A/C condensate trap allowed or not

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Old 10-13-20, 10:17 AM
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A/C condensate trap allowed or not

My new (installed July 2020) Carrier AC system (coil is a CNPVP model) has developed a weird smell (presumably Dirty Sock Syndrome). The installer came last week and sprayed some cleaning chemical on it and the smell came right back just as soon as the cleaner dissipated. I'm having duct cleaners come tomorrow, but in the meantime, I've been wondering if the condensate drain could be involved.


The installation instructions read (I'm paraphrasing): "Condensate trap not required unless required by local ordinance)."

I presume its not required because the drain is on the positive side of the air flow and therefore its impossible for water to flow back.

However, the weasel words about unless required by local ordinance makes me think that it is in fact entirely possible for back flow to occur.

So my question is: what would be downside of putting a P-trap on then condensate drain line *somewhere* : either at the coil or at the termination outside my house?

Also, what the heck is going on the left side of that T-connector, what is that vertical pipe going to be used for?
 
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Old 10-13-20, 10:51 AM
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I see a trap on all AC condensate drains. It's there to prevent the air handler from sucking air into the system through the drain piping. The trap is NOT there to prevent water from being sucked back into the unit. Without a trap you could be getting yuckie air sucked through the drain piping and into the HVAC and house.

If you install a trap put it in the horizontal run next to the flue in your photo. Ordinarily I would not glue the trap in place to make it easy to remove for cleaning but since this appears to be an attic I would glue it to remove the chance of leaks.

The vertical pipe to the left is probably a clean out, especially since the cap doesn't appear to be glued in place.
 
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Old 10-13-20, 12:44 PM
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Thank you for the quick reply and correction about air vs. water flow back. Any thoughts as to why the installation manual would not just say to put a trap in place?
 
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Old 10-13-20, 11:27 PM
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It's there to prevent the air handler from sucking air into the system through the drain piping.
Or to keep cold air from blowing out the drain line.

The biggest need for a trap is if your air handler runs in a vacuum. Condensate won't drain under a vacuum so the trap can break that. The trap won't cause a smell like you have.

You posted in the air conditioning forums.
The dirty sock syndrome usually affects heat pump systems.
 
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Old 10-14-20, 07:04 AM
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You posted in the air conditioning forums.
The dirty sock syndrome usually affects heat pump systems.
I have noticed in researching this issue that DSS is most often associated with heat pump systems, but considering that it is believed to be a moisture based problem, I see no reason why it can't happen on a furnace + AC like I have. I can tell you that my AC coil is a very very wet place, many gallons of water are produced by it daily.
 
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Old 10-14-20, 07:30 PM
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I can tell you that my AC coil is a very very wet place, many gallons of water are produced by it daily.
Basically everyone's is the same. The amount of condensate is based on humidity levels.

What causes the DSS is either the coating on the evaporator coil or the type of metal used.
The heat when in heat pump mode usually triggers the problem.

Is your furnace sitting in an overflow pan with a float switch ? If not.... it should be.

You can install a 90 in place of the tee. Make two cuts in the pipe at the red lines. The trap can go in the top and you'd have to cut out a small piece on the floor. OR you could wrap the drain line around the furnace and put the trap on the side we can't see.

The Rectorseal EZT is a great pre-made trap or you can make your own.

 
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Old 10-19-20, 10:57 AM
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Thanks again to everyone for the responses.

To update the situation, last Wednesday a duct cleaning company came and using some serious compressed air and vacuum machines, cleaned both the supply and return ducts. They also cleaned the blower and ac coil. And the smell returned by Sunday

Regarding the p-trap, I've got the equipment and supplies all ready to go, but I'm not sure it will help too much now. When the system is running and I remove the cap on the current setup, and a very strong stream of air comes out of it. So much so that I find it highly unlikely that there is much air going back into the coil housing from the rest of the pipe work. But I'm not ruling it out entirely.

If I do decide to install a p-trap, I'll be sure to share my handiwork here for everyone's edification.

But right now, I think my next DIY step will be to remove the coil cabinet access panel and try to see exactly what is going on on the coil and drain pan.
 
 

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