I've learned a great deal in the past week about what can/will go wrong with a sand filter. Many, many times on this forum people have complained about sand blowing back into the pool, and many times we (I) have advised people to go look for a broken or cracked lateral. I will in the future recommend that while looking for a crack in the lateral, that you also check the space between the lateral and the hub they fit onto. There shouldn't be one.

I have just had extensive work and time put into my filter because when it was first installed 6 seasons ago, it was apparently "pieced together" by the installer, and installed in such a way that there was tension on the hub containing the laterals. This caused three of the laterals to have enough space between them and the hub to allow sand through and up into the pipe carrying the return water. The sand got into the joint of this pipe where it connects to the hub, and proceeded to sand down the inside pipe, loosening the fit between the two, allowing progressively more and more sand to be blown into the return water and eventually into my pool.

The only symptom I had was a little sand appearing in the pool the first two seasons, and then becoming progressively more and more until this season, when if I turn the pump off and let it settle, I actually have about 1 - 1/2 inches of sand around the edges of the deep end. Once we figured out what the problem was and correctly installed a new hub and siliconed the pipe back into the hub, the problem is completely resolved. We vac'd it to waste, left the filter running for 2 days straight after that, and for the first time since the pool was built, have NO sand in it.

So--long story short--if you're having recurring problems with sand and don't find a crack or break in your lateral, carefully inspect where the laterals fit onto the hub. That may very well be your problem.

Janet