Sand Filter Backwashing, am I missing something?

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Bob2010

Active member
May 30, 2023
27
Ontario
Pool Size
15000
Surface
Vinyl
Chlorine
Salt Water Generator
I've been reading the wealth of information contained here and from what I can tell I should backwash When my filter pressure is 25% above baseline. I have 1.5 inch plumbing with a 1.5 hp VS pump. I use 3200 rpm to gauge and post backwash pressure is around 18. Based on that I should be backwashing somewhere around 22-23 PSI.

It never seems to get that high, maybe I'm backwashing too frequently but it was about a month since my last backwash and the filter pressure was still 18-19, but I noticed significantly less flow at the return jets probably 50% less than previous for a given RPM my VS runs 1500-2500 max rpm. Is this normal? Should I just let the low flow at the returns ride until the pressure creeps up to 22-23? I didn't want to cause any damage so I have just been backwashing when I notice the flow is significantly less since the pressure never goes up. I don't think my filter is oversized for my pool so I wasn't expecting to get the 3-4 month frequency between backwash that some people here have reported.

Thanks for the help.
 
The 25% reccomendation is just that, and a good start point for folks learning. No two pools or systems are the same and we certainly can't know how they'll all respond, but 25% covers most of them.

If you're seeing reduced flow, it's time. (y)

Does the gauge fall to 0 when the pump is off ? If not, the gauge is bad.
 
Bob2010 said:
I've been reading the wealth of information contained here and from what I can tell I should backwash When my filter pressure is 25% above baseline. I have 1.5 inch plumbing with a 1.5 hp VS pump. I use 3200 rpm to gauge and post backwash pressure is around 18. Based on that I should be backwashing somewhere around 22-23 PSI.

It never seems to get that high, maybe I'm backwashing too frequently but it was about a month since my last backwash and the filter pressure was still 18-19, but I noticed significantly less flow at the return jets probably 50% less than previous for a given RPM my VS runs 1500-2500 max rpm. Is this normal? Should I just let the low flow at the returns ride until the pressure creeps up to 22-23? I didn't want to cause any damage so I have just been backwashing when I notice the flow is significantly less since the pressure never goes up. I don't think my filter is oversized for my pool so I wasn't expecting to get the 3-4 month frequency between backwash that some people here have reported.

Thanks for the help.
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I wouldn’t use the 3200 rpm as your baseline. That’s crazy fast. Use whatever pressure your pump spends the most time running at. If it’s 1500, use that as the baseline pressure. My sand filter can go most of the season without needing a backwash. (Though there’s no water in the pool at the moment so I haven’t had to backwash all season so far. 🤣)
 
Bperry said:
I wouldn’t use the 3200 rpm as your baseline. That’s crazy fast
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It's much easier to see the 25% increase at 18psi than it is in the single digits at low RPM. Many of us are at 3 or 4 psi under normal operating conditions. (y)
 
Newdude said:
It's much easier to see the 25% increase at 18psi than it is in the single digits at low RPM. Many of us are at 3 or 4 psi under normal operating conditions. (y)
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Technically true, but sand filters don’t work as efficiently at such high flow rates. They filter better when the pressure is lower and filtering better by definition it means it’s catching more particles. Catching “more” particles should cause the pressure to rise faster and be noticeable on the pressure gauge. Maybe a different % increase is needed in this case.

Running such high pressure tends to just force particles that would otherwise be caught in the filter to be pushed back into the pool.
 
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^^^^^^^ what a GREAT thought. Bravo. :salut:

But it opens up other thoughts, such as the single speed pumps with no choice for decades. Did they still work well enough for the task at hand, even with the inefficiency of too much flow, or was it simply accepted as a necessary evil at the time.

Meh. Either way, running a mid range RPM in the 2ks would satisfy both sides when checking for PSI.
 
Newdude said:
^^^^^^^ what a GREAT thought. Bravo. :salut:

But it opens up other thoughts, such as the single speed pumps with no choice for decades. Did they still work well enough for the task at hand, even with the inefficiency of too much flow, or was it simply accepted as a necessary evil at the time.

Meh. Either way, running a mid range RPM in the 2ks would satisfy both sides when checking for PSI.
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All good questions. Maybe those folks just backwashed once a week like the pool store told them to and didn’t worry about pressure differences? In my case, 25% pressure rise is very slow and would take months maybe even the whole season to reach. Maybe it would be a fun experiment to see the difference of 1800rpm (my minimum) and 3k rpm on filter pressure. I was also keeping the water very clean so there may be something to it there as well. If folks are keeping the FC at 1ppm with 100CYA then the filter pressure is going to go up pretty quickly regardless of pump speed. :laughblue:
 
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Hi thanks for all the info and discussion.

I want to clarify that I don't run the pump at 3200, only for 30 seconds or so to check my pressure every few days. I could use a different baseline but I'm not sure that would change the original dilemma I was having because I haven't noticed the pressure increase from clean operating pressure even at say 2300 rpm.

I find post backwash my filter and heater can both run consistently at around 1300 rpm, so for that reason I tend to run the pump at the lowest around 1800 rpm, it spends most of the time there with some increases to the 2300-2500 max for a few hours for skimming.

Bperry, what size sand filter do you have? Mine listed below is 2.6 sqft from what I can tell ( 250 lbs sand). I'm not sure what type of filtering capacity I can expect with this. I will say that my pool now post SLAM has not had any issues ( the last backwash I did included some of a SLAM time with the filtering leading up to it though, so I may see some difference in flow differentials this time around). It also has an automatic cover most of the time so overall Debris load is minimal.

I'm curious as to your thoughts to my original dilemma. My filter regardless off RPM doesn't seem to change pressure, but I do notice significant reduction in the return jets for any given RPM which is restored post backwash. Should I just continue to ride it out until pressure goes up or backwash based on flow do you think?
 
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Bob2010 said:
Yes sorry it does and gradually goes up as I cycle through the rpm
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Then I see no reason to backwash if the pressure isn’t rising and your pool returns still have good flow. A sand filter supposedly works better when it’s a little dirty.
 
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