Pump/Filter Sizing Mytery

[kingbud]

We bought a place with a gunnite or concrete pool 20 x 40 x 12’ deep, knowing that we were going to have to work on it. 45,000 gallons. Call it Lake Woebegon. The 1.5 hp pump is shot, it cavitates, it leaks, it is loud. The DE filter is also shot – amazingly, it is a 36 square foot Sta-Rite unit which I discovered at startup had two grid ports permanently plugged, reducing capacity by something like 15%. When you pick the grid assembly up it tinkles like Christmas ornaments, there is so much broken plastic rattling around. No kidding. And the grid’s fabric is punctured in quite a few places. Another delightful surprise we found is that the main drain is plugged with screw-in plugs. To recapitulate: the pool in its present state is being serviced by a severely crippled filter, an oversized pump that is slamming it with air, and one 1-1/2” suction line coming from the skimmer. By the way, there are three return lines, two at each end opposite the skimmer (which is located near the corner of the deep end), and one on the same side as the skimmer in the shallow end.

Those who have read my other recent posts might recall my tale of a massive algae bloom, which with patience and a lot of chlorine we managed to kill off. Well, here’s the kicker: the existing filter/motor setup cleared up the water after the bloom, if not overnight then over two nights. The water is crystal clear.

What am I not getting here? A quick glance at filter sizing charts recommends 72 square feet, a 2 hp motor, and three 2” suctions for 45K gallons. How is my crazy system doing such a good job of clearing the water? Is this a testament to the intrinsic superiority of the DE filtering concept?

Another question: assuming that I can get the main drain opened up and repair or replace the broken sections of plastic pipe, thus giving us two 1-1/2” suction lines, then what minimum filter/motor combination can I “reasonably” get by with when I go to buy new equipment? (I must confess, I am squeamish about punching holes in the pool walls to add suction lines.) Incidentally, I am seriously thinking about converting to a vinyl liner – one big advantage (I assume) would be that I can fill the bottom 4 feet of the deep end with sand and shrink the pool volume to roughly 36K gallons.

Talk about your fun in the sun!!

Let me say this, I am grateful for having the support of the many kind and knowledgeable people that participate in this forum. Where would we be without the internet?