When do i need to backwash?

BIC

Well-known member
Jul 7, 2007
108
Hi everyone,

I have a 24' AG pool with SWG.

I'm just wondering when do I need to do a backwash? I have looked around and found different answers.

1) backwask every week.

2) backwask when your pressure is 4 tp 6 psi higher than the normal pressure.

3) backwash when the pressure is 8 to 10 psi higher than the normal pressure.

My pool is new and my starting pressure was 12 or 14 psi. After 2, 3 weeks, the pressure is now at 16.

What are the signs that tell me to do a backwash?

BIC
 

donaldm823

LifeTime Supporter
May 21, 2007
148
Cape Coral, FL
BIC said:
Hi everyone,

I have a 24' AG pool with SWG.

I'm just wondering when do I need to do a backwash? I have looked around and found different answers.

1) backwask every week.

2) backwask when your pressure is 4 tp 6 psi higher than the normal pressure.

3) backwash when the pressure is 8 to 10 psi higher than the normal pressure.

My pool is new and my starting pressure was 12 or 14 psi. After 2, 3 weeks, the pressure is now at 16.

What are the signs that tell me to do a backwash?

BIC
My pool builder said I should backwash my DE filter at least 1x per month or when backpress rises 8psi. I have found that it has never in 2 yrs has risen that high. On several occassions, it went up 5-6psi after shocking the pool-at which time I also backwash. I have a 60ft3 DE filter/2hp pump and 30000 gal concrete
 

JasonLion

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May 7, 2007
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Silver Spring, MD
I backwash when the water stops being crystal clear and there isn't any other obvious cause (like low FC levels). That ends up being about once a month and 8 psi for me. At the same time I normally tell people to backwash when the pressure gets 10 psi above the starting pressure. The difference in how often you backwash is going to be minor in every way except water usage. More frequent backwashing uses more water, which can be a big issue in some areas.
 

BIC

Well-known member
Jul 7, 2007
108
JasonLion said:
I backwash when the water stops being crystal clear and there isn't any other obvious cause (like low FC levels). That ends up being about once a month and 8 psi for me. At the same time I normally tell people to backwash when the pressure gets 10 psi above the starting pressure. The difference in how often you backwash is going to be minor in every way except water usage. More frequent backwashing uses more water, which can be a big issue in some areas.
How come your pump runs at 8 psi and mine at 12 psi (normal)? My filter is a Hayward 210T and my pump is an Ultra Max.

The difference between psi seems to be big.
 

tagprod

LifeTime Supporter
Jul 3, 2007
489
Tomball, Texas
BIC said:
JasonLion said:
I backwash when the water stops being crystal clear and there isn't any other obvious cause (like low FC levels). That ends up being about once a month and 8 psi for me. At the same time I normally tell people to backwash when the pressure gets 10 psi above the starting pressure. The difference in how often you backwash is going to be minor in every way except water usage. More frequent backwashing uses more water, which can be a big issue in some areas.
How come your pump runs at 8 psi and mine at 12 psi (normal)? My filter is a Hayward T210 and my pump is an Ultra Max.

The difference between psi seems to be big.
One more thing - how long are you supposed to backwash?
 

JasonLion

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May 7, 2007
37,879
Silver Spring, MD
I was trying to say that I backwash when there is an 8 psi increase in pressure, but as it happens I also start at 8 psi right after backwashing, so I backwash when it gets to 16 psi.

The pressure reading right after backwashing has to do with the size of your pump relative to the dynamic head of your plumbing system. A larger pump will result in a higher starting psi reading. Smaller pipes, and/or longer pipes, and/or more fittings (right angle bends, heater, SWG, check valve, etc) will all result in a higher starting psi reading. I have a rather small pump, so low starting psi reading.

You backwash till the water runs clear in the sight glass, plus a little just to be sure. Depending on the relative size of the pump and filter and how dirty the filter is that can be anything from a few seconds to several minutes.
 

