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Old 06-26-2011, 10:09 PM   #1
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Burning smell from vents when ac turns on


New to us house.

There is a burning smell that comes out of the vents when it first turns on. Just for a second or two. If you are standing by a vent, you can smell it when it turns on for just a second. Then it goes away as the air continues to blow. Has anyone seen this type of behavior before? Possibly a circuit board shorting for a few seconds?

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Old 06-26-2011, 10:19 PM   #2
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Burning smell from vents when ac turns on


Have you changed the air filter lately? I would get a Pro to do a checkup on it and see if the evaporator indoor coil is dirty or has bacteria in the drain pan and check the freon level etc. We cannot fix smells over the net. Circuit boards don't burn and smell.

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Old 06-26-2011, 10:23 PM   #3
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Burning smell from vents when ac turns on


Coil is clean and so is pan. This is definitely not a mold smell. Smells like 'gun powder'.
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Old 06-26-2011, 10:33 PM   #4
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Burning smell from vents when ac turns on


How new to the house are you and is it a newly built home. I have seen everything from half eaten ham sandwiches kicked by tradesmen into cold air return vents to dead hamsters etc in there, and a dope stash. Something rotten may be in there or in the fan compartment. May want to get the ducts cleaned (robotic duct cleaning is good and some have a video camera on the end of the machine to see in there) unless you have an attic unit with flex ducting.
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Old 06-26-2011, 10:41 PM   #5
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Burning smell from vents when ac turns on


I have had multiple issues like this. Usually turns out to be circuit board or fan motor. You could disconnect power to air handler and inspect back of circuit board for dark black burn marks. If this is the case the board will be going out very soon. If u do this be sure u can put it back together. Another possibility is the t stat, smell the stat and see if that's where its comin from. Really u can just follow ur nose and find which component the smell is coming from. There are other possible causes like burnt insulation in the heat chamber. Good luck
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Old 06-26-2011, 11:38 PM   #6
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Burning smell from vents when ac turns on


Just opened up the air handler and stuck my nose around the circuit board and blower. Let it cycle with it open and couldn't smell anything. And I sniffed hard. Weird. I don't know, maybe it's the new UV bulbs burning stuff off in there? Have uv bulb in the inside and outside of the coil. It's all I can think it could be at the moment even though i havent noticed this on the other 2 units which have much dimmer, older uv bulbs.
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Old 07-12-2011, 08:22 AM   #7
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Burning smell from vents when ac turns on


I have now noticed this slight burning smell from other vents supplied by a different air handler. Same smell, a burnt smell for a few seconds when it first cycles on. When I open up the air handler, I don't smell anything burning, and can't see any fried boards or components. At this point, I can only speculate that it's the blower motor putting out that burnt smell as it winds up.

How can I test the blower motor on these two units?

Anything I can lube or oil?

These units are over 15 years old.

Thanks.

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Old 07-12-2011, 02:44 PM   #8
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Burning smell from vents when ac turns on


Has anyone else experienced this burning smell on an older blower before it goes? Is 15.5 years past a blower's rated life?

Again, the burning smell comes from the vents only for a few seconds when the ac first kicks on. I now notice this smell from vents supplied by 2 of 3 air handlers in this house.

I'd really appreciate any further info on how I can make this smell go away as cheaply as possible, hopefully avoiding having to buy new blowers.
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Old 07-12-2011, 03:02 PM   #9
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Burning smell from vents when ac turns on


You said "air-handler". Do you happen to have a heat pump and electric heat strips?

If the heat strips are coming on for some reason you might get that "toaster oven" smell. They shouldn't be coming on though if you are just using the AC.

Disregard this if you have no electric heat strips.
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Old 07-12-2011, 04:21 PM   #10
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Burning smell from vents when ac turns on


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Originally Posted by fabrk8r View Post
You said "air-handler". Do you happen to have a heat pump and electric heat strips?

If the heat strips are coming on for some reason you might get that "toaster oven" smell. They shouldn't be coming on though if you are just using the AC.

Disregard this if you have no electric heat strips.
I'm not sure, to be honest. When I open up the Lennox box, it has only a coil in it from what I can tell. I know that you can select heat from the thermostat, but to be honest, I'm not sure how the heat side works with these units
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Old 07-12-2011, 04:31 PM   #11
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Burning smell from vents when ac turns on


here is what i have inside
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Old 07-12-2011, 04:47 PM   #12
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Burning smell from vents when ac turns on


I just cycled the heat.

First came the familiar "burnt motor" smell as the blower first kicked on, then followed by the usual "burnt dust" smell that comes when you first turn on the heater after it hadn't been used in a while.

I'm still thinking its the blower motor, or something related to that when it first turns on. But I can't smell anything apparent when I stick my head in there close to the blower when it turns on. It's as if whatever burns at the start gets pushed into the vents too quick for me to smell there.
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Old 07-12-2011, 07:45 PM   #13
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Burning smell from vents when ac turns on


Can the motor or any wiring "see" the UV lights? A good UV light destroy any plastic, motor winding coatings,wire insulation ect. that it comes in direct contact with. Guessing the one above the coil is causing damage since that's where the motor is.
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Old 07-18-2011, 09:03 PM   #14
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Burning smell from vents when ac turns on


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Originally Posted by Marty S. View Post
Can the motor or any wiring "see" the UV lights? A good UV light destroy any plastic, motor winding coatings,wire insulation ect. that it comes in direct contact with. Guessing the one above the coil is causing damage since that's where the motor is.
It actually turned out to be the UV lights causing the burning smell. I don't see any degradation on any of the wires' insulation that comes into contact with the upper UV light. Very strange. Looks like more options are to somehow shield everything, run the blower fan 24/7, or shut them off. Any ideas how to 'shield' the wires from UV light? Wrap them with tin foil?
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Old 07-19-2011, 07:10 AM   #15
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Burning smell from vents when ac turns on


You can build a shield, not sure about foil though. I'd just move the upper light into the return.

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