Gurgling new toilet - but no blockage

[ChrisMKUK]

I posted this on the Screwfix Community Forum but depsite many views, there were no replies!

My wife is a wheelchair user and we previously had a toilet fixed to a plinth to raise it so she could transfer more easily. We have had it replaced with a comfort toilet which is the same height as the previous one (including the plinth).
The waste pipe outlet was lower than the previous one so the builders made adjustments to the pipework.
The original cistern was kept.

It gurgles a few seconds after flushing which lasts for a good 5-6 seconds. There is no sign of air bubbles in the bowl. There is no blockage in the waste pipe and water flows freely into the drain. They have also replaced the flushing system.

However, we have discovered that if the cistern handle remains down after flushing, the gurgling is much quieter and lasts for 1-2 seconds at most! They suspect the new toilet is the problem (for whatever reason) and this morning brought an old one, put it on a temporary plinth and the problem disappeared. They have admitted defeat! Clearly, the new toilet and old cistern do not get on!

Before I contact the supplier for advice, can anyone enlighten me as to what might be the problem?
This is the new model:
https://www.victorianplumbing.co.uk/burlington-regal-back-to-wall-pan-p15

 

[Madrab]

It's got to be the rate at which the water is passing the flush (drop) valve and that's creating turbulence, that's probably why it's less when the handle is held as that's holding the valve up/open a bit more than it normally would, creating a smoother flow. That's being said to gurgle usually requires air to be drawn in, unless the turbulence actually being created at the flush pipe/pan connection

I would be tempted to open the cistern up and see/feel where the gurgling is actually coming from and then see if it changes when manipulating it by hand. It may need a different type of flush valve, either a flapper or syphon to alleviate.

 

[ChrisMKUK]

Thank you for the reply.
Can I safely rule out that the issue isn't with the toilet itself; ie, it isn't faulty, but rather how it is connected to and the type of flushing system?

 

[Madrab]

Ah ok, it's a syphon, I presumed it was a drop type valve, that actually may be your problem, the syphon uses air to control the flush volumes and to break the syphon so they normal gurgle a bit at the end of the flush.

I'd maybe look to change that to another type of flush valve - a drop type valve valve replacement would be the most common.

 

[ChrisMKUK]

Thanks Rob - much appreciated!