BIC

Well-known member
Jul 7, 2007
108
JasonLion said:
I was trying to say that I backwash when there is an 8 psi increase in pressure, but as it happens I also start at 8 psi right after backwashing, so I backwash when it gets to 16 psi.

The pressure reading right after backwashing has to do with the size of your pump relative to the dynamic head of your plumbing system. A larger pump will result in a higher starting psi reading. Smaller pipes, and/or longer pipes, and/or more fittings (right angle bends, heater, SWG, check valve, etc) will all result in a higher starting psi reading. I have a rather small pump, so low starting psi reading.

You backwash till the water runs clear in the sight glass, plus a little just to be sure. Depending on the relative size of the pump and filter and how dirty the filter is that can be anything from a few seconds to several minutes.
Thank you again for the info Jason 8)
 

AnnaK

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Jul 15, 2007
1,138
Eastern Pennsylvania
BIC said:
What are the signs that tell me to do a backwash?

Hi BIC,

I use two primary indicators:
1. The pump pressure is 8 psi above 'normal' -- with 'normal being the pressure shortly after a backwash/rinse.
2. The water in the sight glass is getting murky.

I have a 2-speed pump and I look for the pressure differential when the pump runs on high speed.

When I backwash I use the trick I learned on one of the forums: catch some of the discharge water in a white plastic pitcher every now and then during the process. Initially, this water will be dirty, murky, and will have lots of floaters. When it's clean, the sand is clean and you can go to the rinse cycle.

When I have a lot of water in the pool such as after a heavy rain, I backwash whether it needs it or not, and I shoot for two cycles, the second one shorter and just to give it an extra blast. It seems to remove those last few grains of sand that seem to collect in the sight glass after a rinse.

Sometimes, after vacuuming a heavy load of pollen and tree flower debris, I'll do a backwash though I always vacuum with a skimmer sock in which catches this stuff. I've never had to shock my pool but if I did, I would backwash afterwards.

Anna
 

duraleigh

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Obviously, everyone's pool circulatory system is different. It seems a percentage increase in pressure form "clean" condition could be applicable in every case....I'm not sure.

For Mas985 and you other "inginears", Is it reasonably easy to predict the resultant loss in flow if your filter pressure increases say 40% or 100%. My ignorant guess is that a 100% increase in filter pressure would result in a pretty significant flow loss but I simply don't know.

Refill water is fairly hard for me to come by (low gpm well) so I would love to backwash as infrequently as possible.....just don't know how much efficiency I'm losing with high filter pressure.
 

JasonLion

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May 7, 2007
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Silver Spring, MD
There is no easy way to guess the flow rate to filter psi relationship without knowing some of the other numbers. Basically, it makes a large difference what the shape of your pumps head curve is and where on the pump curve you are currently falling. If you are near the max dynamic head for your pump, towards the left in a typical pump head curve, the flow reduction will be dramatic. If you are near the max flow rate for your pump, towards the right in a typical pump head curve, the flow reduction will be much more gradual.

Further, even though a well designed system will fall somewhere around the middle of the head curve, you still can't learn very much, since the slope of the curve near the middle varies from pump to pump. Here is a typical set of pump head curve charts if you want to see what I am talking about.

The 3 HP pump in that diagram going from 60 feet of head to 80 feet of head will go from 175 GPM down to 135 GPM, a 23% loss of flow. Meanwhile, the 2 HP pump going from 70 feet of head to 90 feet of head will go from 108 GPM to 55 GPM, a 49% loss of flow. So the pump with the larger starting pressure does worse at identical psi increases. That is of course a contrived example but it shows the kind of variation you can see in the real world.

Real world pools seem to fall all kinds of different places on their pump's head curve so I expect that if you could measure flow rate on many different pools you would get all kinds of different changes in flow for the same change in filter pressure.

