Differential Draining of Pool Water

[JoyfulNoise]

Hi Matt,

I also live in Tucson and my pool has high CYA that needs to be diluted. I'm thinking to do it this week using your method. I want to double check that water goes out from the deep end and comes in at the shallow end. Some other posts did it the other way around, that's why I'm asking. Thanks!

Where you drain and fill from has to do with water temperature. Colder water is more dense (heavier) than warm water. So if your fill water is colder than your pool water, then you want to put your fill hose in the deep end and drain from a step on your shallow end. If the pool water is colder than your fill water, then you can fill through the skimmer (no pump running) and drain from the deep end well.

 

[J Level]

So I thought I would start this thread for the purposes of logging useful information and, perhaps, teaching others. I am currently doing a differential drain of my swimming pool whereby I am removing water from my deep end well using a submersible pump and some PVC plumbing while I am adding fill water to the skimmer. As this process is running, my pool pumps and automation system is completely shutdown for safety and water flow reasons. Note - this idea is NOT new nor do I claim any privilege over this process, it has been discussed in passing many times here on TFP.

The idea is simple - by removing colder and higher TDS water from the deep end, one can then add warmer, lower TDS water to the skimmer and the two layers should remain separated enough to effect an efficient drain of the water and closely approximate the results of a complete drain and refill. There are many reasons why a pool owner might want to do it this way but a significant reason is that here, in HOT Arizona, you cannot fully drain a plaster pool when the air temps are above 86F as damage can occur to the plaster surface. And, in Arizona, air temps consistently can get above 80F starting as early as March....heck, there have random days in January where the mercury has reached 75-80F.

Here's some shots of the setup

Deep end removal -

rtskD4N2QUu08Ri9BUyJRw.jpg

• JoyfulNoise
• Apr 16, 2019

Deep end pump out

And flow meter -

gmRmamFHRJOOst6pjY1Z8Q.jpg

• JoyfulNoise
• Apr 16, 2019

Flow meter for deep end pump

Shallow end skimmer fill -

MS5TOn6XQcy1cTIgA5bZUA.jpg

• JoyfulNoise
• Apr 16, 2019

Skimmer fill pot

Skimmer fill flow meter -

Oq6hMPHZQUWq9X%1JAldeQ.jpg

• JoyfulNoise
• Apr 16, 2019

Flow meter on skimmer fill

Some info -

Deep end pump out flow - 22-23GPM

Skimmer fill flow rate - 5GPM

Water temperature of deep end well - 69F
Input Water temp from fill hose - 77.5F

You can look at my signature for my latest test results. I have a salt water pool with a nominal salinity of 3400ppm and currently my CH is around 1100ppm. So the pool water has very high TDS and it's about 8 deg F cooler than the input water. My hose fill water (softener shut off) has a pH of around 7.6-7.8, TA 100ppm and a CH of 180ppm (TH 200-220ppm). My end goal is to get about half the water out of my pool and lower my CH to less than 800ppm or better. While that's still about 2X the recommended level, I'll happily take that over where I was last year at an eye-popping 1500ppm CH.

Thanks for sharing your process! I am looking to perform a similar function -- sharing my info and a few questions....

Bought house somewhat recently (also in Tucson) - was having pool guy who used to service for prior owner continue for a couple of months. Tried to have him explain what he was doing/educate me. Didn't appear to be doing much but adding a stabilized chlor tab or two, light skimming, emptying cleaning robot filter bag and skimmers. He stopped showing up consistently and I ended service and dug in.

Since, I have had water tested at pool store twice over past two weeks and added some of their stuff which stabilized ALK ad PH, but based on their readings (2nd reading took to a different store and were pretty consistent) - and my learning here it appears I will need to perform at least a partial drainage. Pool Store readings (CYA - 149, FCL - 23, TCL - 23, PH - 7.3, ALK - 90 (up from 40), Hardness - 677, Phos - 3500). I have TF-100 coming in mail now - to learn to accurately test myself - but trust them somewhat...as they have all been around this range, and some of the junk I threw in did what it was supposed to (Alk up, and No Phos mostly). Using strips on the side in the interim.

