Condensate line question

This home is 2-story, air handler is on 2nd story, condensate drain comes out and straight down the side of the house. I was always told it needed a P trap - but I was never sure why. Other than the fact that something can crawl up the pipe…

The trap is usually located at the unit. Did you not see one there?


It is to keep conditioned air from being blown out the condensate line (and therefore lost in most cases). This is assuming a unit with positive air pressure at the drain pan.


Another point…it needs to be extended away from the home. It’s causing damage.


May not apply in this config, but a secondary or auxillary overflow drain line may be connected to the drain pan installed with the equipment. This overflow drain should though, discharge to a conspicuous point of disposal to alert occupants in the event of a stoppage of the primary drain. Just something to be aware of. But like Marcel said, tough to see

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Thomas, no there isn’t one there. I guess it could be inside the plenum or in the subfloor. And yes, I see the water damage to the stucco - not a good thing. I’m going to recommend it be extended to ground level, then out away from the foundation. maybe a P-trap inside? Notably, but It didn’t register at the time, but there was cold air coming out of the end… imagine that!

I believe its actually the opposite. When the unit is running, it has a negative air pressure. It can pull air into the unit, which can then prevent the water from draining, and could lead to backups.

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My comment was for a positive pressure system, which is what I’m used to seeing. A negative pressure system will have the effect you describe. Here is a good article on the difference…


It also keeps from sucking in.

Any air that comes in or out causes the building pressures to change elsewhere in the building. Something you may want to avoid, aside from energy loss scenario.

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It’s not always negative, and there are several potential issues like the one you mentioned.


So do you recommend there always be a trap installed, even on a positive pressure system?
Because I see them missing all the time, but I always see the positive pressure systems

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Condensate lines have been required to discharge a minimum of 1’ from the structure in Florida since 2010 (including 2014, 2017, 2020 and [draft 2023], reference 2010 FBC Residential R318.6 Protection against decay and termites.
Condensate lines, irrigation/sprinkler system risers for spray heads, and roof downspouts shall discharge at least 1 foot (305 mm) away from the structure sidewall, whether by underground piping, tail extensions or splash blocks. Gutters with downspouts are required on all buildings with eaves of less than 6 inches (152 mm) horizontal projection except for gable end rakes or on a roof above another roof. That looks like a Lennox Compressor label (although I can’t actually read it), but if it is a Lennox system, the installation instructions for the unit typically call for the trap to be installed at the unit.

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Dave - thanks for the info about the condensate line.,… one of those things I know,and it actually makes sense- but had forgotten I knew it!. That is actually a Rheem system. Here’s what it looks like inside…

Think about Radon, mold, moisture pulled through the building envelope condensing inside the walls, back drafting CAZ, and of course energy loss. Just to mention a few.