Slow drain basin – no block

[CBW]

No you’ve got a six inch soil stack and a 38mm waste going into it so plenty of room for the foul waste to get around it.

Looks like standard 4 inch to me.

 

[Hugh Jaleak]

I'm pretty confident the basin waste is the lower of the 2 going into the stack, OP suggests it runs under the floor for some part of its journey to the stack, so makes sense, if bath is against the wall, then bath waste could well be higher.

Basin waste may not be all the way across the stack, but if it's sticking out too far into the stack, crossflow could have partially blocked it inside the stack. I'd be interested to know what the basin flow would be like with the pipe removed from the boss adaptor.

 

[dal5band]

I had a similar case and it was due to too much solvent glue being used by the original installer. It had formed a crust on inside of the pipe which collected gunge and slowed the flow. Luckily this was all inside so just replaced the section of pipe and joint.

 

[Johnny Allround]

I’m looking at Liam,s pictures and he indicates that the basin is above the bath waste. Then taking Hugh,s theory the waste pipe coming out of the trap suggests it is 32mm and looking at the photo again you have a 32 mm waste and a 38mm waste or do my eyes deceive me. My point being the 32mm waste is below and could prove which is the basin waste.