I know nothing and am not sure I'm understanding this problem, but that said, I'm wondering if the toilet is set properly with a proper wax seal, which conectes the toilet to the waste pipe in the floor which goes to the septic tank. The sollution may be as simple as replacing the wax seal with a new one, taking care to seat it properly. Not sure what they mean by "dry trap", their should be a pipe up from the waste pipe which goes straight up through the roof the sewer gases escape.

Hopefully someone who knows more than I do will reply, but I am having a similar problem with the drain in the floor in our laundry room. Basically, this drain only exists in case the washer floods the floor. I think it is a "dry" drain meaning that water goes down it only rarely. My contractor seemed to imply that the bad smell came from standing water in the system under the drain. He also said to pour water down the drain occasionally. (Not for 15 minutes, though!)

Astridh, can you see the drain pipe an where it goes to connect with the sewer/septic main pipe? Do you see a vertical pipe for gases to escape?

Well, some basics from my understanding and experience... not a professional.
One purpose of a trap is to prevent normal gas from the sewer from wafting up into the house; if there is nothing blocking the gas, it will... the trap should be filled with water, thus fulfilling the job. If a drain is not used, it will dry up eventually and thus allow gas to flow freely- smells bad, not safe or healthy.

We had (I don't know all the technical words) a house trap that was at the point of exit from the house that allowed access to the drain line, and the cap popped off (was not set back correctly by a plumber) and a terrible odor was wafting (I like that work) throughout the house, was difficult to locate where it was coming from. I had the whole fire dept (told them it smelled like gas;-) there investigating... lots of cute rescue-type men in cute fire gear ;->... they determined it was sewer gas and firmly tapped the cap back on, and voila, smell gone.

bodica has a good point, although I would think there would be some moisture leaking as well as gas if the seal is not intact. Again, the wax seal is after the toilet trap, thus gas could flow freely. Check all drains in that room- any sinks, shower/tubs, etc for either "dry" or break in the line.

As far as the septic is concerned (had to learn all about them when we bought our current home with a failed septic), it should not be an issue if this is occurring in only one area- a failed septic will show itself either in the clogged leach field, backing up, slow drains, etc. The type of system you describe- pump-up- just means that the tank is likely on higher ground than the drain, and needs to be actively pumped into the tank. If that were not functioning, the problem would be similar to a failing system, with back-up on the lower floors first. Not likely what she seems to be experiencing.

If the smell is detectable in ANY other part of the house, perhaps it is just collecting in that bathroom? Could it be coming up through the walls? Is the drain line located below that bathroom? that it may just be drifting to that high space?

Hope this gives you some things to investigate. Try posting on the plumbing forum, they have been most helpful.

I have this issue too with my downstairs emitting a strong foul odor occasionally. I fix mine by flushing the toilet periodically in the basement. It fixes the problem. I think the pump has to kick in to evacuate the waste from the upstairs bathrooms. Makes sense to me but I am no professional....

Running the water for 15 minutes to fill a trap is foolish. A half-gallon once every two weeks is sufficient.

With new construction I'd make sure that the main vent coming out of the roof isn't obscured.

She's in a cold climate, if excess moisture is coming out of the vent the vent can sometimes "frost over" and obscure some or completely close a good portion of the vent pipe's cross section, restricting the amount of free venting.

Limited or restricted venting can then cause a primary fixture (toilet, for example) siphon another fixture's trap (sink, for example) dry when the primary fixture is used.

That's not a common occurrence, but it does happen, and I thought I'd mention it because it is new construction in a cold climate.

If the trap is going dry from non-use, a trap primer can be installed. These are devices that emit an occasional drip of water to keep the trap wet. Another option would be something like Trapguard, I've never used that product though so I can't vouch for it.

The easiest and least expensive? As other have mentioned, use the unused bathroom once a week to keep the trap wet. 15 minutes of running water is excessive, 15 seconds would be more appropriate. Walk through the unused bathroom and turn the tub/shower on, turn the sink on, flush the toilet, then turn the tub/shower and sink off.

A "pump up" septic system usually has a small accumulation tank in the basement that collects the household waste, then "pumps it up" a few feet so that the run from the house to the septic tank will have the proper vertical pitch.

As to any septic "caps" being loose...I'm not sure what they are referring to; cleanout plugs, or the cap to the pump up cannister...who knows.

Mongo

Bodica, I can't see into the drain. All the walls and flooring are finished.

Mongo, thank you so much! This sounds like just what I need.

You can float mineral oil on the water in a seldom used to trap to slow the evaporation of the water.

Well, the original plumbers came back and, though they couldn't find a problem the first two visits, replaced the johnny seal and voila, it is fixed, after a year and a half. Then they sent an outrageous bill, which is funny. thanks for all your help.

So it was the wax seal? Is that a johnny seal? Didn't know if they were called that. Want to learn.