Sand to DE filter help

[jcowart]

Just want to say I love this forum. I have an 18k AGP. One skimmer and one return...so no water features. Right now I have a powerline "DB" 1.5hp with a Aquaview 20" sand filter. The pad is right their and is plumbed with 1.5" PVC. Total of 8' from skimmer to return. The water is very clear in the day but skim milk at night with the light on. My OCD has convinced me that it could be better. I keep my water balanced and had no problems last year other than at night with the light on. I learned that my pool uses about 1.5 ppm a day in the hot southeast Texas sun, and could even judge it by the cloud cover. Right now my levels are:
FC 6
CC 0
Ph 7.5
TA 90
CYA 50
CH 180
I took my filter apart and pulled out cotton like stuff and washed the sand with a hose. After I started it back up and let it settle I added some DE. The problem is as soon as I started to add it I could see a white haze from the return. I found a DE filter for sale for $350. It's a Hayward EC50C92S 1hp with 3 way valve. The specs show it as 25 sq ft with a flow of 50gpm. What I come up with is 18000gal/8hours/60mins is 37.5gpm. Is the average DE 2gpm per sq ft? If that's right then I would need a min of 18.75 or say 20 sq ft.

 

[jblizzle]

For an 18k gallon pool, we would generally recommend a 3.2 sqft (24") sand filter or a 48 sqft DE filter at a minimum.

So, your current filter is slightly small and your pump is pretty oversized ... a common problem we see. That DE filter is much too small. Our filter recommendations are based on pool size and the pump size is based on water flow needs ... you do not seem to have high flow rate needs, so the smaller the pump the better. Rarely is the filter recommendation too small for the pump recommendation.

It sounds like something may be off in your current filter. It may be worth a thorough inspection to see if anything is cracked/displaced and allowing the debris through the sand.

 

[jcowart]

I have searched and searched. Where or how do you come up with 48 sq ft. I figured the filter and pump was not sized right but that's what I have to work with. I did notice that the sand was lower on the right side "away from the pump". Once washed, it was level all the way around and up to where the tank starts to round up. I just pulled the valve again and the sand is lower on the right side again.

 

[jblizzle]

There is a table here: hydraulics-101-have-you-lost-your-head-t915.html#p6543 for filter sizing. We generally recommend the table value for sand and double the table value for cartridge and DE to reduce the frequency of filter cleaning.

Really you can go as large as you want for a cartridge filter, but for sand and DE you have to be sure you have enough pump power to adequately backwash.