I am having my pool closed for the first time on Friday. The ladies at the pool store have advised me to have the water lowered to 4" below returns prior to my appointment. I have read that a DE filter should be backwashed prior to closing, yet when i am asked them they said no.

So what is correct? Is the pool guy going to show up and want to backwash my filter after i have already lowered level, thereby causing the level to be lowered more?

Use the backwashing process to start lowering the water level. Closing would be a good time to open the filter and clean and inspect the grids so you are ready to go in the spring.

John is perfectly correct!

You want to take the DE filter apart at least once a year and fully/properly clean it! Doing it on closing means that you will be all set to open the pool and swim come next spring

Since you guys are at it, what's the proper dilution factor for cleaning the filter with muriatic acid? I saw some copper tubing inside.

I think the standard is 10:1 for a few hour soak, I use closer to 25:1 (water: acid ) because I soak them for 10 - 16 hours at a time and don't want the acid to attack the metal nuts and bolts and washers that hold them together

PLEASE also remember to do the degreasing soak PRIOR to acid washing them!!!

Degrease what and with what?

Before! you put the grids/ fingers - filter elements into an acid solution, degrease them in a solution of either electric dishwasher powder or TSP (Tri - Sodium- Phosphate). 1 cup of either per 10 gal. water should be enough. Allow a few hours to soak, agitating the mix certainly wouldn't hurt , rinse them and then put them in the acid mix.

Unless you have calcium in the water, many pools do, you probably won't even need the acid wash {Chem 201 moment (not that I'm qualified ) - acid coagulates oils on the membranes, causing worse clogging than you started with and acid only attacks metals, like calcium, that are congesting the membrane }

The thing to look for before doing a chem soak, is how well the membrane is letting the water flow out of it, after you've rinsed them with the hose (if they still hold water for ~ 30 seconds, it's time to soak them).

I hope this clarifies what I didn't say in the earlier post