Sizing replacement filter

[mas985]

For cartridges, bigger is always better. A larger filter not only means less frequent cleaning, but because the surface area is larger, there is less head loss in the filter (i.e. higher efficiency), and there is less stress on the filter fabric meaning the filters will generally last longer. Also, since there is a little degradation with each cleaning, longer times between cleanings mean longer cartridge life as well. Although the cost is more than the smaller filter, the lifetime cost is likely to be less with the larger filter. Also, scaling should not be an issue if you keep your pool chemistry balanced.

I have a 420 sqft filter on a 20k pool so similar to your situation. I clean the filters once per year and now on my 7th year and still going strong. I would highly recommend a larger filter.

[Billj]

As a general rule we like to go large as possible when replacing filters for the reasons stated above. I however am from the school of thought that says If it ain't broke... It is hard to argue with success. If that 150 sf filter worked all those years I don't see a real good reason to up-size, particularly since your water condition requires frequent cleaning. Pentair makes a model called "Clean & Clear" that comes in a 150sf and 200sf. The tanks are the same size, just different pleat count on the (single) filter element inside. It also has a twist lock ring that requires no tools and is very, very easy to open and close. Just a heads up if you decide to stay with the same square footage.