Re: Can a holding tank fill itself or is somebody living on my boat?
You might have an anti syphon loop....But is the syphon break at the top of the loop unblocked. If this is blocked abd the tank is below sea level within your boat then the water will have no problem syphoning back in. [img]/forums/images/graemlins/laugh.gif[/img]
Check your air vent and filter is clear as this could help start the syphon process if partially blocked and causing a partial vacuum after draining the tank.
Eastern Scotland and beyond.