Help with a physics/plumbing mystery?

I can think of one thing that might cause that.

This might take a bit of explaining, so be patient.

In most plumbing there are traps that hold water in them. (P-trap or S-trap)

Places where the drain hangs in a U shape. You can see these traps under your kitchen sink.

The idea is that by being full of water they stop sewer gases from coming out of the drain where you can smell them.

For some reason some people forget to put them into shower drains. Or they put them to far from the vent(I will explain that in a moment)

The toilet moves a lot of water down its drain. This makes any other drains connected to that have a big change in air pressure in them. A flushing toilet can make sink drains burp trap water up into the sink or suck the trap dry.

If the trap is dry sewer gas can come up it and make a stink.

To prevent the pressure change from water moving in the drain affecting the water in the traps plumbers put drain vents on the drain system. They provide relief for the air pressure or vacuum in the pipes.

If you look at your roof you should see a piece of pipe sticking up, a piece of sewer pipe. That is the main sewer vent.

So, maybe you have no trap in the shower drain.

It needs one.

Or maybe it is to far from proper drain venting. (5 feet is about the limit)

This might be something to ask your plumber about.

There are a few ways of fixing the problem. One trick is mechanical vents that prevent vacuum forming in the drain system. If you have a small cottage it might be fairly easy to install a proper vent for the shower drain.

And yes, I can imagine that weather and temperature changes might make a difference.

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Try this experiment.

Run a bit of water in the shower. You only need enough to fill the trap, if there is a trap.

While you listen near the shower have the toilet flushed.

You should not hear water gurgle at the shower drain at all.

You should not be able to hear the water from the toilet running down the drain from the shower drain either.

If you hear water being sucked out of a P-trap you have a vent problem.

If you can not hear that, but hear the toilet water running down the sewer line you have no P-trap between the shower and sewer.

Is it possible that it isn't your drain? But rather the space around the drain? Or the area just behind your shower, next to your drain? Or even something right next to your drain pipe, just below the shower but not in the drain itself?

Because your space heater wouldn't do anything for your drain (except for the first few inches down) - but it's would do a whole lot for whatever is in between your walls or just beneath the floor. And clearly everything that you've tried doing to the drain itself hasn't worked.

Could something have died / rotted / be growing in the spaces around your shower?

Don't pour bleach down the drain, it will corrode your pipes. It could be a couple of things. You may have sulfur in your water source, ask your neighbors if they are having the same problem. Second and probably the cause is your vent stack may be clogged or improperly positioned. Your drains all have vents on them so sewer gases to get into the building but if they get clogged it doesn't work and hold the gases in. Call a plumber and ask them the check that out.

We had a similar problem when we moved into our house. The bathroom smelled so bad that I would gag. Unfortunately for us, we went to replace the bathtub and found that the cause of the smell was that all of the subflooring was rotten and had to be replaced. We ended up gutting the entire bathroom and doing a full remodel, although I think we could have gotten by with replacing just the subflooring. I would check to be sure that you don't have any rotten wood anywhere. Good luck!

If you have put clorox down the drain & it odor still comes out..the drain may not have a trap..If it's worse, when its cold outside..I would gues cold air is coming down the vent system. You mention a small cottage, with a space heater..I would guess that a "draft" is being created..& cold air from outdoors is some-how coming down thru the vent system.

my first thought is that perhaps your toilet has a leaky johnny ring, and the floor is rotting. the reason it would smell at certain times (like when everything is closed up, or there is no ventilation) is the always always present smell has no where to go.