Double Trap Siphonic Pan - Help Needed

11 Jan 2014
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United Kingdom
Hi there, my toilet is an old double trap siphonic pan and up until last week, has worked fine for the last 27 years. When I flushed it, the cistern would flush water into the pan but it was no longer sucking the water out of the pan although the water level would stay at its own found level.

After fitting a Dudley Turbo 88 siphon along with a Syvac Aspirator it is now flushing perfectly with the suction working as it should but for some reason the water level in the toilet pan is not filling up to the level it used to.

I'm not a plumber but I do understand the toilets u bend determines the max height of the water level which is why when I've added enough water to reach that level it doesn't keep getting higher, it empties out what is over and above the amount.

I also know that in my case it has nothing to do with there not being enough water in the cistern. Normally, during the flush cycle it would stop sucking the water out a few seconds before it finished filling up but now it seems to still be sucking after its finished filling.

Does anyone know what might be the cause and how to put it right?
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It sound like the syphonic action is being continued after the time when it needs to be broken. So this is very likely to do with the "bomb" in the syphonic system.

I will have to go back to school on that one to remember the sequence but towards the end of the flush when there is no longer water in the syphon the bomb should release air back into the traps to break the syphonic action and the remaining water in the pan rim, refill resevoir etc reseals the main trap.

I would be checking that the "bomb" is not getting stuck down - Certainly it must be sealing OK as it is the "Bomb" seal that is all that usually needs replacement when syphonic action fails to work properly.
Thanks for your reply, as far as I know the 'bomb' bit doesn't move atall, once it's in place and the cistern secured back to the toilet there's no moving parts in it and nowhere for it to go.
Here's the links to the 2 parts []

Are you saying if you add more water after the flush finishes (e.g. from a jug or bucket) the level in the pan still drops to below that to which it has been prior to the recent repairs?
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When I add water to the toilet pan from a jug the water level goes up to the level it used to be and stays there. I was trying to explain that if I was to keep adding water that it finds its own level cos of the u bend bit and its own level is how it used to be before I changed the siphon and aspirator. For instance if the pan holds 2ltrs at normal level it doesn't make any difference if I add 5ltrs it drains back down to 2.
Odd. :confused: The bomb is a simple (but effective) device that harnesses the water passing over it when a flush begins to draw the air from the centre trap in the pan, thus creating a slight vacuum in the centre trap. This vacuum is sufficient to allow atmospheric pressure to push the contents of the pan through the outlet. (In theory anyway!) I cannot see how the suction created can be continued once the flush has ceased, and lack of water flow across the aspirator will break the vacuum effect created by the aspirator.

Did wonder if a build up of limescale could be causing enough of an additional restriction to cause the effect described, but think that can be ruled out as the problem has suddenly occurred and the pan would block very easily if this was the case..... Incorrect siting of the bomb usually causes the syphonic action to fail to operate, cannot see how it could cause it to continue after the flush...... :eek: Out of curiosity, is it a Royal Doulton pan?

Found a diagram if it helps. This shows a low level pan with horizontal aspirator in the flush pipe, but principle is the same. Dome washer on tail of bomb needs to make an airtight seal over hole into middle section A or syphonic action wont occur. The area 'D' is designed to fill with water during flushing, then this water slowly drains to refill the first trap when flush is complete.
As I feel sure this must be related to the bomb's action, is the position of the bomb seal adjustable on the stem? if so has it been set correctly.

I would be inclined to inspect the bomb itself to see if there is any obvious manufacturing defect such as a plastic flash obstruction.

I have only ever had to deal with one! - That was a simple replace the seal on the bomb and all was well again. It certainly wasn't made of plastic.

I may still have an antique book in the loft with drawings from my college days. I will have a look but don't hold your breath! I have a gut feeling that if the bomb is set too low it may still be sealed as the refill reservoir starts to empty. Either way I would play with the height of the seal to check the effect.

Image pinched from a Doitall post so credit to him!

Also check the cistern is flushing the correct volume as the "88" comes with different settings. Unlikely but perhaps the re-fill reservoir is not getting fully charged.
Valid point Blagard, I am sure these need the full 2 gallon (9 litre) flush to operate correctly. If siphon is only giving 6 or 7 litres there may lie the problem. :eek:

Had many years of aggro with, (and got to learn the finer points of) a Doulton double trap syphonic pan at my parents place. Fitted new in '79 when the house was built, it never worked effectively. Blocked for a pastime, builders actually returned and removed it/checked soil pipework twice in the first year but nothing could be found wrong with their installation, always put down to the pan. We actually kept a drain rod with 4" plunger next to the pan to save embarrassment when it inevitably blocked.....
I also forgot to mention that as the "88" can be set dual flush or single flush it must be set as single flush. Not only because the WC will need a full flush to work, but it is actually against the Water Regs to replace a single flush cistern with a dual flush. - Presumably this has something to do with a need to ensure old pan designs get a flush they were designed to work with.
Thanks to all on this thread especially Blagard who posted that very clear drawing of the dual-trap close-coupled syphonic toilet.
I'm about to try fixing one of these that is displaying classic symptoms of malfunction: water flushes into and nearly fills the pan, but there is no suction to clear the contents of the pan. Looks like the bomb seal has perished after decades of use. The drawing is huge help in understanding how the toilet works.

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