Yes I've seen siphoning like this before. One of the holiday flats we stayed
in in Guernsey had this problem. Successive plumbers had no luck with it as
the owners told me, so could it be in this case that the pipes are now
clearer than in living memory and the true design issue is now apparent?
Brian

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"Dave W" wrote in message
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On Mon, 10 May 2021 10:38:45 -0700 (PDT), Murmansk
wrote:

I've not had much experience of blocked toilets I'm glad to say - solving
them has been limited to pouring a bucket of water in from a height or
using a plunger.

I had to do one recently that was really badly blocked and I used a drain
unblocking hose attachment on my Nilfisk pressure washer.

The strange thing is that once it was unblocked, when I flushed the water
level ended up much lower than normal and there was a kind of "motor boat"
burping sound!! Now even after a few flushes the water level always ends
up a fair bit lower than it should be.

The toilet is ground floor, goes sharp right after pan, along half a
metre, then sharp left then through some kind of boxing in and out through
a wall 23 inches thick to emerge below ground level outside where it joins
the soil pipe from upstairs.

Background: we just had some professional drain jetters in as the pipes
were blocked by sand (seaside location!).

If you're saying that the water level ends up below the level of the
U-bend, perhaps the U-bend has been displaced, or a hole has been
created in it, effectively lowering it.

Another cause of low level is if the exit pipe goes very deep and has
small diameter, so that the falling water sucks water out of the
U-bend.
--
Dave W