The larger the pump, the greater your pumping and maintenance costs. Therefore, you want to use the smallest size pump possible for your swimming pool. To choose the right size pump, you can consult a pool supplier's design chart. Using the chart, match the hydraulic characteristics of the pump to both the piping and the pool's flow characteristics. For a solar pool heating system, you also need to consider the need to pump the pool's water to and through the collector(s).The Florida study shows that a 0.75 horsepower or smaller pump is generally sufficient for residential pools. Smaller pumps, which cost less, can be used if you decrease the pool circulation system's hydraulic resistance by doing the following:
Substituting a large filter (rated to at least 50% higher than the pool's design flow rate)Increasing the diameter or decreasing the length of the pipes, or replacing abrupt 90-degree elbow pipes with 45-degree ones or flexible pipes.By decreasing the pool circulation system's hydraulic resistance, you can reduce the pump's electricity use by up to 40%.
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I will agree on the first point that larger pumps will cost more to operate.
In the early 1980's, manufacturers pushed bigger pumps saying they were better and dealers took the bait. We would customarily find systems with 2 HP pumps on 48 SF DE filters. The grids were usually crushed and rendered useless. Plus, you were not getting the full benefit of that pump's GPM as it as was being held back.
A simple solution to efficiency and keeping to code for VGB, is to use larger pipe especially on your suction side to decrease your FPS. We use 2.5" and 3" on our suction lines whether a skimmer or floor or strip drain. It depends on the application - pool or spa set up. Most of our "typical" pools our filter pump is 1 HP. Pools with multiple pumps for water features, etc.. require crunching numbers to determine proper pipe size to match pump size.
We always use the largest filter whether DE or cartridge (Cartridge filters can handle a higher flow rate) on any size pool or spa. Here, I believe bigger filters are better.
There's nothing wrong with using 90 degree elbows provided the pipe length before the turn is at least twice the distance of the pipe's diameter. We usually go 3X's. As an example on 2" pipe, our pipe nipples are minimum of 6" long. This is of course if space allows. 45's don't really offer much of a reduction plus you have additional glue joints to assemble and rely on.
We prefer using sweep pressure 90's (From Waterway) or we heat bend a predominant amount of our pipe to make turns that are not quite capable of having fittings. We can and have, heat bended our pipes into sweeps when we have the room in a trench.
Flexible pipes? Sounds like you are referring to flexible PVC which is banned from any of our projects. It's known to have insect infestation and chlorine can destroy it over time. It also can become mis-shapened, egg-shaped, and very brittle when buried making it difficult to glue and seal a fitting during a repair.
We only use Sch. 40 rigid PVC pipe. Some applications call out for Sch. 80.
Variable speed pumps can help overcome systems with existing smaller pipe allowing you to "dial in" a flow rate and still maintain a proper turnover rate. They work well for solar systems when they call for heat as they can ramp up the speed to allow for the climb to solar panels on a structure's roof.