Sizing replacement filter

I'm refurbisihing me 25 year old pool. I bought the house 15 years ago. Just had the plaster re-done and want to replace my obsolete Mytilus Pacfab 150 cartridge filter because 2 years ago I bought the last replacement dome in the county. The pool holds about 23000 gallons, has a solar heater, no covers, and pop-up cleaners in the floor. Piping is 2.25" OD plastic pipe. I have a Hayward single speed 1.5 HP motor that was replaced about 5 years ago. It runs 4 hours/day in winter and 8 hours a day in summer. I clean my filter monthly unless there was a bad dirt storm which happens about twice a year. When I clean the filter, it is rarely dirty to the top of the cartridge. The water here is very hard (if you do not dry your car with a towel it will look like a spotted leapord) and one old pro told me to clean the cartridge monthly so that minerals did not harden and ruin the cartridge. Water pressure in the system is virtually constant at about 25 psi irrespective of the cleanliness of the filter. I had had a similar vintage pool before and could tell by the rising pressure whether the filter was clogged but not so on this pool. I am assuming this is due to the pop-ups which my old pool did not have. I once did a quick experiment and the pressure remained the same irrespective of whether the cartridge was in the filter or not.

The question is what size filter to replace my old one? Most salesmen here are telling me pool design has evolved since my pool installation and that I should be at about 400 sq ft instead of my present 150. They say I will have a cleaner pool, less pressure, and will have to clean less often. Using Pentair as examples, a 420 will be twice the cost of a 150, and replacement filters will be about triple the 150. Frankly I am dubious about the claims, except for the longer times between cleanings which may not be a good thing due to the minerals. I believe a larger surface area does not necessarily mean better filtering if my present cartridge is rarely full. It just means the larger cartridge will take longer to get dirty.

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.

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.