Skimmers
Skimmer Weir Doors
Skimmers need a weir door to do their skimming function. The weir door assists greatly in skimming by creating a current in to the skimmer.
A skimmer will draw in water just fine without a weir door, but it won't skim debris off the surface any where near as well.
The door is to increase the amount of water taken from the top of the water where the floating debris are. Without the door on, most of the water is taken to the filter from beneath the surface and would leave a lot of the floating stuff on your water.
The skimmer weir limits the flow coming in to maybe the top 1/2" of water.[1] If the cross section area of the skimmer throat is 3"X12", that's .25 sq ft. Let's say the skimmer is pulling a measly 10 gallons per minute. That's 1.3 cubic feet per minute. That means the flow coming through the skimmer throat will have a velocity of 5.2 ft/minute or .087 feet per second to maintain the water level. Now reduce that opening to 1/2"X 12". That's 1/6 the area so to maintain the water level - and water will seek its own level - that water is moving six times as fast or 5.2 feet per second. That's a pretty good clip - 3.5 miles per hour - enough to overcome slight breezes and currents and pull water into the opening. Unless you have a return jet pointing straight at the skimmer throat, any surface debris being shoved along by the return flow will just go right on past the skimmer otherwise.
Skimmer Crack Repairs
Skimmer cracks are generally the result of settling/movement around and underneath. Ideally it's best to dig up the area, replace any bad items, and re-pack to prevent further movement. It can be worthwhile trying to fix a skimmer crack with epoxy and seeing if it holds.
Repair products members have used include[2]:
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