Bathtub fills up for a bit when running water and then drains normally

[sotiris]

Hi all,

I am a newbie DIYer and have a question I was hoping someone might be able to help me with. When we run the water in our bathtub it drains very slowly at the start, as if it is clogged or something, and then after a minute or two it drains normally as if something has been dislodged inside. This happens every time so I struggle to understand what the reason might be as I don't believe it is clogged. I find it quite annoying and haven't been able to find something online (but maybe I am searching for the wrong thing).

If anyone has any suggestions/experienced this would be great to get your thoughts.

Many thanks,
Sotiris

 

[oz-plumber]

Probably building up pressure and creating a syphon.

Causes: Crappy waste outlet, undersized drain, inadequate ventilation of drain.
But being a bathtub, the residual water has enough volume to fill the trap after the main water has drained.