Barring the introduction of odd chemicals like paint or Baquacil to the water, you may never need to replace it. If you have to service the internals of the filter, it might not hurt to change the sand.
There are two schools of thought on replacing the sand. Fresh sand has lots of sharp corners and edges which slowly get worn off as the filter is used. Some people recommend replacing the sand every five years. I just keep using the sand forever, as long as it doesn't get all clumped up with scale (which is rare), and add a little DE to make up for the smooth edges when the sand has gotten older.
as i recall sand works best when it has those sharp edges and has dirt on it, it will work worse when new that used for a season or two. mine is starting to channel so i guess i will have to get new sand or just flat out get a new pump and filter this year.
Of course, this debate goes on forever about sand. It makes sense to me to NEVER replace your sand but simply take care of it. I believe sometimes we over-think what makes a good filter medium:
1. Dirty sand filters better..........hmmmm. maybe, but how dirty? If you believe that verbatim then you will never backwash 'til your filter explodes from running 200psi and filtering no water (I'm kidding...it won't do that but I'm making a point.....you certainly can't let it get too dirty so how much is enough?)
2. New sand has sharp edges and, as a result, catches more dirt......hmmm, maybe, again. Until I see some new sand and then 10 y/o sand under a microscope, I'm not sure the sand gets that much wear and tear.
Even if it's true, couldn't you make the argument that the old, smooth sand fits together more tightly creating smaller spaces for the water to pass through and, thereby, catching more dirt?
Different brands of sand would certainly have different characteristics....I believe the only criterium to be called "pool" sand is the the particles be in between some size values but itr doesn't say what percaentage of those particles can be smaller or larger.
Until my filter starts spitting dirt back into the pool, I'm gonna keep this sand forever. BTW, Vince, if your sand has channeled, simpy open the top of the filter and break it up. Backwash thoroughly, and you're good to go for another season.
I've often felt that channeling is a possible sign of an overdriven filter, i.e. the pump pushes the water thru the filter with too much force but contaminates in the sand could also be a culprit.
From what I've read here and my personal experience, I'm pretty convinced that a big, big filter will solve most of these issues. Whether that filter is cartridge, DE, or sand bigger is simply better in virtually every case.
You don't need to replace the sand when it starts channelizing, you need to resettle it. This can be done by opening up the filter and very gently poking the sand everywhere with a gently running garden hose. Try to rinse all of the sand but be careful not to apply any significant pressure to the laterals (which are buried in the sand) with the garden hose. Any obvious debris should be removed, usually it will float to the top. Any clumps that have developed should be broken up. If there are clumps that you can't break up by hand then you must replace the sand, though that is very rare.
I actually remove the lint cover (top) off the sand filter and reach an arm in and stir the sand around. Best to do it with the pump running and in the backwash mode. Granted this is a mess to do but it really results in clean filters if you can force yourself to do it.
I've switched all our filters to the Zeosand media, I have noticed it will clump or solidify a lot more than sand did but I still like the benefits that it supposedly gives by reducing ammonias naturally from our water.
I always think about how many millions of years the ocean has been washing up on a sandy beach and thinking that this sand doesn't look any worse for wear than any other sand I've even seen. Makes you wonder doesn't it?
There is a difference between water washing up on a beach and water running through a bed of sand. I you have any doubts just look at how running water can erode a mountain! I HAVE seen both new and old sand through a microscope and old sand is more rounded. Granted it was from a sand filter on an aquarium but then again Pentair makes aquaruim sand filters and they look JUST LIKE the ones they make for pools! HMMMM.