Water flow decreases after 4-5 hours

[Jess3311]

Hi! Maybe someone can help me...

Our pool (30ft above ground round) is about 4 years old and we were advised to change the sand in our filter this year (Pentair sand dollar filter.)

We also ended up having to replace our pump (Pentair dynamo 1.5 hp) due to a bolt getting loose inside and wrecking the motor.

We are currently still in the process of getting our water clear for the summer, and at this time it is going well. The water is still a little cloudy but clear enough to see the patterns on the liner at the bottom.

The PROBLEM:
The water flow through the return jet is greatly decreasing after running pump for 4-5 hours. I’m having to turn off the pump and let it sit for a bit and then turn back on for the water to flow harder again.
This is even after backwashing and making sure all strainer baskets are clean (skimmer and pump.)

The pressure gauge at initial turning on of the pump is usually between 15-17 psi and will remain there for a few hours. Then if I come back, say 6 hours later, the pressure has risen to 20 psi and the water flow has greatly decreased to the point the pool water is almost completely still. I have heard from many pool owners that the pressure gauge is often not the best thing to look at to determine problems but I wanted to post all details here just in case!

At the time when I notice the water flow is very low, I’m also hearing a whistling noise that seems to be coming from the top of the filter. The sound stops after turning off the pump and turning it back on and bleeding off air at start up. I am seeing no bubbles coming into the pool at return jet and I am not losing prime on my pump. Also I am not seeing any water leaking from any joint or pipe.

Could this problem be from a leak somewhere, either on suction or pressure side? Since we basically replaced all of our equipment is it possible we did something wrong when adding the sand to filter?

[Teapot]

Hi Jess3311 and welcome,
Who advised you needed to change your sand after 4 years?
What is the diameter of your sand dollar filter?
What is the depth of your pool, I am guessing at 4ft?
How long are you backwashing for?
I am not surprised at the drop in flow with that pressure increase. Flow gauges are the better way of observing but pressure gauges help somewhat.

OK lots of questions for you to answer, please pop back with the answers.

[Jess3311]

We were told to change our sand out by the owner of the pool supply store we use.

I’m not sure what the diameter of the filter is but I do know it is the sand dollar SD60.

Yes, depth of the pool is 4ft and we typically backwash until the water runs clear...probably about 2-3 minutes.

When I explained what was happening to the owner of the pool supply store, she told me it was normal for the water flow to be dropping quickly because of the new sand and it’s ability to capture more material from the water. She said to wait until pressure reads 25 psi on the pressure gauge before backwashing. She did not know about the whistling sound without doing a service call to my house.

[Teapot]

Your pool store owner is full of BS!
Sand is millions of years old when you fit it, It absolutely does not wear out in 4 years or 24 years. It can get very dirty and stuck together called mud balling and can channel (worm hole) but most of it is stationary except when backwashing and water cannot wear out sand!
Holding a piece of printed text behind the sight glass on the filter you will be able to see how clear it is running. May I suggest backwashing for around twice as long as you may release the dirt from the top of the filter but may find it runs dirty again a minute or two later. Don't forget the rinse position on your valve, worth hoping backward and forward to release more dirt. The normal pressure 15-17 psi is due to your 1.5hp pump feeding a 22" filter, basically that's friction you are creating and just wasting electricity doing so. Sadly it's popular to have too larger pump and too smaller filter and use more electricity than is need to do a simple job. In Europe we tend to use larger filters and smaller pumps. Me for example 12x24 pool 24" filter but my pump runs on filtration at 0.08hp and the water is fantastic but I run my pump 24/7 because it's so cheap to do so as I use only a light bulbs worth of electricity oh and my pressure gauge reads 1/2-1 psi so low it's difficult to actually read. Obviously I run full speed for backwashing.

Back to you, backwash as you are, don't wait until 25psi you put more strain on the pump, use more electricity and far less water flow so achieving absolutely nothing positive. If you have dirty pool water you need to backwash more frequently to clear the dirt but do it really well.

Lastly for now, ditch your pool shop and hang around here, we will put you on the right track for pool management you control at a fraction of the cost the pool shop will cost you.

Try the longer backwash and report back, your filter is small for the size of your pool so only has a small capacity for dirt hence the frequent pressure gauge rises, as the water gets clearer you will see less pressure gauge rises but they will happen.

[Jess3311]

I will do the longer backwash and report back.

One more question, if the longer backwash doesn’t help and the pressure continues to rise to 20 within a few hours, you’re saying I should go ahead and backwash and not wait until 25 psi...that would leave me backwashing atleast 2, maybe 3x per day. I understand backwashing more frequently when trying to get the water clear, but 2-3x per day seems excessive??

Also, any opinion on what is causing my filter to whistle? It’s never made that sound before.

