Two ground floor toilets on one branch pipe?

Hi,

Hoping someone can help me to work this out.

I'm renovating a house which currently has an upstairs toilet and a ground floor toilet in the main bathroom. Both are attached to the same soil stack which is vented externally above the roof. We're splitting the ground floor room into two to create a bathroom and an ensuite. So we plan to replace the existing toilet in the bathroom and add a new one to the ensuite just a couple of metres away through a new partition wall.

The easiest way to do this looks to be to add an external horizontal branch pipe, connect both toilets to that branch pipe and the branch pipe to the stack. However, I've read that this may create problems with siphonage. I.e. flush one toilet and the water level in the other changes....or worse. One possible solution I've read about involves adding a vent pipe with an air admittance valve (I know it would have to be one specifically suited to external use) to the branch pipe further away from the stack than the ground floor toilet which is furthest away from the stack. In other words the vent would be at the end of branch. I think I can see the logic to this, but is it the best solution and how high would the air admittance valve need to be? Would it cause a problem if it was lower than the existing vent above the roof line?

Thanks in advance for your help folks.

 

Thank you both.

 

Thanks for your response GW. I know that a Durgo is an AAV but I don't quite follow what you're trying to say. Could you explain a bit more please?

 

Thanks very much GW