A swimming pool main drain uses suction to pull pool water through it. Heavy particulate matter that sinks to a pool's bottom is often sucked away through a main drain.
Main drains are not required in an inground swimming pool, however, they are strongly recommended and can be extremely useful in some circumstances. The advantages of main drains are to give you the ability to drain the water from your pool without the use of a sump pump. This is useful when replacing your pool liner.
For proper water circulation, it is best to position the flap so that it is about 2/3 closed. This will allow for more flow through the main drain than through the skimmer, which will promote a "bottom-to-top" circulation throughout the pool.
Skimmer Vs. Drain. If the pool receives a lot of leaves and other debris, more suction may be required at the skimmer than the main drain to adequately strain debris floating on the surface. Conversely, if dirt on the bottom of the pool is the principal contaminant, more suction is required at the main drain.
When constructing a new swimming pool, if you are going to use a main drain it is required that you use two. This is due to safety regulations to help prevent Main Trap Entrapment. Main drain entrapment occurs when the suction is so great through a single main drain that a person can become stuck on or in the drain.
MAIN DRAIN INSTALLATION:
The two main drains should be at least 3 ft apart from each other at the bottom center of your deep end hopper pad.
The main drain for a pool is a suction port that should be located at the deepest point on the pool floor. A main drain is responsible for aiding in the circulation and filtration of the pool water. Having a properly working and dedicated main drain line can also allow you to fully drain your pool if necessary.
For me personally, I have found running around 60%-75% suction from the main drains and 25%-40% from the skimmers is adequate for proper skimming and adequate filtration.
So, if you see that you pressure quickly drops to 2 psi, it's likely that there's a leak in the main drain line somewhere between 4′ – 5′ under the water level of the pool. If there is a leak in the section of the line at or above the water level of the pool, you'll see a quick drop to 0 psi.
Most of the dirt and debris that sinks exits the pool through these drains. To keep people from getting their hair or limbs caught in the plumbing, the drains are almost always covered with grates or antivortex covers (a cover that diverts the flow of water to prevent a dangerous vortex from forming).
Most pools have two main drains, but a small pool may have only one. The drains are positioned at the pool's deepest point.
For testing only the main drain pipe for instance, cut the main drain and skimmer pipes (all of them), so you can move the suction manifold pipes out of the way. Then push on a SlipXFPT female adapter onto the cut pipe(s), and screw your pressure testing stick into the FTA fitting.
Pool suction drain injury, also known as suction entrapment, occurs when the drain of a wading pool, swimming pool, or hot tub suck in a swimmer's jewelry, torso, limbs, hair or buttocks. Evisceration, also known as disembowelment, could happen in case of buttock entrapment.
It's important that pipes are buried deep so that they don't freeze. It is usually 3 to 3.5 ft deep.
Because there is no main drain on most aboveground pools, you will only be able to lower the water to the bottom of the intake or skimmer, using your aboveground pool filter system. To drain an aboveground pools, either a). Use small submersible electric pump and garden hose, or b).
Single Drain - Unblockable means there is one drain approved to be unblockable so that a human body cannot sufficiently block it to create a suction hazard.
Locate the small hole at the bottom of the main drain. This is the plumbing line to the filter. Plug the small hole with an expandable rubber plug or a plastic threaded plug, if the hole is threaded. Replace the main drain lid if needed.
Important: swimming pool drains are often built with short radius elbows that are normally found in cold water supply lines. These tight elbows can easily get clogged by any debris that flows past a dislodged strainer basket, and can also increase the risk that a drain cleaner gets stuck in the line.
Look closely at the filter, pump, heater, and pipe valves, which is where leaks often tend to occur. If you have a vinyl pool liner, look for tears or separations around the fittings, lights, steps, and corners.