The often overlooked, sometimes misunderstood, but ever important system of return inlets in a swimming pool is a critical function of the complete pool water circulation system. It may seem obvious, but for the everyday lap swimmer or child splashing in the leisure pool, the details of how the pool water is kept sanitary and balanced may be out of sight and out of mind. Be assured, however, that for every well managed pool there is a sophisticated back-of-house system maintaining water quality for the benefit of those swimmers enjoying the pool. Of course, as the pool water is circulated through the various components of the recirculation system (filtration, sanitation, heating, etc.) it must not only be drawn from the pool, but then returned to the pool. It is returned through a system of inlets – multiple orifices located in the pool to distribute the filtered and treated water back through the total pool water volume.
There are many different approaches to return inlet design. The two most common, which this post will focus on comparing, are wall inlets and floor inlets. We will discuss floor inlets first. Floor inlets use a network of piping beneath the pool slab to supply inlet nozzles located in the floor of the pool. The nozzles are typically set flush with the pool floor and designed to distribute flow in a 360° pattern around the inlet location. One of the major advantages of a floor inlet system is that the inlets can be spaced evenly throughout the entire floor of the pool. This is typically preferred for larger pools in that treated water can be more thoroughly distributed across the width of the pool, rather than being limited to the perimeter of the pool as with wall inlets. It is also advantageous to return treated water at the bottom of the pool depth profile, especially for perimeter overflow pools, because the treated water is then mixed through the entire depth profile of the pool as the returned water makes its way from the bottom of the pool up through the vertical column to where water overflows at the pool gutter lip. This action promotes optimal mixing of the returned water within the pool volume. Returning at the pool floor can also help to prevent small debris and dirt from settling out on the pool floor, by providing a sweeping action tending to carry particulate to the main drains where it is then conveyed back to the filtration plant. The water velocity at the return inlet nozzles is critical in providing this sweeping effect, and also for proper distribution of return water throughout the pool. By design, floor inlets tend to provide distribution of return water from the nozzle location. The flow and velocity from each individual floor inlet can be adjusted by opening or closing the inlet orifice (typically a threaded device with a set screw) and it is critical that each inlet be adjusted for evenly distributed flow upon start-up or initial installation.
All that is not to say that wall inlets do not have their place as well. Wall inlets were the industry standard for many years and still have advantages for certain pool types. Wall inlets are generally a less expensive system because the network of return piping does not have to be routed beneath the pool slab. Also, for outdoor pools in freeze/thaw climates, wall inlets are much simpler to winterize than floor inlets because for wall inlets the inlet orifice can be the low point of the system, so the piping drains by gravity of any residual water when the pool is not in operation. A traditional wall inlet design includes a directional eyeball orifice at each inlet, so they can strategically located and also aimed to aid in carrying debris to skimmers or overflow weirs. With this type, the flow and velocity from an individual wall inlet can be adjusted to some degree by changing out the eyeball orifice size. Most health codes still require that wall inlets are provided in stair alcoves or other isolated areas where floor inlets may not be practical.
A second type of wall inlet design is common with stainless steel gutter systems. Known as a “pressure tube return”, the stainless gutter system includes an integral channel within its construction which serves as the conduit for filtered water return around the perimeter of the pool. This conduit is pressurized just as a traditional pipe would be, and it has many small orifices at the bottom face of the stainless steel gutter profile where returned water is distributed around the perimeter of the pool. This is a very effective solution for certain pool types and renovation projects where the construction cost of installing an extensive network of pool return piping may be prohibitive.
In many cases a combination of wall inlets and floor inlets is best for free form leisure pools, where wall inlets may effectively serve a current channel area or a lazy river, and floor inlets are necessary for a wide open water zone or are required by code for a shallow play area. Both inlets styles have their purpose, however, as stated previously, floor inlets are often preferred for larger bodies of water. I have observed on large competition pools with continuous perimeter gutter systems and only wall inlets for return of filtered water to the pool, that especially during times of minimal pool usage the wall inlet system does not effectively distribute treated water through the center of the pool volume. As with any component of the pool recirculation system, sound design practices must be considered for each specific pool type to ensure adequate performance and operational success.