Sewer vent

Verify that's the problem by lifting one of your manhole covers to act as a temporary vent.

The water company is now legally responsible for private sewers such as this. Take it up with them.

As it's a shared sewer, the local sewage network operator owns it, not the home owner. They need the owner's permission to modify it.

It seems most unlikely that was the only vent point, so I'm surprised it causes the effect you see. It is almost certainly venting the last inspection chamber before an interceptor trap. Usually, these aren't actually operating as interceptor traps anymore as the rodding plug is no longer fitted (lost, or removed to act as an overflow if the trap keeps blocking), in which case the trap vent isn't required either.

When you lift your manhole cover, the only way I could see this causing the problem is if you see backing up, either before flushing, or caused by the flushing. It usually causes the water level in the pan to change too, at least for a short time after flushing. This would be more likely caused by a blocked sewer than the interceptor trap vent being blocked off.

Tim Watts wrote

And stand well back...

Thanks. Good to know that the vent isn't needed any more, and probably isn't the cause of the problem. None of the manholes is in a position where you could leave it lifted for any length of time, so that test really isn't an option. The sewer does get blocked every 2-3 years with the usual symptoms and, yes, Thames Water come and jet it for free. This time I have tried to induce backing up by running a hose down the pan at full blast, but no sign of a blockage. I suppose I'm going to have to bite the bullet and ask the neighbour's teenage daughters whether they're having the same problem :-(

Yes, maybe one of them has let some of their crap, ie not human crap but builders crap, go down the vent and block the sewer partially, or you could have a fat burg. Got any Chinese around there? Brian

No symptoms of a blockage. Toilet flushes normally- just leaves stuff behind

stuart noble wrote in news:GIzBv.372021$ snipped-for-privacy@fx33.am:

Subtle change of diet?

Could it be a poor molding of the flush of the pan?

We have two identical toilet pans, one up one down. The downstairs one is a much more effective result, than the one upstairs.

No, this is a 2 part Victorian pan (which exits downwards, so not easy to replace)

Other than it not flushing properly?

I would suspect a partial blockage somewhere. Have you lifted a manhole cover to see if it's backed up at all?

Tim

Maybe when the weather cools down :-)