Hmmm. First, I don't like caulking the toilet to the floor. If you end up with a leak under the toilet, I'd prefer the liquid to come out to where you can see it instead of being captured and pooling under the toilet, and slowly rotting the floor out sight unseen.

As to the smell? With the house being used on a part-time basis, you may have a trap that is going dry.

Traps are designed to hold water, and the slug of water that is held in the trap acts as a barrier between the free air in the house's drain/waste/vent piping and the free air in the living space.

If the trap dries out, which it may due to evaporation, then sewer gases can enter the house through a fixtures drain. It'll be even worse when you turn the vent fan on and depressurize the room, the room may be getting it's makeup air from the vent pipe.

Not good.

The next time you visit, if the room doesn't smell, first prime all the traps by running water into each plumbing fixture. Run the water in each sink, each tub, each shower, flush each toilet.

Wait a bit, see if you notice any odor.

Then run the fan.

Since the odor seems concentrated in the shower, as a worst case, I know that sometimes showers are plumbed without a trap. After you run water into the shower's drain, can you look down the drain and see standing water?

Best of luck with this...

Mongo

Mongo,

Is it required to put a trap in a shower? Above you said

"After you run water into the shower's drain, can you look down the drain and see standing water?"

Should you see standing water? The reason I ask is because I too have an odor in the shower (I'm pretty certain it's sewer gas) and I want to know what to ask the contractor and be prepared with questions and answers should he try to blame the odor on something else.

Thanks in Advance
Vern

I'll check for water in the drain tonight. Our (college student) kids are supposed to run water from each faucet & flush the toilet in the master bathroom once a week or so, but I doubt that they remember.

Should the shower have a trap? If so, how is that done when the house is built on slab? Should there be water in the shower drain?

I did not smell the odor when we first arrived, and I ran water in the shower, tub & both sinks. The next morning, after the shower & fan had been used, I noticed the odor. Initially, it seemed to be coming from the corner tub or right side vanity. Now the odor is stronger near the shower. There was no odor in the water closet last night or this morning.

I'll try to check back soon, but the WiFi connection at the hospital is weak. (I'm taking day shift while dh takes night shift for one of our children.)

Thanks for your help.

I had the same trouble a few months back.

We have a bathroom with a shower that gets very little use. An odor developed in the bathroom.

I tore it apart, bleached and scrubbed everything to no avail. Turns out the trap had gone dry.

All drains must have a trap.

Just pour some water down the drain and it'll close the trap. I'm told down here where it's so warm people some times pour a bit of oil down the drain. It floats to the top of the water and the water doesn't evaporate so quickly.

oil down the drain. Vegetable oil only! (I don't know if it works.)

If the new fan exhaust is anywhere near a plumbing vent, you might be getting a backdraft.

"If the new fan exhaust is anywhere near a plumbing vent, you might be getting a backdraft. "

That's one of my fears, Worthy. It will be a while before I'm at the house during daylight hours and can look to see how close the two are on the roof. Meanwhile, I've put electrical tape over the switch so that no one turns on that exhaust fan.

Okay, next dumb questions:

How much water should it take to refill a dried trap? I've been running water in the tub for only a minute or two.

I just peered down the shower drain with a flashlight. There a space, maybe 3/8" wide and 1/2" deep around the white pvc drain pipe, under the metal cover, that's mostly grey and it collected a little of the water from the shower. The drain itself curves sharply about 4" down from the shower floor. All I can see is purple stains (sealant?) inside the white pipe.
I could not see any water, even immediately after running the shower, probably because of the bend in the pipe.
Would it make sense to pour a bleach solution down the shower drain?

I'm baffled why the odor has begun now, when the house is about 3 1/2 yrs. old and never had this problem. The only things different now compared to prior years are the new exhaust fan and the extreme drought. I'm blaming the drought for the recent increase in floorboards & crown molding separating. In a nearby neighborhood, it also caused a sinkhole that destroyed a house. I don't suppose I could blame it for the odor, though.

It doesn't take much water at all to prime a dry trap, a couple of cups will do it.

The trap should look something like this:

Except it will be a solvent welded trap instead of a threaded one as shown in the picture.

Premature posting. I wanted to add more to the above post before I submitted it.

The "U" part of the trap is what holds water. If you look down the drain you might be able to see a portion of the pool of water that is held in the trap.

The odor could also be hair and oil conglomeration that has been brewing in the waste pipe for a while. Hair from your head, oils from shampoos, conditioners, and soaps.

I've snaked some waste pipes and had slugs of "stuff" several feet long come sliding out of the waste pipe. Ugh.

On occasion I have seen vent pipes on the roof plugged up. Anything from bird nests to frost build up during cold weather.

If the house is on a slab, an area about 18" square around the proposed drain location is normally boxed out. No concrete is poured in the boxed out area when the slab is poured. The drain pipe is piped into that box. Later the plumber can come in and set the trap in the boxed out area, then fill the boxed area as the shower pan is built.

So...it could be a dry trap caused from lack of use/evaporation. That's the most common in a seldom used bath.

It could be a blocked vent pipe.

If your vent pipe is blocked, sometimes water sent down a drain in one part of the house can cause that same trap or a trap in another part of the house to siphon dry. An indication of that problem might show itself when you flush a toilet upstairs and see the water in a downstairs toilet bowl shimmer or bubble. Upstairs laundry can cause that as well.

Siphoning can also be from the horizontal pipe extending off your shower's drain being too far from the vertical pipe that vents the shower.

It could be a hair ball that's been smoldering on the pipes for a while.

Do you get a good pull from your exhaust fan? When it's running, is there enough suction to hold a paper towel or a section of toilet paper up against the grill?

Mongo

"Do you get a good pull from your exhaust fan? When it's running, is there enough suction to hold a paper towel or a section of toilet paper up against the grill?"

I checked it again, this time with a little toilet paper since it didn't pull the dryer sheet. It did finally pull the tp to the exhaust fan when I held it right next to the center (it's a Panasonic WhisperCeiling, not sure of the CFM rating.)

This is a one story house, so maybe dh can check the vent pipes from the roof this weekend. Meanwhile, I'll try some drain cleaner; it would probably be a good idea in the kids' tub/showers anyway.

Thanks again. Right now, every little problem like this is magnified by sleep deprivation and emotional exhaustion.
I'd like to call the plumber back, but can't be here to meet anyone. Maybe the next tme someone asks what they can do for us, I'll ask if they can housesit for a few hours to meet the plumber & give them a check to fill in.

We had a similar problem. It took months to figure it out. Sometimes there was odor-sometimes not. When it would smell, we would try to figure out where it originated,etc. I even kept a journal. What it ended up being was (in the crawl space) a connection on a drain pipe was just ever so slightly loose. This was discovered when DH checked for a second time. It wasn't loose enough to leave any drips or other signs. I think he turned it less than a 1/4 turn. What a relief!

Well, this odor problem may be resolved with a new bathroom installation. Another plumber came today, and after much exploration, using a camera, etc., he said that that the drain for the tub and sink(s) is holding waste water in an area about 12 - 15 inches long. The odor was horrible when the plumber removed the side panel of the garden tub, so it appears there's a crack someplace but the camera didn't pick it up. Making the repair, because this house is on slab, requires removing the tub, vanity, tile, etc.

We have to find a remodeling contractor to do the removal/demo, then the plumber will make the repairs, and finally the contractor will replace the tub, vanity, floor, etc. The plumber is bringing out his boss on Monday to confirm his findings. Meanwhile, I have to find a contractor or two and get estimates.

I will never, ever, build or buy a house on slab again.