Odor from tub drain. Cant find a leak or clog.

So, guest bath has sewage odor out of the drain. Goes away when water is run. Comes back in a couple of weeks. Trap probably going dry.

All other tubs currently and previously would not stink up for months when not used. I opened up the wall behind the tub and can see the trap. No discoloration of drywall underneath so no evidence of drips.

Trap itself doesn't look huge but not sure if there's even a standard for something like that. Any suggestions?

Was thinking of pouring dyed red water down just to make sure nothing is leaking. Is there anything downstream that would effect the effectiveness of a trap?

The trap must be vented to prevent siphoning. Can you see that there is a vent ?

Could be a pinhole leak in the bottom of the trap.

I was trying to find a vent in the pics I took of the construction but cant fund anything. How close must the vent be to the trap? I know I have vent pipe(s) in the house but have no idea how this particular trap is connected.

Is there a way to test for siphon?

Pinhole at the bottom of the pipe? Think I'd see some evidence of that.

How old is this house? How long have you lived in it? How many bathrooms? How many people live in it from day to day? Any new construction?

All other tubs currently and previously would not stink up for months when not used

Click to expand...

That's why I ask.

A tub would commonly be plumbed with a 1-1/2" drain, and the vent could be up to 5' from the trap, depending on which code applies to your location. Most of the U.S. would have it within 3.5'.

Whenever I go into a home, and find a garden tub in the Master with potted plants in it, I run a bit of water to refill the tub trap.
Traps dry out when not used. That's why floor drains are done with "trap-primers".
All you have to do, is run a bit of water every week.

Sometimes when I'm working in a commercial building, and the men's room has a bad odor, I will take a cup and fill the floor drain. Problem fixed!

Thanks folks for the quick responses. Here's a bit more info.

1 - New construction. 15 months old. That tub is never used but the one in the master is used less than 3-4 times a year and has no smell.
2 - Yep, pouring water in once every week or two would likely solve the problem. The engineer in me just takes over sometimes to try to figure out what it's different than any other tub I've had (that I've also not used). I mean the master tub will not stink for months at a time.
3 - Just 2 people in the house. There is a sink and a toilet next to that tub, the toilet and sink get used every day but the tub doesn't get used ever.

And now for a slight twist .... I don't smell it anymore. It stunk 2-3 days ago. I haven't added water but the odor is gone.

Do people really have trap primers? Never seen them before

An ounce of vegetable oil poured into the drain will float on top of the water and greatly increase the amount of time it takes for a trap to dry out.

Yes, we really do install trap primers. Next time you're in a commerical space, check to see if one of these is under the lav counter.



They may be exposed, or we may have a closet to put them in.
A trap primer like the one above will work with a few floor drains.

It's like I tell my younger son when he's trying to figure out an easier way to mow the lawn. Engineers have already worked on that one, it's called a "lawn mower".
There really are solutions for plumbing problems that have been written into the codes.

If you have smells in a bathroom.
Is the toilet sealed to the flange. Unless the closet flange had been installed "over" the finished flooring, it should have two wax rings. You can have smell through an un-sealed closet flange to toilet bowl connection.
Flushing a toilet "may" siphon a tub trap if it isn't plumbed right.
Given time, all bets are off and the water in the trap evaporates.



There is no trap primer requirement for tubs and showers.
We do have to prime floor drains.

Think I'm tossing in the towel on this one. I had the plumber come out under warranty and he poked around. Used the flexible camera thing I had to check water level inside pipe and it pretty much looked like it did when I looked at it last. There was water above the joint so water at the weir level. If you flush a toilet or run water, you can see the water in the tub trap ripple but not suction out or anything. Previously, I could smell it from the drain ... I mean I had my nose on the drain so don't think it's the toilet or anything.

If it continues, I'm pulling the toilet but for now I'm going to keep water in the drain and try to forget about it for a bit. Makes zero sense. If I pull the toilet, I'll be tempted to redo the floor

One more tidbit. So, back to the water ripple in the trap when the toilet is flushed. Not exactly accurate the way I described it the first time. The water moves up and down a little. Maybe 1/2" but it's hard to tell by looking at the camera. I can't remember what the plumber said exactly but I think the water moving wasn't a bad thing, if there wasn't enough water in the trap it wouldn't form ANY suction to move the water up and down. If there's too much suction because of a vent issue then it'd pull the water out and create a secondary vent.

In case that changes any of the diagnostics, I'll eagerly await a response otherwise back to the original planb of throwing in the tower and filling with water every few weeks.