I have an existing central AC unit and the condensate line exiting the inside air handler is 3/4" SCH 40 PVC pipe. From there is goes below grade into the concrete slab.

25' away it where it exits the house it is a 1-1/4" PVC pipe. I don't know why there is a size change, but I supposed it is OK.

Normally I would pipe in a p-trap there to stop ants, spiders, roaches from going up the condensate line. But in this case the line is 1-1/4" in size, and the bottom of this 1-1/4" pipe is only 1" above a concrete pad. There is no room to put in a 1-1/4" p-trap.

A 3/4" p-trap (or running trap) may work but I can't reduce the 1-1/4" to 3/4" because it will create a "step"...unless there is such an animal as a 1-1/4"X3/4" coupling that is "eccentric" such that the bottom of the 3/4" is at the same level as the bottom of the 1-1/4" side?

Why not a reducing el sum?

My experiece is that the p trap is located somewhere near the unit.

why not a PVC vent screen with fine wire mesh to at least keep the bigger spiders out?

Some I don't know if the fire ants in Florida are more ingenious in the fire ants in Texas or if you even have them but I doubt that they would be able to negotiate up a pipe that long I would have common sense to go around it to invade your house.

Paul48, a reducing el 1-1/4"X3/4" may work, but it's not a common item at a big box or even local plumbing supply stores. The center of the pipe is less than 3" from the top of slab.



I end up using a 90 elbow, followed by two 45s. I assembled them first, then turned the 90 until it touches the slab. The first 45 (labeled "A" in the picture) becomes the trap, then I "rolled up" the second 45 until the water surface in it appears to be slightly higher than the inside of the elbow A.



That's kind of my "V" trap for now. I didn't glue anything because I need to take this thing apart every month to rinse out the algae build up.

NP16 down here in south Florida we typically put the p-trap outside where it exits the foundation. It could be located inside near the air handler I guess but most depends on how it's plumbed and whether a condensate pump is used. I prefer to do it outside because due to slow condensate flowing it tends to build up slimmy glooey stuff and if you have it inside you can't really examine it easily, if it's outside you can just take it apart and rinse it with a garden hose.

a lot of insects - ants, termites, carpenter ants etc...like to follow water.

Yeah!! I got your Point. But, why don't you fix a PVC vent screen with fine wirework to keep the greater bugs out? These creepy pests can get inside from anywhere, but, its a work of contractors to fix the PVC pipes in such a manner that no one can get inside the hole of Pipe. Always hire a professional HVAC contractor like commercial AC repair Santa Barbara ( [www.santabarbarahvaccontractor.com] ). The experts know how to deal with issues and to fix the problems.



Edited 3 times.