Plumbing – Adding bug screen to 3/4 PVC

No, there should not ever be two traps on a single drain line.

The point of the trap is to create a water barrier which prevents sewer gasses from coming up the empty pipe and into the house.

When you have two traps on the same line, you end up creating a kind of vapor block in the line which prevents it from draining properly.

What do you do?

Get yourself some 1 1/2" PVC (not CPVC, which is yellow, PVC which is white) with a 90 deg elbow, and also get two Fernco couplings (black plastic couplings with straps on each end that can be tightened) that will fit the pvc and iron drain pipe.

Use a metal blade on a sawz-all and cut out the p-trap in your basement. (Have a bucket handy!) Leave room to connect the Fernco coupling on both sides. I'd cut the right hand side out just past the coupling.

Measure and cut the PVC so that with the 90 elbow the ends line up with the newly cut drain lines. Glue the PVC together and confirm the fit, then connect to the old drains with the Fernco.

Viola - a proper drain line that will drain smoothly w/ no vapor lock.

You are lucky there is a vertical pipe. The teapot lid is often at the bottom of the chamber, so it needs to be pumped out for access. As you suspect, this should enable cleaning out the main house trap. The pipe with the flap that is under water is to access the pipe beyond the trap. Either the pipe beyond or the trap can be the cause of the blockage. Dealing with the pipe beyond likely requires professional service with root cutting equipment.

So lets deal with the trap and hope that fixes it. You need to feed a plumber's snake down the pipe and through the trap to break up the blockage. I've also had luck simply jetting out the trap with a common garden hose with maximum water flow. How successful you are depends on the nature of the blockage. It may also require professional service with the proper equipment.

If you do require assistance, enquire about the cost of removing the main trap. It is typically no longer required by building codes and would eliminate a common trouble spot. Sometimes it's an easy job and other times it means digging up your garden.