How do I connect up a hayward pro grid de4820 to a pool pump

while on recirculate, the water is not being filtered but just returning to the pool. the first thing i would suggest is doing a bump of the grids and let them recoat with media.

Suction loss is due to one or more of the following;

• Skimmer wier stuck in upright position
• Water level too low
• Clogged skimmer baskets
• Obstruction in suction line(s)
• Breech in suction line
• Missing/ cracked/ dried out pump lid o-ring
• Pump pot basket full of debris
• Clogged impeller
• Broken pump seal
  

These account for 99% of suction loss problems, it may require a bit of detective work to find the problem.

There's a hole in the filter allowing DE and dirt to pass. Find hole and repair or buy new. I just bought new grids off of ebay, great deal. I payed $112 for complete set by Unicel and saved over $100 My filter is a Hayward DE4820

You did release the catch ... right? It should just lift out ... off the pipe and off the rod holder onthe other side ... wiggle it if necessary.

Air in line or dirty filter

You might have torn a grid or grids in the filter when you backwashed..

One of two things can cause this. Either the spider gasket is bad or there is a rip or tear in one or more of the filter grids.
P.S.
(the spider gasket is inside the valve you turn to backwash filter)

Chances are you have a torn grid inside, sometimes debris will cover the tear while it's running, then when the pump shuts off the debris settles to the bottom of the tank. Then when the pump turns back on, it shoots DE and debris through the tear until something else plugs it. Tear the filter down and inspect the grids, replace as necessary.

Here is your manual. Hope it helps... Rob http://www.haywardnet.com/products/Manuals/pdfs/Manual362.pdf