Ideally every pool would have a flow rate meter and people could clean the filter when the flow rate fell by 1/3. Since almost no one has a flow rate meter, the best we can do is guess at an average psi increase that might correspond to a 1/3 loss of flow in the average pool to recommend as a starting point. From there people who are patient and observant can adjust based on watching for small differences in water clarity at different psi changes.
 

Rangerman

LifeTime Supporter
May 27, 2007
229
Fayetteville, Arkansas
duraleigh said:
Refill water is fairly hard for me to come by (low gpm well) so I would love to backwash as infrequently as possible.....just don't know how much efficiency I'm losing with high filter pressure.
My pool has a french drain under the main drain. I route the french drain water through the filter to backwash. No waste from the pool this way. Seems to work well for me. :)
 

duraleigh

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Rangerman,

My pool has a french drain under the main drain. I route the french drain water through the filter to backwash. No waste from the pool this way.
That's very interesting but I can't picture your system. Do you have an alternate intake to the pump and pick up water from another source (french drain) other than your pool?
 

Rangerman

LifeTime Supporter
May 27, 2007
229
Fayetteville, Arkansas
duraleigh said:
Rangerman,

My pool has a french drain under the main drain. I route the french drain water through the filter to backwash. No waste from the pool this way.
That's very interesting but I can't picture your system. Do you have an alternate intake to the pump and pick up water from another source (french drain) other than your pool?
Yes - the french drain is a line by itself. I close Jandys from both the main and skimmers (one valve set to close line into filter), open the french drain line into the multi port to backwash, making the water source the french drain. No pic, sorry.
 

duraleigh

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Rangerman said:
duraleigh said:
Rangerman,

My pool has a french drain under the main drain. I route the french drain water through the filter to backwash. No waste from the pool this way.
That's very interesting but I can't picture your system. Do you have an alternate intake to the pump and pick up water from another source (french drain) other than your pool?
Yes - the french drain is a line by itself. I close Jandys from both the main and skimmers (one valve set to close line into filter), open the french drain line into the multi port to backwash, making the water source the french drain. No pic, sorry.
What is the source water for the french drain....just ground water?
 

Rangerman

LifeTime Supporter
May 27, 2007
229
Fayetteville, Arkansas
duraleigh said:
Rangerman said:
duraleigh said:
Rangerman,

My pool has a french drain under the main drain. I route the french drain water through the filter to backwash. No waste from the pool this way.
That's very interesting but I can't picture your system. Do you have an alternate intake to the pump and pick up water from another source (french drain) other than your pool?
Yes - the french drain is a line by itself. I close Jandys from both the main and skimmers (one valve set to close line into filter), open the french drain line into the multi port to backwash, making the water source the french drain. No pic, sorry.
What is the source water for the french drain....just ground water?
Yup - our home is built in what was an apple orchard. The natural ground water was enough of an indicator to install the french drain during construction. Floating liner = :cry:
 

BIC

Well-known member
Jul 7, 2007
108
AnnaK said:
BIC said:
What are the signs that tell me to do a backwash?

Hi BIC,

I use two primary indicators:
1. The pump pressure is 8 psi above 'normal' -- with 'normal being the pressure shortly after a backwash/rinse.
2. The water in the sight glass is getting murky.

I have a 2-speed pump and I look for the pressure differential when the pump runs on high speed.

When I backwash I use the trick I learned on one of the forums: catch some of the discharge water in a white plastic pitcher every now and then during the process. Initially, this water will be dirty, murky, and will have lots of floaters. When it's clean, the sand is clean and you can go to the rinse cycle.

When I have a lot of water in the pool such as after a heavy rain, I backwash whether it needs it or not, and I shoot for two cycles, the second one shorter and just to give it an extra blast. It seems to remove those last few grains of sand that seem to collect in the sight glass after a rinse.

Sometimes, after vacuuming a heavy load of pollen and tree flower debris, I'll do a backwash though I always vacuum with a skimmer sock in which catches this stuff. I've never had to shock my pool but if I did, I would backwash afterwards.

Anna
Thanks for the guide 8)