Dirty pool + sand filter = no rise in pressure?

[flv7a]

Hello--

I'm 6 weeks into pool ownership and it's been a rough ride. On a tip from a friend this has become my new go to site, so hopefully things are going to get better from here. At any rate, I've just finished killing off my third algae bloom of the young season. It took extremely high levels because I didn't understand the CYA effects of tabs for the first few weeks. This latest round may have been mustard algae, but I can't be definitive on that. After getting the pool to the 'milky' stage my sand filter cleared the first bloom in about 24 hours, with the corresponding pressure rises/back washes that I expected. Unfortunately for blooms 2 & 3 the story has been different- despite a healthy outflow from the return, the sand filter shows neither an increase in pressure, nor an indication that it's filtering anything out. My local pool store is happy enough to roll out a 'side system' DE filter, which clears the water, but functions more as a sales pitch than a helpful diagnosis of my current sand filter. Any ideas on why my filter has gone from perfect operation to 'stuck at 18 psi' would be greatly appreciated.

Thanks,

-flv7a

 

[JohnT]

Sand filters don't usually clog quickly from algae. They have fairly large capacity for and tolerance of dirty water. If you want to check, you can backwash the filter and see if the backwash water is dirty a few seconds after you start the pump.

Sand filters work better removing fine debris when they are slightly dirty. You can simulate this dirty state by adding a small amount of DE to the skimmer until you get a 1PSI rise in filter pressure to enhance the cleaning ability of your filter.

 

[flv7a]

Interesting--

So the local company has me back washing twice a day when the water is not clear, but that would obviously run contrary to what you've pointed out below. Is there an alternate theory on back washing frequency while trying to clear?

Much appreciated,

-flv7a

 

[JohnN]

Backwash when the pressure rises 25% of its clean pressure.

 

[CUTiger78]

Any ideas on why my filter has gone from perfect operation to 'stuck at 18 psi' would be greatly appreciated.

One idea - "stuck at 18 psi" makes me think the pressure gauge has failed and needs to be replaced. The pressure gauge should go to zero when the pump is off, go to it's "normal" pressure when the pump is turned on, and rise as debris accumulates in the filter.

 

[jblizzle]

As the sand gets dirtier, it will be able to filter smaller particles. But if you wait too long, the flow is reduced too much. Thus the recommendation of only backwashing when the pressure rises 25% over the clean pressure and not on some arbitrary schedule.

 

[flv7a]

Thanks all for the replies - it confirmed my suspicion - that backwashing constantly (irrespective of pressure) makes no sense and I was getting bad advice. The gauge is certainly working - it goes to 0 with the pump off and reads differently on backwash/rinse - it just never seems to rise above 18 when on filter. I'm going to give the DE additive a try. Much appreciated.

-flv7a

 

[duraleigh]

flv7a,

What does your water look like?

 

[flv7a]

Dave--

The pool looks great now (see picture 1), but about 10 days ago I was nuclear waste bad (see picture 2). The whole saga really exposed two big flaws in what the local store was telling me:
1. They couldn't explain why I was getting algae with reasonable (2-3ppm) chlorine levels, or why their shock levels "2 lbs per 10k gallons" weren't killing it up - a couple of minutes with the CYA (80) article in pool school cleared that up.
2. They were adamant that I needed to replace my sand filter with a DE filter - all the while telling me to backwash the sand filter twice a day while clearing regardless of pressure - not surprising from someone who sells DE filters.

A tip from a friend led me to TFP and the pool calculator - and even though I was able to clear the pool the folks at the store still look at me like I'm crazy when I tell them I was taking the free chlorine up into the low 20's to kill things off. I'm on the road to recovery now - thanks to the self education I've put myself through here with pool school - awesome resource.

Thanks,

-flv7a

 

[Big_kid]

Could your sand be channeled? That would explain the lack of pressure increase and filtration.

Adding a little DE helps our sand filter from time to time, but I usually end up backwashing it out once it's done it's job.

 

[flv7a]

"Could your sand be channeled?" - Perhaps, how is the best way to determine? Now that I've stopped the practice of "proactively" backwashing my pressure is increasing again - when I hit the magic 25% mark I'll return to a normal backwash cycle.

Thanks,

-flv7a

 

[duraleigh]

If your psi is back on the rise, you do not have channeling.

If you want to clean the sand, inspect for channeling, etc, you have to remove the top.

 

[Smykowski]

Search the forum for "channeled" or "channeling." You will find the procedure to deep clean and resettle the sand.

 

[john redcorn]

As the sand gets dirtier, it will be able to filter smaller particles. But if you wait too long, the flow is reduced too much. Thus the recommendation of only backwashing when the pressure rises 25% over the clean pressure and not on some arbitrary schedule.

Question on this, I'm new to sand filters as of this year.

Organics in pool vs. organics trapped in filter? If one is not regularly backwashing (I don't, I follow your above advice) are the organics trapped in the filter of any concern to FC levels or do these organics "die" quickly being trapped in the filter with a constant flow of FC-containing water?