The “weir” is the flapper door on your thru wall skimmer. The weir serves three basic functions:
(1) It prevents debris from flowing back out into the pool once it’s captured, essentially functioning like a check value.
(2) It moderates the flow into the skimmer as water rises and falls so that it receives enough water to function, but not so much that it floods.
(3) It ensures that the top ½ inch of water flows into the skimmer.(that is where all the dirt starts)
Skimmer weir doors are specific to each manufacturer. These examples are for both standard and widemouth aboveground pool Waterway skimmers. In-ground Waterway Renegade skimmers use a different part number. All skimmer weir doors,and skimmer baskets are available at http://www.waterwayparts.com