Syphoning bath trap

[bobapcoed]

Having had a new bath installed, it unfortunately had a shallow p trap fitted. Flushing the toilet syphons water out of the trap, causing smells. Unfortunately not enough room to fit standard P trap or bottle A/S trap. I've fitted an in-line non return valve which has helped but not cured completely.
Various questions.
Is there an alternative to what I've already tried?

There is too much water in the cistern. Would reducing the amount help against syphoning.

I'm reluctand to cutting a hole in the floorbords under the new bath, as you can understand.

Is it ok to make up a trap using fittings, to get the maximum possible depth of seal.

Although the bath is mounted at the same height as previous one, which had a normal trap, it's obviously deeper thus reducing space. That bit got past me. Any help please.

 

[kevplumb]

one of these might work

low anti syphon trap

 

[bobapcoed]

Thanks for that Kevplumb, and your prompt response.

 

[bathjobby]

You don't say which type of soil stack your going into; open vented or Air Admittance Valve ? If AAV and new, check that the plastic cover has been removed.
You can use whatever to form the trap under the bath and cutting hole in floor boards to get a deeper trap is common practice.

There is too much water in the cistern. Would reducing the amount help against syphoning.

What is the definition of too much water in the cistern ? But yes, probably but it all sounds like it stems from the shallow bath trap, so fix that, not everything else in the bathroom to accommodate it.