Issues with water Levels in Toilet pans

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

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....

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.

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.

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.

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?

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.

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.

You forgot S*it, sorry Organic Matter, on the left side of that formula.

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)

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.

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

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