The laws of nature play a role in every aspect of our lives. We can either work with these forces, or against them. When it comes to gravity, pool builders must do both.

Sometimes builders can use gravity to their advantage. In other Tinstances, they must work around this most basic natural law. Here, let's discuss the ways gravity affects pool systems and how builders use it when possible—and combat it when necessary.

Against the Law. Sometimes, gravity sends the water where you don't want it. Check valves can help. Placed in the plumbing, these devices only allow water to flow in the right direction. But they aren't perfect. Sometimes check valves don't close all the way, and they wear over time, says Ray Alderete, owner and president of Alderete Pools & Spas in San Clemente, Calif. That's why many pool builders try to minimize the need for them.

The most basic way that pool builders must combat gravity is in placing the circulation equipment. The goal is to keep the pump flooded so it doesn't lose prime. First and foremost, you don't want to make the pump pull water up to it, which causes stress on the motor. Combating gravity in that way is too hard and when the pump is turned off, the water is compelled to move down and possibly leave the unit dry. If builders must elevate the equipment, they have to upsize the plumbing and install plenty of check valves to make sure the water doesn't fall away from the pump when it's turned off.

Gravity offers opportunities—such as creating the waterfalls and vanishing edges that beautify so many yards. But it also poses roadblocks, which builders work around every day. Courtesy NSPI/DAL Pino Quality Pools Many builders place the equipment so it's lower than the pool's water level. This uses gravity in the system's favor, allowing it to pull water to the pump, where you want it. But there is a downside: When it's time to open and empty the pump's hair and lint trap, you risk having the pool drain back through the opening. Valves mitigate this risk by closing off the water to the pump. To work on the unit, technicians simply turn the valve to block off the flow.

Some builders prefer to place the equipment at deck level. This way, the pump doesn't have to pull water up, and the pool can't drain back out during maintenance.

Gravity also poses a special hazard for projects that have one body of water elevated higher than another, such as raised spas, waterfalls, or vanishing-edge pools. When the pump is turned off, pressure is removed from the lines and the water can escape from the upper pool through the return inlets. It would then move backward through the plumbing into the lower vessel until the water in both pools is at the same level. This is called equalization. Remember, gravity sends water to its lowest point—even if it's through pipe.

The risk of equalization is especially dangerous in vanishing-edge applications, where the upper pool is usually dramatically larger than the catch basin below. If the upper vessel were to drain back, the lower one would overflow. All the excess would spill to the ground and possibly undermine soil stability.

If a vanishing-edge pool is built incorrectly, gravity can coax water backward through the plumbing, from the vast upper pool to the smaller catch basin. A Hartford loop and vacuum relief valve are essential to preventing this. This hazard requires a more reliable solution than check valves. Experienced builders will use a Hartford loop with a vacuum breaker on all vanishing-edge projects. The loop, which looks like an upside-down U, provides a barrier by running above the upper pool's water level, then curving back down. This actually uses gravity to combat itself: Unless the renegade water were to move backward quickly enough to cause a siphon, it couldn't overcome the loop. If suction is created, the vacuum breaker will stop it by introducing air into the pipe.

Another option for builders to prevent equalization is by feeding water into the upper pool with a water feature rather than inlets. This breaks the connection between the water and the pipe, so there are no return inlets through which the water can creep back.