Quote Originally Posted by DennisP
What, if any is the "down side" to a main drain???
This forum and others are peppered with testaments about MD clogs and leaks. It is much more serious in an IG pool than an AG pool but still an issue that seems to be quite frequent. Given that every attribute that you assign to a MD can be accomplished in other ways, why have it? A MD is a little like an appendix, it serves no real purpose but it can be a real PITA when it goes bad.


Quote Originally Posted by Teapot
so if you have a skimmer it should be matched with the same size in square inches of returns so 1 return for a skimmer is not rational, of course it will still work but the flow restriction from 1 return limits by definition the flow into the skimmer.
I have a little bit of a problem with this statement. You actually don't want the returns to have the same area as the suction ports. Pool plumbing is not the same as HVAC. If the return head loss gets too low when compared to the suction side head loss, the pump has a tendency to leak air into the pump basket and even cavitate. In general, you want the return head loss to be several times that of the suction head loss and restricting the returns with eyeballs is a good way to ensure that while still providing useful work.

Plus return eyeballs serve another useful purpose. With higher exit velocity the water reaches more distant parts of the pool faster so mixing and circulation are a little better with smaller eyeballs. It does reduce flow rate but not by much and studies have shown (see sig) that flow rate and turnover are not all that important to pool sanitation and circulation. Also, the higher pressure does not cause more stress on the pump. In fact it causes less stress on the pump motor. Higher pressure means more head loss which in turn means lower flow rates and since a pump draws less power the further to left on the head curve (i.e. higher head loss), this also means there is less energy consumption and less heat to cause stress on the pump motor. It is also true that the efficiency drops a bit but if the run time is kept the same, the energy draw is still less, albeit less water is filtered per minute but again, I don't think that matters much.