I have a bathroom that is usually odor-free but occasionally sewer odor creeps in. When it does, it’s usually not intense but on rare occasions it’s intense enough to smell from the next room.

Any ideas? I cannot connect it with any activity.¹ It seems to hit randomly. Traps are good. It seems to close to the toilet connection to the pipework but there are no waste water leaks in sight.

Do I need to remove the toilet and dig up the tiles? I could hire a plumber to scope the drain, but that would likely cost more me digging up the bathroom myself. Should I look into renting a drain camera? Or would it make sense to rent an infrared camera and pour hot water in the drain?

I think the kitchen drains may be upstream from the bathroom.

¹ (update) the odor seems to hit after I run hot water in the kitchen just upstream to the bathroom.

Has anyone encountered this? I didn’t talk directly to the plumber but was told they will not flush a 30+ year old tank. I wonder if the plumber is concerned that it’s so fragile that flushing would cause leaks.

This diagram is from the service manual of a combi boiler. It’s a flow sensor which detects whether hot water is running, which is then used to trigger on-demand heat and switch a diverter to take radiators out of the loop.

In English, the diagram shows:

• X ⅔ red wire (+5V)
• X 2/2 black wire (ground)
• X 2/6 green wire (signal)

I need to know what those fractions mean. I took the voltage measurements in this video:

• https://iv.ggtyler.dev/watch?v=d8ucufoyUlQ

I cannot necessarily trust the model in that video to have the same specs as mine. My voltmeter detected 4.68 V on the red input wire showing that the sensor is well fed. The green “signal” wire is supposed to be 0 V at rest and 2 V with water running (or I think the reverse of that is used in some models). In my case the green wire is ~1.33 V at rest and ~0.66 V when water is running. I need to know if these readings are normal as I troubleshoot this problem.

update

As the resp