Washing Machine Trap Not Working

[depawl]

My wife recently purchased a new washer and dryer, I had to do a bit of work on the inlet piping but the drain line is essentially the same as it's been for 20 years. It's 1.5" PVC with an S trap that goes down through the laundry room floor to the main house drain line in the basement. I did have to move the trap a bit closer to the floor (it's about a foot above floor level) to accomodate the new inlet piping. It's been working fine for a couple months, the other day my wife said she smelled something in the laundry room. After so investingation, I found that there is no water in the trap. This has me stumped. There is no evidence of leakage anywhere, so I can't see how the trap would not remain full. Any ideas?
Thankss.

[phishfood]

My best guess from your description is that the new washing machine is discharging water at a faster rate than the old one, and this increased flow rate is causing the trap to siphon itself dry. With a relatively small pipe size like 1 1/2", and no vent to allow air to the back of the trap, it wouldn't surprise me at all if this is what is happening.

What I would do, if at all possible, is to change the drain line out to 2", and use a sanitary tee above the floor. Out of the side of the T install a 2" Ptrap with a riser of at least 2 feet to accept the discharge hose from the washer. Out of the top of the T install a vent riser that sticks up several inches above the washing machine. Install an air admittance valve on that to allow air to be sucked into the pipe to vent the trap.

[depawl]

phishfood:
Thanks for the reply. I had thought about this also, but the new washer is one of those front loaders that uses very little water. It seems like the old one used so much more water and drained at at much higher volume for several minutes. This one uses so little water it's hard to tell when it's draining. Although currently I can think of no other possibilities.
thanks.

[majakdragon]

Phishfood is correct. Even though the new machine uses less water, the rate of flow when draining is probably twice as fast. "S" traps have been removed from many Codes because they are prone to being sucked dry and allowing sewer gas to enter the house. New Codes for washer drains require 2" pipe.