Vacuum in drain?

Hi.
I hope someone can explain the odd behaviour of my downstairs loo. After it is flushed the water level slowly drops a few inches.
The only explanation I can think of is that the pressure in the drain is lower than atmospheric and the water is "sucked" out.
I can't understand how this can happen. Surely the drain is open to the atmosphere somewhere? And why only the downstairs loo?
Regards
Nick Wilson
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Nick Wilson wrote:

2 explanations.
(i) its blocked, and teh als bit gently seeps through. (ii) the soils tsack vent is blocked, leading to vaccuum in teh pipe as you surmise.
thre should be an open ended pipe or air admittance valve in teh pipe above the loo somewhere to stop this.
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Hi
Not (i) as the water level ends up a couple of inches below normal. (I should have explained better.)
So must be (ii). I'll have to look and see if there is "one of those pipes that goes up to the roof, then has an elabotate anti-birds-nest thingmy". Still don't see how there would be a vacuum in there...
Cheers
Nick
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When you flush the bog, the plug of water (and any (umm) effluvium), effectively becomes a temporary descending piston in the vertical pipe. The vent at the top of the vertical pipe allows air in, so that the piston does not pull a vacuum on the water in the bowl of the bog.
--
Tony Williams.

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Ok I can understand that. But (and I should have mentioned this in the original post) if, say 1/2 hour after I last flushed it, I pour a jug of water in very slowly - it gets sucked out.
Also, there is no "big pipe going up to above the roof" for the downstairs loo, so I guess that there is "some other arrangement" and that it's blocked. What should I be looking for? I guess a pipe branching off the soil pipe??
Nick
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You are looking for an air admitance valve. This sits on the top of the shortened stack and just lets air in when required. They do have a tendency to block.
Another alternative is that you have a macerator pump with small bore drainage (i.e. Saniflo). However, you'd probably know if you had one of these from the noise.
My final suggestion is that the toilet is design to have a lower water level in the bowl than you are used to. A syphonic action pan may take some time to return to its standard level.
Christian.
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Nick Wilson wrote:

see next post. Depends on whether there is a dead bird stuck in the pipe blocking it.

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On 19 Jan 2004 14:24:33 -0800, nick_k snipped-for-privacy@yahoo.co.uk (Nick Wilson) wrote:

Wind.
DG
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Well it has been windy recently, so I thought this, at first. However it does it every time - even if it's not windy.
Cheers
Nick
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> > > > Wind. > > > > Well it has been windy recently, so I thought this, at first. However > it does it every time - even if it's not windy. > > Cheers > > Nick
Just wondering -are you on mains sewer or sceptical tank ? I experience a similar phenomenon of low level with my loo when the inpection pit between the bog and the tank starts to fill with effluent due to a faulty pipe blocking, between pit and tank. I then now that its time to get the rods ready to clear matters as the next stage is downstairs toilet overflowing. I suppose that a similar thing could happen even if on mains sewers if the pit fills with effluent.
Regards Pete
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On 19 Jan 2004 14:24:33 -0800, nick_k snipped-for-privacy@yahoo.co.uk (Nick Wilson) wrote:

Thirsty rats?
Sorry, couldn't help it :)
PoP
Sending email to my published email address isn't guaranteed to reach me.
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