As is (or used to be?) normal, this WC in our downstairs bathroom is connected direct to the drains. No vent pipe. Occasionally the sewer surcharges sufficiently for water in the chamber through which the effluent from this WC passes to rise and cover the inlet into the chamber from the WC. This traps air in the pipe between the chamber and the WC's own outlet. If you flush the WC, or if the water level in the chamber continues rising, the water in the WC pan rises also.
That is all "normal". BUT there is a puzzle. The drain from the WC has a second pipe connected to it (under the ground). This from a gully. I've never had to dig down, so I assume that the connection from the one to the other is an offset tee or a Y connector. The bath and basin in the downstairs bathroom, where the WC is, empty through the wall into this gully. So waste from the gully and waste from the WC end up running along the same pipe into the chamber. (I have verified that this is the case.)
If the sewer surcharges, I would expect the water in the trap of the gully to lift before the water in the trap of the WC starts to rise. In fact, I would expect the water in the WC NOT to rise, and all the back pressure to be relieved via the trap of the gully. The gully is at least 200 mm lower than the WC trap and does not appear to have a deeper trap than the WC's own trap. It also contains considerably less water than the WC's.
But - and this puzzles me very much - if the sewer backfills enough to create back pressure in the shared drain leading into it from both the WC and the gully, ONLY the water in the WC trap rises, NOT the water in the gully!
This plumbing is the age of the house - 51 years. Is it possible that there is a non-return valve in the pipe from the gully, which closes under back air pressure? If there is, I would be surprised if it is still working after so long - especially working well enough to prevent the water level in this gully being affected at all. All the lifting force (compressed air) seems to go exclusively to the WC outlet .
As regards the possible presence of a non-return valve, I don't know if the following is relevant. This gully (uniquely around our house) is not outside. It is in the concrete floor of the (lean-to) garage.
That is all "normal". BUT there is a puzzle. The drain from the WC has a second pipe connected to it (under the ground). This from a gully. I've never had to dig down, so I assume that the connection from the one to the other is an offset tee or a Y connector. The bath and basin in the downstairs bathroom, where the WC is, empty through the wall into this gully. So waste from the gully and waste from the WC end up running along the same pipe into the chamber. (I have verified that this is the case.)
If the sewer surcharges, I would expect the water in the trap of the gully to lift before the water in the trap of the WC starts to rise. In fact, I would expect the water in the WC NOT to rise, and all the back pressure to be relieved via the trap of the gully. The gully is at least 200 mm lower than the WC trap and does not appear to have a deeper trap than the WC's own trap. It also contains considerably less water than the WC's.
But - and this puzzles me very much - if the sewer backfills enough to create back pressure in the shared drain leading into it from both the WC and the gully, ONLY the water in the WC trap rises, NOT the water in the gully!
This plumbing is the age of the house - 51 years. Is it possible that there is a non-return valve in the pipe from the gully, which closes under back air pressure? If there is, I would be surprised if it is still working after so long - especially working well enough to prevent the water level in this gully being affected at all. All the lifting force (compressed air) seems to go exclusively to the WC outlet .
As regards the possible presence of a non-return valve, I don't know if the following is relevant. This gully (uniquely around our house) is not outside. It is in the concrete floor of the (lean-to) garage.