down? Anyone know?  Seems to me it was about to burn up. Middle of the night? Scary thought. Hopefully the 15A breaker for it’s circuit would have tripped. 

I’m no electrician. A few encouraging words about built-in safety would be reassuring.  
Thanks. Keith 





Edited by keithb7
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  • keithb7 changed the title to An Uneasy Evening...HVAC & Electrical Guys

I think you dodged a bullet there, at 350 F it was above what a motor with Class B Insulation is rated for. The thermal protector is not functioning in that motor, it should have opened before it got that hot. Here is a copy paste..


"Another point: for safety’s sake, no one should be touching most electric motors in the first place, unless they are specially designed to have safe surface temperatures. Such motors include those used on bench grinders, power saws and the like. For those applications, Underwriters Laboratories sets maximum acceptable surface temperatures for a metal “surface subject to casual contact” at 70 C (158 F) after 30 minutes of operation in a 25 C (77 F) room. Even at that temperature, however, you don’t want to touch the surface for long.

The surface temperature of a continuously (and correctly) operating general purpose industrial electric motor will easily be 80 C (176 F) and perhaps as high as 100 C (212 F)."



Edited by linus6948
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You have me thinking I should check all my breakers with the thermometer, to see if any are heavily loaded.


That 1.12 amp motor wouldn't trip a breaker until the hot wires literally melted and sparked, but it should shut off below the spec of the insulation.

You didn't quite get there, but you likely smelled the dust burning off the motor before it just quit.


Did the bearings gall?



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On 1/10/2021 at 12:25 AM, Ulu said:

Also I hope you changed the capacitor.

The motor is pooched. It is resists any turning by hand. I got a new motor today with a capacitor.  Replacing everything now. 

Yesterday I became a welder. Today I am a HVAC guy. Lol. “Dabbles in everything. Good at nothing.”


All back together working again now. I can tell someone was in there a few years ago and changed 1 of the motors. (There are two)  We’ve owned the house for 3 years.  The Recent failed motor, I’d wager was original. Made in USA. The House is about 25 years old.  The motor changed by previous owner was made in Mexico. The exact same brand-name motor I bought today? Made in PRC.  We just keep digging lower. 


Edited by keithb7
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Good you discovered it before it got ugly. 
jack of all trades, master of none, thats something people have called me. I havent tried bricklaying yet, so who knows? 

guaranteed you wont get 25 years out of the replacement. Too bad,with all the technology and advances in metallurgy and lubrication these things should last longer. And in our “envirnmentally friendly” society longevity should be the mandate. 

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What I was suggesting is that when you put in the new motor you must always put in a new capacitor, even if the old one seems good. They have a variable but limited lifespan.

The HVAC that I bought in 2005 has a computer on the blower motor because it’s a DC servo design.


Contractors won’t replace one without the other, because they don’t know how to test either one and they can’t guarantee their work otherwise.


If they get a bad component and they have changed both parts then they can take both parts back to the vendor. Otherwise the vendor will look at them and say it is the other part.


They’re going to sell you both parts either way. It was the same way with capacitors and motors. The vendor wouldn’t guarantee the motor unless you changed the capacitor too.


Edited by Ulu
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