filter pressure slowly coming down

Diver

Well-known member
May 5, 2011
454
South of Boston
#1
This season I switched the motor on my superpump for a 2 speed one. After I opened the pool I experimented with my inline chlorinator to see how it would work on low speed (my CYA is low so I could use some trichlor&CYA in my pool).

It turns out that the flow is not strong enough and during my experiments the FC levels in the pool were border line low. So I got myself a nascent algae bloom – the water turned pretty hazy, but it didn’t turn green. Mistakenly thinking that I’m not filtering enough, I tried to filter it all out. It worked to some extent.

Last season I backwashed 3 time for entire season (DE filter). This time to get rid of the haziness I backwashed 5 times in a course of 3 days! The pressure was going from 9psi to 20 psi in a matter of few hours. I figured something was wrong and realized that I need to shock it. So I shocked the pool and the water got back to normal.

During this ordeal I noticed something interesting. At the end of the shocking process, pressure on the filter was 16 psi in the middle of one day. I figured I’d come home from work and back wash it again. When I got home, the pressure was 15 psi. The pump was on all this time, so it simply dropped during continuous pump operation. Sometimes pressure changes when the pump is stopped and started again but this was not the case.

Puzzled, I decided to let it run one more day. At the end of the next day the pressure was 12 psi. Next day even lower.

So I’m curious what process might result in the pressure on the filter coming down over time. First I thought that maybe my suction head loss increased so the filter gauge would read lower, but the pump vacuum gauge showed only small increased. Overall the flow improved from one day to another.

The FC levels were still around the shock level (not much sun last few days + cover), so could have FC eliminated (oxidized?) some dead algae in the filter? Would it work like that? Or is it a simple case of some DE settling on the filter grids?
 

JasonLion

TFP Expert
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LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
#3
The other possibility is that there is some kind of blockage on the suction side of the pump, which would lower the filter pressure as the blockage got worse. That could be anything from a full pump strainer basket to a clog in one of the pipes.
 

Diver

Well-known member
May 5, 2011
454
South of Boston
#4
mpacheco72, while it's possible, i doubt that the case here. it's been pretty consistent so far - i could see the flow getting killed as the pressure readings go up, pressure dropping after back wash. i see it matching vacuum reading changes. also reading on low vs high speeds..
 

Diver

Well-known member
May 5, 2011
454
South of Boston
#5
JasonLion said:
The other possibility is that there is some kind of blockage on the suction side of the pump, which would lower the filter pressure as the blockage got worse. That could be anything from a full pump strainer basket to a clog in one of the pipes.
Jason, I would see that on my vacuum gauge, wouldn't I? plus i can see that the flow improved.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
#8
Yes, a blockage would increase the suction pressure. If suction pressure is stable and there isn't any air in the pump strainer basket, then your filter pressure gauge is shot.
 

Diver

Well-known member
May 5, 2011
454
South of Boston
#9
no, gauge is not shot. i had doubts that it was, but decided to err out on a safer side and backwashed filter anyway. after backwashing, pressure dropped to expected 9 psi and vacuum was at 6". that's what i had at the beginning of the season and that's the same clean filter psi i had all last season (i didn't have a vacuum gauge back then).

at low speed the reading are also what i expect. the same goes for numbers during backwashing/rinsing and when I close MD and have it run on skimmer only. the flow on full speed is noticeably more, but it's not such a big jump as i'd expect going from 16 psi on a filter to 9 psi. at 16 my GPM should be about 35, and at 9 it's about 56-57. i definitely didn't get a jump of that magnitude.

so the gauge seem to be functioning properly and there must be something else. we got about 1.5" of rain yesterday but that should account for so much pressure dropping.

i should get a flow meter :)

edit: to play devil's advocate - all the numbers that checked out are below 11 psi, the last high psi reading. so technically my gauge could be shot and not be able to read anything above 11 psi. when pressure got to 16 psi few days ago, it broke the gauge and then it degraded to 11 psi even though the actual pressure could've stayed at 16 psi or even higher. but i thought i'd notice the difference in the flow though. i guess now i need to see if it eventually raises above 11. if it does, i should be ok :)