Pump Pressure low on Filter but great on Recirc. Why?

My pool is 17000 gallons in-ground with a Hayward 1 hp pump and Hayward 2420 DE filter and I can't seem to get any flow when in filter mode , I backwash and it's good for 30 seconds but then back to 18 psi and low flow. My levels and numbers are good cl 6.0 in shock mode, ph 7.5, alk 110 ppm. I am trying to open but can't get it to filter and stay within 5 to 14 psi range that it stayed in for the last 4 years. Please help. By the way , this is the ultimate place to come to for help, so many knowledgeble people and always with the best solutions.

Have you compared your flow between "filter" and "whirlpool". there should not be much difference at the return if your filter is clean .

Have you cleaned the grids on the filter?

Yes I have cleaned them and it will be ok for a few minutes but then go back to high psi and low flow. Only has good flow in recirc mode.

Dj, the grids are clogged with something, this is proven by the fact the system runs well when in 'recirc'. You need to take the grids out, hose them off well, then soak in a degreasing mixture of tri-sodium phosphate ot powdered automatic-dishwashing detergent for 1+ hrs, rinse well and then soak them in a ~10 : 1 water acid mixture for a few hours and rinse them again. This should remove whatever is clogging the grids and return you to being able to filter. Please make sure that you add the proper amt of DE on start-up and after every backwash (if you forgot to add the DE after a backwash that may have caused the problem to begin with)

I am confused about the whole recharging of the grids, I know you need the powder to help filter the water but why does a lack of de powder cause a problem. You would think the flow would increase with no powder to push through.

dj,
waste will correct me if I'm wrong, but I think it's because dirt gets stuck on the grids in the absence of DE. Some of it stays there, restricting the flow, even after you backwash and add DE.

One other question that is puzzling me , why is it that when I backwash and then recharge the grids , the flow is excellent and my psi is between 5 and 14 psi but after a minute or sometimes less it goes back to low flow. To me that would indicate that there is not a clog , otherwise it would always have low flow and would not matter if I backwashed it or not. I am an Auto Mechanic so the theory is what is puzzling me , either it is always clogged or not. I don't see de shooting back into the pool so I don't think I have a torn grid but it seems to slow up very quickly after I add the powder , and I am only adding 2 pounds like my filter calls for. I guess I could keep backwashing but I would spend a fortune in de powder. And I don't think that is a logical solution.

dj,
Again, I'm ready to stand corrected by waste on this, but I think the dirt only partially clogs the grids. The rest of the job is done in those first few minutes after adding DE.

No , I don't mean to disagree with you I'm just confused . Is it possible I should fill to above the skimmer and vacuum to waste to ensure all the crud on the bottom is gone? That was what I originally intended to do but also wanted to circulate the shock (BLEACH) to make sure it was well distributed and not just pooling up in one spot and that is how I noticed this problem.

If the water is really dirty, a DE filter will behave like you describe. They can require almost continuous backwashing. When the grids are dirty, sometimes only a small area is open, and that area can clog pretty quickly since all of the water (and dirt) flows through that area.

Thanks for the help , finally my pool is blue and clear. I filled to above the skimmer and then vacuumed to waste until I could see the bottom. Then when all visible debris was gone I soaked my grids in automatic dishwashing soap and the scum just melted off. Now my pump is at it's normal range of 10 psi and thats where it has been for the last 4 hours without missing a beat, once again thank you for the help.

dj,
Great news. Thanks for letting everyone know how it turned out.