Toliet bowl empties over time

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Help ! I've been scratching my head about this for some time, perhaps someone else out there has experienced this and knows a solution ?
The water in the toilet bowl in our downstairs loo sometimes drains away to nearly empty. Often at least enough water drains away to leave an air passage through the pipe to the drain, so it get a tad smelly in there....
After flushing the water level is fine. Then it can take hours ( sometimes days ) without use, and on return the water level has dropped significantly.
I have checked for blockages in the bowl bend and in the pipe leading to the drain, both of which seem clear.
Also considered it may be an air pressure problem - when the door is closed the air cannot escape anywhere so the pressure pushes the water level down and some drains away. This is evident when the door is swung shut and watching the water level. Leaving the wind open resolves this so it's been discounted.
Any other suggestions / opinions ?
Thanks in advance,
Jon
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Have you got a thirsty Feline?

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nope. No pets involved, and the kids are too young to use the toilet and block it regularly with excess paper....
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Does the drain pipe have an air in take valve on it anywhere. Using other appliances connected to this same drain pipe might be sucking the water out as it passes the branch for this loo. The point about it leaving the small air gap tends to point to this as being the fault, because once the air is getting in, the vacuum stops and the small amount of water left is not enough to cause a proper seal again.
You can solve this by fitting an air in take unit on a piece of smaller bore pipe to the large pipe coming from the back of the loo.
Have a read through this page:
http://www.diydoctor.org.uk/projects/air_admittance_valves.htm
But you may not need the same size units for this problem, and a smaller 32mm or 40mm valve will probably allow enough air in to stop the vacuum from happening in your situation.
Hope this helps a bit.
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This is common on downstairs toilets. Put a HepVo trap on the basin in the toilet. Should solve it. See current thread on this.
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Their is no basin in the toilet. It's a basic loo only room.( used to be an outside loo pre extension of house ) The loo in question is the only thing connected to the drain via this particular route.
<snip>

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There should be a basin there. Install one and fit a HepVo trap on it which acts as an air admittance valve, then two problems sorted.
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Jon wrote:

That is probably the problem. Coupled with maybe a partially blocked sewer or sewer vent.
Its alomst certainly negative pressure in the sewer. Now ssweres are supposed to vent way up high at teh top[ of te house siomewhere. If THAT is blocked then e.g. flusing an upstairs toliet may cause an inital pressure surge as the contents travel towards wherever the downstairs loo branch joins the main sewer, followed by a negative pressure pulse as it travels away. And air admittance vale would alleviate teh neagtive pressure pulse. Thats how they work.
IMM's as usual dogmatic insistence on doing things his way and his way alone is also valid, but is only one option. Not necessarily the best. Mine may not be either, thats up to you to decide.
Its definitely worth while lifting some manholes and poking around. And flusihing any muck through, and then you can observe flows when you flush toilets etc...Good clean drains alwatys lift the spirits :-)
If once everythung is definitely working as it appears it shoudl be, and teh p[rolem persists, you might actualy if hes friendly ask a builkding inspector to suggest teh corect thung. Be fcareful tho. He might insist on the hand basin...and at least a cold tap.

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Behave yourself.

He "needs" a wash basin, as if he sells he will have to fit one anyway. Installing the needed basin and using an HepVO trap solves two problems., Very logical. Have you heard of logic? You certainly never heard of HepVO.
In modern houses the downstairs toilet and the main stack will be on two separate 4" pipes that feed into an outside manhole. The downstairs toilet will have no air admittance valve and not be vented. So when the upstairs bath is emptied the water goes directly into the manhole. The vented mains stack cannot affect the downstairs toilet as it is vented. When waste goes down the main sewer pipe from the manhole it is vented via the main stack.
In this situation I doubt if the toilet and main stack are arranged that way. A HepVO trap is a simple unobtrusive method.
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IMM wrote:

IMM. I didn't say your solution was wrong, and I have indded heard of HepVo and equivalents. I investigated them for my own house.
I am not at all sure that the lack of a basin is a problem when selling a house. Its a building regs issue, and would need to be sorted if any 'material alerations' were made to that area, but there are millions of houses that don't match regs being bought and sold all the time.
Your points about how the suction works are valid IMHO.

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I relative of mine had to put one in to sell as the surveyor noticed one was missing and the purchaser would not buy unless all points noted were rectified. It is a hygiene point, and I'm not sure if this can be forced to be updated (which it should be) in the event of a sale. For a long time now wash basins have to be in toilets.

That is how it is done in modern houses not my opinion on how it is done. Having a HepVo on the downstairs basin in any house would make matters better too.
HepVo is one the biggest unknown recent innovations around (1997 when it won an award). A whole piece of obtrusive roof penetrating stack (which can extract heat from the house) can be eliminated in 4/5 of homes (every 4th house requires an open stack vent and end of cul-de-sacs) and no large clunky air admittance valve near the toilet. HepVo makes waste piping a total doddle. And no water in the HepVo waste trap to run dry and smell if the house is empty for a while. Now if Hepworth can make a dry toilet bowl....
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IMM wrote:

[SNIP]
I like Hep Valve too but a relative would not have one where it would have solved a problem because, "what happens when she drops her earring down the basin again? We normally just retrieve it from the bottle trap...." I gave up and told him to run a separate pipe to the stack
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They just undo and are easily removed.

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IMM wrote:

You still need a stack vent somewhere.
read yer regs.

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Every 4th or 5 th house has a full stack vent, depending on where you live.
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Hi IMM In you wrote:

Ye Gods man! Learn to snip!
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unhook to mail me | http://www.fishter.org.uk /
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My outside loo gets hot in the sun, and over a couple of weeks of no use, the water evaporates away. Doesn't happen in the winter.
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*If you think nobody cares about you, try missing a couple of payments *

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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The water is syphoning out because the drains are blocked. Lift the nearest manhole cover and you'll see.
Dave Baker - Puma Race Engines (www.pumaracing.co.uk) I'm not at all sure why women like men. We're argumentative, childish, unsociable and extremely unappealing naked. I'm quite grateful they do though.
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Jon wrote:

Almost always a suction problem. Its why you have air admittance valves. Often happens in high winds such as we are having. The stack vent can be subject to turbulence and fluctuating wind pressure - often you can stand there and see the water moving up and down as the wind blows.
Probably solution is to fit an air admittance valve to that loo, so any partial vacuum will open the valve.
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There is no stack vent. The toilet is fitted against an outside wall downstairs. Approx 12 inches the other side of the wall is a manhole that it drains into.
Would an air admittance valve still help in this situiation ? ( Will check later on Google for their uses more fully ! )
Jon
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