# Bad smell from central air unit



## sodusme (Nov 23, 2008)

We've noticed a bad smell coming from our central air unit. My brother who has worked in the HVAC industry for quite a while came and checked it out thinking it was the "condensate tray" that may have something growing in it. There is no easy way to access the plenum but we were able to look inside enough to see that the tray is angled enough that my brother says it should be draining 100% and thus have nothing growing in it. The "condensate line" also appeared clear of obstructions. He sprayed some kind of a cleaning mist into the fresh air intake vent and said that should get rid of anything that might be present in the system. It didn't....lol. We still have the bad smell....any other suggestions? I'm no HVAC professional so go easy on me with the terminology....lmao.


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## beenthere (Oct 11, 2008)

What does it smell like?


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## dac122 (Sep 5, 2008)

He checked some basics, which is good. Do you have a condensate pump and/or P trap on your condensate lines somewhere? Sometimes odors can get pulled back up the condensate line - which is what a P trap would prevent, and pump maybe. Also check for a low spot somewhere in your condensate lines as it doesn't take much water for beasties and yucky stuff to begin multiplying.

Please answer beens question as this might be another issue.


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## sodusme (Nov 23, 2008)

beenthere said:


> What does it smell like?


Its kind of a "sulfur" smell. Its a propane furnace and I'm 100% certain its not a leak from there but I would characterize the smell as almost a propane smell. We had a new secondary heat exchanger put in over the winter. I just asked the wife if we had this smell before this repair and neither of us can remember if we have....



dac122 said:


> He checked some basics, which is good. Do you have a condensate pump and/or P trap on your condensate lines somewhere? Sometimes odors can get pulled back up the condensate line - which is what a P trap would prevent, and pump maybe. Also check for a low spot somewhere in your condensate lines as it doesn't take much water for beasties and yucky stuff to begin multiplying.
> 
> Please answer beens question as this might be another issue.


There is a line that comes out at the bottom of the plenum that the condensate tray drains into.....that angles down into a T fitting and I noticed that the top of that T fitting is not covered with a cap should it be? Here is a diagram of the tubing as it leaves the plenum....


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## beenthere (Oct 11, 2008)

Have them check for a gas leak.

Ethyl Mercaptan may be what your smelling. Its the odorant added to propane so you can smell it if you have a gas leak. Could be your gas valve is leaking through. Or, a fitting is leaking.

Have it checked ASAP!!!


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## sodusme (Nov 23, 2008)

Wouldn't I smell it other times then when the central air is on? This smell is present only when the central air is on and blowing air through the register. Not to be a smart alec but wouldn't we have blown up by now....I mean my brother has been down there smoking and I've had a generator running near the door on one occasion.


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## beenthere (Oct 11, 2008)

Nope.

Need X PPM of gas for it to ignite.

The odorant is a lot stronger smelling, then the amount of gas in the air. The gas is also heavier then air. So it wouldn't rise into the house without help from another source of force to move it to there.

Your coil is on top of your furnace. So teh blower is before the coil. So I doubt your pulling in an odor through your drain lines since they are positive pressure while the A/C is running and blowing air through your registers.

It may be from something other then a gas leak.

Not to be a S. A. But.
Hopefully, you think enough of your family, to have a leak check done.


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## Flashheatingand (Sep 7, 2009)

You mentioned that the smell occurs only when the a/c is on. Did your brother spray the coil with a bleach solution, or just spray the intake. It's possible that the funk is in the coil.


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## sodusme (Nov 23, 2008)

beenthere said:


> Nope.
> 
> Need X PPM of gas for it to ignite.
> 
> ...


I'll check into the gas leak scenario this week.....




Flashheatingand said:


> You mentioned that the smell occurs only when the a/c is on. Did your brother spray the coil with a bleach solution, or just spray the intake. It's possible that the funk is in the coil.


No it was all we could do to actually look inside the plenum to see the condensate tray...there was no door or any access panel left to get easily inside. It should be noted also that the smell doesn't occur all the time when the a/c is on....it tends to come and go. 

