I have recently installed a new tub. The drain pipe (1 1/2 ") in the floor has a p trap under the concrete. I also noticed that the tub itself has a small p trap just after the outlet from the tub. These two p traps are approx 3 feet apart from each other.

The problem I have is that when we fill the tub fully and open the drain, it all drains fine... no problems. However, if we use just the shower or put very little water in the tub (such as to clean it), then the water doesn't drain (or drains very slowly).

Is this problem related to having two p traps along the same drain or is there some other cause for this? How do you recommend to check it and/or solve the problem?

Thanks

Since it's a new tub you should still have the brand name and model number.
Would you care to share the information?
Standard Pumbing Code, Chapter 7,Section 703.6 titled "DoubleTrapping states, "No fixture should be double trapped.

The problem I have is that when we fill the tub fully and open the drain, it all drains fine... no problems. However, if we use just the shower or put very little water in the tub (such as to clean it), then the water doesn't drain (or drains very slowly).
the reason the tub drains when fuull is the head pressure and the weight of the water pusheing the discharge through both traps. With little or no head pressure,such as a shower, you are are air locked by the two traps.
The solution would be to remove one of the traps. Could you take a picture of this "small "P" trap" for me? Regards, Tom

Thanks. That was exactly the problem.

I simply connected the vent in between the two traps and now it works fine.

The tub model is Christina Moda (Christina is the brand). I didn't really want to modify the tub since it is a fairly expensive whirlpool tub. Venting was the fastest way to fix it. We still have double traps, but siphoning and/or air locking should not be an issue any more.

Thanks again!