We recently bought a house with a pool; I think I have the basic chemistry under control thanks to this site, now moving on to address some of the equipment oddities, particularly the filter (and later I will have some skimmer/main drain questions for another thread).
Basics: 21000(est) gallon pool, plaster finish, likely built in the late 50s/early 60s, probably resurfaced more recently. 2 pumps: a 3/4hp main pump (circa 2005 from a phone call to AO Smith) and 3/4hp boost pump (circa 2011) connected to a relatively new jet on the side with a Polaris pressure side cleaner. Seems to keep the bottom pretty clean (when I remember to zip the bag, doh!).
My questions are about the filter -- DE, of unknown spec and age. The actual container (not including stand) is green metal, 24Ē high and 18Ē in diameter. It has a ďLeslieís Diatomaceous Earth FilterĒ label, but all of the actual info fields appear to be blank (faded off with age?). Some rust all around the filter, and it (or something nearby) is leaking. There is a backwash valve hard-piped underground to the street (to a drainage ditch that flows to a creek then to the river). All of the house drains and sewer cleanout are around the front, around 200í away.
(view of the pressure gauge from the back)
All of this is under a very low structure that was clearly built on top of the equipment after it was installed. The roof is permanently installed, and there are a couple of access doors not quite in the right place (I had to reach my phone in to get a picture of the pressure gauge). Iím guessing the filter hasnít been opened since then. Iím going to take the structure apart, at least opening up the roof. Unfortunately all of the electrical is (badly) done on one of the sides, so Iíll need to keep part of it for now.
The ďhutĒ: http://tinypic.com/r/dbnn2a/8
So, finally, the question:
Iím hesitant to even try to backwash this thing -- is there a simple way to figure out/estimate how much DE to add? Any idea how old it is? The guy at Leslieís said it was so old he didnít know, suggested opening it up to measure the grids. Iím afraid to do that for fear that more things will get broken/contaminated and now be an emergency repair.
Iím inclined to just replace the whole filter now, most likely with a cartridge filter (mainly to avoid the backwash issues), and have read some about sizing etc. I was planning to do the installation myself if possible. Are there any gotchas I should know about? Is it as simple (hah!) as removing the multiport valve and repiping the pump/filter outputs/inputs directly? Should I expect any issues/changes with the water if I do this? Or am I looking at this wrong and this one is not end-of-life and worth saving?