Issues with water Levels in Toilet pans

[Tom]

(Set up is 2 Toilets on one waste pipe and exterior venting through roof.)

Noticed that after flushing loos that water level gradually drops about 2cm over a couple of mins - Looks like siphon effect and I understand this can be caused by a semi block in either waste pipe or roof vent) or windy weather. However doesn't appear to be a block in waste pipe, weather conditions have been variable and I cannot see a block in the top of the vent. (Issue is same in both Loos and prob appears to be recent).

One additional point - I can top up the level in the pan at any time after flushing and it will still drop approx 2cm no matter how little or how slowly I add the water. Suggests to me no siphon effect.

Am I missing something obvious...anybody have any ideas please?

[meow2222]

Yes, this is a blockage where the blockage is not 100%. Needs to be sorted asap before you get a total hard block and lose the use of both bogs.

NT

[Tom]

Ive had a go with Acid so its caustic soda next. The only thing that makes me think it is not a block the drian side of the pan is the fact I can very slowly replace this small amount of lost water (and so not create any flushing siphon effect) and the level still reduces. Im thinking it points toward block on the vent side but dont properly understand the physics involved....

[Alan]

Is there an auto air valve at the top of the stack? If so remove it and see if the fault goes away. It may be that it is stuck, and there is a slight vacuum on the soil pipe from the sewer which is sucking the water from the bog - which explains your symptoms exactly.

Alan.

[NRH]

If you put caustic in after you've used acid, which will leave traces in the blockage, you will probalby have an explosion in the waste pipe, which will be very unpleasant.

[PJ]

Why? Do you not remember your chemistry lessons? Acid + Base = Salt + Water.

It will form weak solutions of sodium sulphate, sodium nitrate or sodium chloride depending upon which acid was used.

[Tom]

Yep, got the Chemistry covered thanks.

Alan - are you thinking of an air admittence valve that might be used when the pipe terminates in a loft space? Mine goes out through the roof - I thought these would all be open vented?

[The Natural Philosopher]

Not really. A few flushes in between dilute things to a pathetically innocuous degree ;-)

Hydrochloric acid at 30% (brick acid) plus caustic soda (sodium hydroxide) = warm salt water. Sodium chloride. No gas and its not hugely exorthermic either. At our concentrations anyway.

[Alan]

Yes, I was. The pipe could be blocked by a birds nest or similar however. Only a blockage in the vent explains your symptoms.

Alan.

[NRH]

You forgot Shit, sorry Organic Matter, on the left side of that formula.

[Tom]

Cheers Alan. I will get up on the roof and have a look. Not too sure how this pipework is set up as its all inaccesable but presumably I could pour a bucket of water into the roof vent to flush through anything down into the soil pipe. Im thinking that maybe something could have backed up from the first pan partially blocking the vent pipe (although presumably the vent should drop into the top of the soil pipe to prevent this)

.

[F]

We had this. It was an *almost* complete blockage outside the house. Lift the manhole cover and have a look. Ours was 5 feet deep and full to the top.

[Andrew Gabriel]

All it often takes is enough standing in the bottom to seal off any air flow through the sewer pipe.

[Tom]

Thanks but it doesnt look like the blockage is between the pan and the sewer...