[Teapot]

If you leave the backwashing until the pressure reaches 25psi you won't have to backwash as frequently that's true, it will also take a lot longer to reach 25psi as the flow will have reduced by so much you be wasting electricity but not water. Lets see how you get on with the longer backwash first.
I had similar with a customer, he backwashed for a couple of minutes regularly but over 1/4 of the season the flow dropped and pressure rose. When I arrived turned the pump up to full power and backwashed after the initial dirt went it also ran clear and then a lot more dirt came out and everything settled down again nicely.

Tricky to diagnose a noise over the PC but pressure rise could be some of it, the water under 20psi can't get through the dirty filter so looks to leak anywhere and on the sand dollar those clamps around the valve do not always seal perfectly, could be the vibration through the valve or even the gauge. When your filter is clean the lower pressure built up in the filter is less and the noise goes away.

You have things to deal with but in your situation I would be looking to purchase a pump speed controller, it would pay for itself in a season, stop noises, reduce unwanted system pressure, improve filtration and save loads of electricity hence money. You could also keep your eyes peeled for a second matching filter and run both, for a pool your size that is how I would spec it if I were building it but also I would only be using a 0.75hp pump.

[Denniswiseman]

Just think if the presure builds up your filter is doing it's job clearing the dirt from your pool and needs to be backwashed out to make room for more. As your water clears the backwashing would decrease
The rule of thumb for backwashing is "Backwash your filter when the pressure rises by 25% over clean pressure."

[Teapot]

With a clean filter pressure of 17psi the dirt is being driven deeper into the filter bed than say 3psi (which is still sufficient) and broken up into smaller particles under such pressure so dirt can be made smaller and squeezed back into the pool. Pressure only has to be sufficient to hold the sand in position, excess is just a waste of energy (electricity in this case)

Naturally the deeper into the filter the dirt ends up the harder it is to backwash it out again and the longer the backwash will need to be run for.

[Jess3311]

Compared to previous year’s pool openings, the pressure build up seems to be happening much more quickly. I have never seen the water flow come to almost a complete stop with a pressure rise of only 5psi. Then again, maybe it is because the sand is new. The filter is absolutely doing it’s job and my water is almost completely clear... maybe I just need to stop worrying!

[Denniswiseman]

Compared to previous year’s pool openings, the pressure build up seems to be happening much more quickly. I have never seen the water flow come to almost a complete stop with a pressure rise of only 5psi. Then again, maybe it is because the sand is new. The filter is absolutely doing it’s job and my water is almost completely clear... maybe I just need to stop worrying!

Maybe this year the water is more contaminated
As you backwash you will notice that it will get longer before having to backwash again

[Teapot]

Then again, maybe it is because the sand is new.

No it's at least 65 million years old.
Pool shop lies, sand wears out!

Good thing is your water is clearing, bad thing is you are wasting hundreds of dollars a year in electricity.

[Robteat]

I have a similar issue. Mine is a DE pool in ground and salt water. But my pressure doesn’t rise. However, I’m getting a low flow reading from my salt generator. While my water is moving it is not moving strong. If I backwash all is normal...but after an hour back to low flow. I’ve replaced my DE Grids...helped some..but still having the issue.

[Teapot]

Robteat, pressure won't rise if the pump isn't as large as Jess's, you just get low flow. Replacing grids won't help, that is just a support mechanism for the DE. if the flow reduces sufficiently to cause the chlorinator flow switch to not open, your filter is clogging so you have to backwash it and re-coat with DE, let us know when you get a) bored of doing that. b)broke from doing that. c)fed up with all that mess.

[Nemesis]

Robteat, pressure won't rise if the pump isn't as large as Jess's, you just get low flow. Replacing grids won't help, that is just a support mechanism for the DE. if the flow reduces sufficiently to cause the chlorinator flow switch to not open, your filter is clogging so you have to backwash it and re-coat with DE, let us know when you get a) bored of doing that. b)broke from doing that. c)fed up with all that mess.

What would you recommend doing in this situation? I'm in a similar boat. DE filter 24k in-ground pool vinly. I took the filter apart and cleaned it and all. The pressure starts around 15psi water flow is strong after a few hours 22psi and very weak flow and almost no suction. I usually stop the filter and start it 5 min later it goes back to 15psi and strong flow for a few minutes then back again.
If I backwash it I need to add a bunch of DE powder and I have to do this several times a day. It sounds very expensive. Is there an alternate option?

My multi-point valve has a minor crack I see small air bubbles near the rinse option. Could it be due to that?

[Teapot1]

Small air bubbles from the crack are after the pump but if no water appears then its ok, water molecules being larger than air.
To cure the the problem, I find DE hard work, messy and expensive. You cannot use flocculent if you need to so all in all I would replace with a sand filter filled with Dr Drydens Activate AFM ng media, youll only buy it once, it backwashes like a sand filter so no grids to top up, filterd to 1 micron and. below that with flocculent. Never wears out or biofouls. To make everything filter better a speed controller on your pump will improve filtration and save you money on electricity, slower filtration is better filtration. Worth fitting a flow gauge as well, something like the FlowViz.