Now when he sprayed the fresh air intake my wife and I both thought the smell disappeared for a while....its been cool enough here that we haven't really ran the a/c that much but when we started it up recently...about 2-3 weeks after he sprayed the smell is back. We poured a small amount of bleach into the sump pit...my brothers' reasoning was if the plenum or any of the piping is not sealed 100% tight (which it isn't) that it could be sucking up stagnant air from the sump pit or musty air from the basement as its quite damp in the basement. We both noticed the smell in the basement around the a/c-furnace and although it smells somewhat like propane...we both concurred that its a little different but still an obnoxious odor.


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## Flashheatingand (Sep 7, 2009)

I don't know about the smell. You just mentioned how it only occurs during cooling. If so, To eliminate the a/c, I would clean the coil. You might might suggest to your brother, that pulling the heat exchanger may help with access. It's can be a pain, but if he is a hvac guy, it should be feasible.


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## Flashheatingand (Sep 7, 2009)

Just looking at that pipe drawing, I noticed there was a tee. It's better to have an air gap. As it is, the sump smell can get inside of the furnace. With an air gap, the smell goes into the room, but not necessarily into the furnace/ air handler.


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## sodusme (Nov 23, 2008)

What is an air gap? I've suspected that it is coming from the sump pit....that water is pretty stagnant and doesn't get pumped out enough in my opinion.


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## dac122 (Sep 5, 2008)

If you're certain the odor is coming from the drain then just throw in a P-trap.


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## beenthere (Oct 11, 2008)

dac122 said:


> If you're certain the odor is coming from the drain then just throw in a P-trap.


Spray Febreeze or some other cented spray in it. And see if the odor spreads through the house.


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## dac122 (Sep 5, 2008)

beenthere said:


> Spray Febreeze or some other cented spray in it. And see if the odor spreads through the house.


I like this idea. :thumbup: Make sure whatever it is has a strong scent.


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## beenthere (Oct 11, 2008)

dac122 said:


> I like this idea. :thumbup: Make sure whatever it is has a strong scent.


Not too strong, or you could get a false result. Just from natural infiltration from the basement to the main floor.

If the odor is coming from the sump. Its not being pulled in through a positive pressure drain line. Its from return air duct/system leaks, and or an undersized return system.


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## Flashheatingand (Sep 7, 2009)

As dac 22 mentioned, a simple trap (Up-stream of the tee) on the condensate run should eliminate the sump smell.

As for the question about an air gap, for example... your washing machine. The plumbing waste line has a trap, then a riser in which the washer hose is inserted. The air gap is the separation between the waste-line and the washer hose.

Upon further review, of the drawing, it is likely there is already an air gap, but, there isn't any trap. Anyhow, you want to have both an air gap and a trap.

B.T., I like your advise about the Febreeze in the pit, I will have to put that one in the mental tool box.


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## sodusme (Nov 23, 2008)

So if I spray say some febreeze in through the pipe that drains into the sump pit....and that smell then comes into the house I can be pretty certain then that the smell is either the stagnant air from the basement or from the sump pit? Now how do I add a "p-trap" in the line...what do I need to do differently from how its made now?


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## beenthere (Oct 11, 2008)

Extreme;y doubtful its coming in from your positive pressure drain line(means air blows out that pipe, not drawn in).

Spray some Febreze in the sump, turn stat fan switch to on.

If no Febreze odor. Hope you have someone coming out to leak check for a gas leak.

If you get a Febreze odor. You need to seal ALL return openings in the basement and at the furnace.

P trap is self explainatory once you have it.


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## Flashheatingand (Sep 7, 2009)

I like the advise as for sealing everything up and troubleshooting. I would definately recommend adding a p-trap. Just google p-traps, it's pretty simple. 

Based on the readings, I think the smell is from the sump pit, but being there physically would help. In troubleshooting, it's a process of elimination. Make sure the gas line is sealed, make sure the coil is free of funk, and trap the condensate run.


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