We are getting a septic smell from the washing machine drain (which is
in the wall of the main floor).
It's the only place in the house we get this.
Could this be due to the water blasting through the trap with too much
force? If so, how do we rectify it?
It could indeed be what you suggest. If so, it means the trap was
The trap should have a vent connection just downstream from it.
This allows air to be sucked in as the flood of water from the
washer passes by. That prevents the water in the trap from all being
siphoned out at the end of the flow.
Try this test when you have the odor:
Slowly pour a quart of water into the drain opening where the hose goes
If the odor goes away after a while, the trap was dry.
If the odor does *not* go away, there is no trap <ugh>.
How you fix this will depend on what access you can gain
to the trap connections.
Pick up a basic plumbing repair book (library or even at HD)
to see how traps and vents are supposed to be connected.
If no vent, you *may* be able to use a mechanical vent to
solve the problem:
I think Jim and the OP is on the correct answer..
The codes I have worked with (40 yrs ago) requied a vent within 18-24" of
the trap to prevent a siphon.. However, contrators and appliance installers
tend to ignore this requirement (in fact most consider the vent stack as a
method of dispersing the sewer smell). One of the most important purposes of
the vent is to prevent the siphoning of water from the traps.. The water in
the traps prevent the sewer gas/smell from getting back into the house.
I you find that the trap is more than 24" from the vent, then you may have
to tee in closer to the trap and run a branch vent to the main vent stack.
Sorry for the toppost, Google's follow-up is confused today.
I suspect, as you do, that the force of the washer pumpout is
siphoning water out of the trap - assuming there *is* a trap!
The quick and easy fix is to keep jug of water handy to manually
refill the trap after each time the washer runs. If this doesn't fix
it, then maybe there is no trap in this drain, or the trap was made
wrong; I'm told that a trap needs a horizontal section after it, and
that traps that go straight into vertical drops are not effective,
though I don't understand why (more prone to siphoning, mebbe?).
You should investigate to see if this drain is properly vented, which
is also important in preventing siphoning. (If the trap or vent are
very wrong, it might be that flushing the upstairs toilet is siphoning
the trap or pushing fumes out it.)
Finally, make sure there's an "air gap" where the washer hose enters
the drain. Basically this means the hose has to fit loosely into the
drain, it shouldn't fill the drain. This is mostly to prevent
back-suction into the washer (ick).
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