My feedback on main drains in an above ground pool...

[Anonymous [GDPR] European in the UK]

What I really meant by that is that an HVAC fan will do fine with equal friction loss on both side but a pool pump will not. But I bet if I did a head loss calc on your setup, the head loss is not exactly the same on both sides of the pump and therefore the two sides are not exactly equal. If they were, you would probably have cavitation at higher RPM. Low RPM doesn't matter because the pump cannot cavitate on lower speeds.

Also, a PO on another forum thought he was being smart and energy efficient by running a separate line for every return (i.e. keep the pipe size AND ports the same on both sides of the pump). Combining that with a low head loss cartridge filter and the pump was cavitating severely. The only options for him was to either throttle the return with a valve or down size the eyeballs. At least with the latter, the higher exit velocity does some useful work while throttling just adds head loss.

Yes that person's idea was floored, not really from the plumbing point of view except the single lines for the returns is way too much as any flow calculator and headloss calculator would show but the issue is the wrong pump. We think pool, we think pool pump we buy off the shelf without thinking the issue through. Baring in mind I don't run the pump fast except for back washing cavitation isn't an issue. I have a slow start mechanism for the pump to build up to speed which is better than 0-2800 rpm but in the main the filter provides sufficient reduction in flow to prevent cavitation with the slow start and reduce power I use. At 30 watts the motor RPM is 1400, at normal pool speed 1800 RPM (69 watts). I agree they are probably not exactly the same but close and no cavitation as the power being used is controlled, unlike so many of the variable speed pumps that start with a burst of maximum RPM to prime presumably, I try to work with nature and not against it.

A bullet has far higher density than water and so it will sink before reaching the other side. Higher exit velocity of water will simply move in a straight line and the faster it moves, the faster it will get to the other side. One way to make a spa jet feel stronger is to use a smaller orifice spa jet which increase the force per unit area. When I changed my return eyeballs from 1" to 3/4" and I saw a noticeable difference in debris movement to the skimmer on low speed. However, there was only about a 1/2 PSI rise on pressure but I do have 4 returns. The smaller returns make the flow more directional but it also helps to move the surface water better at lower RPM for better skimming action. If the larger eyeballs are working for you then fine, use them. I was only saying that the smaller eyeballs can have benefits for certain situations.

Yes the bullet does but it also has a way higher velocity, it's path decays so quickly as the water put's up an opposing force (Newton's 3rd law of motion). I agree the directional flow does help so adjusting the size to suit is necessary. The flow doesn't move in a straight line though, it forms a turbulent street shedding it's power as it goes. It may move in the general direction but the turbulent flow helps mix the chemicals rather than pushing a jet of freshly chlorinated water into the skimmer which could upset the chemical redox probe. I too have 4 returns on my pool, another much bigger had 7 and 3 skimmers, that really got the water mixing and near no dead spots. My point about the eyeballs is they are crap from a fluid design perspective as they don't smoothly transfer from one diameter to another, there are shelves and steps everywhere so more back pressure is applied to the flow. I too use the smaller eyeballs where necessary but the total area of all the holes is near equal to the area of the pipe/s delivering it and the eyeballs are taper machined to the right size to achieve the best result.

My spend on electricity per season is around $32 at the current euro/dollar exchange rate. I refuse to go back to old ideas by old pool builders who say I have built pools for 30-40 years but really they haven't learnt a thing in that time they just repeat 1 years experience 30-40 times.