I live in a house that was built in 1978. In both bathtubs we could hear the septic aerator echoing up through the pipes along with an odor. I placed a line trap out side and a clean out. This helped out a lot. Now no echo but still odor from time to time. I just noticed the drain for the central ac unit was poorly put together (about a 1.5" pipe placed into a 2.5" pipe just laid over into it). I don't think it's the vent because I don't have problems draining. I hope you might have some ideas, maybe messed up drum trap or p trap.

This is why every fixture needs a trap (and a properly vented one at that). Otherwise, you will get the odors from the waste that builds up over time in the pipe. Do you have traps under all sinks, tub, showers etc (excluding toilets, as the traps are built into the bowls.)

I Have no way of knowing if there is any kind of trap under the tubs without tearing the tub out and digging down into the slab. That is why I need ideas other than that. Digging into the slab will probably the last resort.

You said " 1.5" pipe laid into 2" pipe". If I understand it correctly, you have A/C condensation line connected into sewer vent. If that's the case than it is wrong. A/C discharge has to be connected into drain before trap otherwise sewer gas will use A/C condensation line as a sewer vent. Please, confirm it is not the case. Back to you. Milo

This sounds like a venting problem to me. Septic tank aerator is probably contributing to odor problem if vent is partially blocked.

Milo I think you are right the pipe goes tees down to Sewer and up to vent. I assume there is a trap under the slab but I will not know without tearing up the slab. I guess I could make a trap before the t and secure it air tight. I'm not sure this will make the bathtubs smell on occasion though.

hkstroud: I vented the cap on the tank thinking the same thing. It has a .25 hole and you can feel the air blowing out and still have the problem.

I was thinking a block vent in your plumbing system.