From researching, I like the method and set up you used - to avoid full drainage - although still is somewhat cool here. Thinking to rent submersible at Home Depot, put in deeper end, run a couple of hoses to the closed off skimmer on shallow side.. Now to the questions...

1) I've seen folks say to do the process in reverse (sub pump in shallow end, fresh water in deep) -- thoughts here on why chemically which way would work best?
2) Based on my numbers am I chasing a losing battle and is it just better to perform full drainage, or is part of the water still salvageable? (Looks crystal clear - as guessing one might expect with such high chlorine.). If full drain, still can get in this season?
3) Your set up looked pretty fancy -- pvc pipes, water volume measures ingress and egress, etc. Can I just drop in the pump, run hose into yard, turn on -- and turn on fill hoses from spigots that will just run in on the shallow side/skimmer?

Thanks in advanced - appreciate your input.

 

[mas985]

I assume you are refilling due to CYA.

Replacing water in-place process depends on several factors.

Pool Water TDS (PWTDS)
Pool Water Temperature (PWT)

Fill Water TDS (FWTDS)
Fill Water Temperature (FWT)

DeltaT = (PWT - FWTDS * 0.006) - (FWT - FWTDS * 0.006)

If DeltaT is positive, then you want to fill at the lowest point and extract at the surface and at the greatest distance from the lowest point.

If DeltaT is negative, then you want to extract at the lowest point and fill at the surface and at the greatest distance from the lowest point. <-Preferred as it is easier to do with a drain pump and you can fill inside one of the skimmers.

If |DeltaT| < 5, then there will be a lot of mixing.

 

[J Level]

I assume you are refilling due to CYA.

Replacing water in-place process depends on several factors.

Pool Water TDS (PWTDS)
Pool Water Temperature (PWT)

Fill Water TDS (FWTDS)
Fill Water Temperature (FWT)

DeltaT = (PWT - FWTDS * 0.006) - (FWT - FWTDS * 0.006)

If DeltaT is positive, then you want to fill at the lowest point and extract at the surface and at the greatest distance from the lowest point.

If DeltaT is negative, then you want to extract at the lowest point and fill at the surface and at the greatest distance from the lowest point. <-Preferred as it is easier to do with a drain pump and you can fill inside one of the skimmers.

If |DeltaT| < 5, then there will be a lot of mixing.

Thanks for the reply - great, scientific info! Yes - looking to refill to address/reduce high CYA (149) and high phosphates (3500) (from reading have learned phosphates aren't too important, but at 3500 don't mind getting lower). Not certain how to test for TDS...but assuming there's a correlation to phosphates & CH - so probably high Also, my FCL is 30! I pulled out chlor tabs out about 2 weeks ago, expecting it to go down but, it seems to be going up...not certain how without a source. My thought was to leave it - let FC burn off/out a bit...not seeming to work.

TF -100 came in and tested myself...

PH 7.3
FC 30
CC 0
(TC 30)
CH 1150
ALK 220 (this shot up, or i didn't perform correctly)
CYA - 100+ (dot disappeared around 2/3 or 3/4 of the way to the 100 mark. Guessing store reading from a week ago - 149 was accurate close). Phosphates not tested but showed the 3500 in store.

Water temp inverted as it has started to get hot during the day here...in pool is 72 deg, from faucet is 71 deg...probably not enough temp difference to create effective shield for partial draining.

So, as new pool owner thinking...chemistry is really messed up, and maybe best to start clean? Probably need to either drain completely (now is getting into mid-80's to low 90's during the day) or hire reverse osmosis h2o recycler ($795 for 15,000 gallons).

If I drain completely - would it avoid plaster cracking issues if I perform at dusk at sun goes down and refill overnight? I have a number of outside faucets that I would be able to run to the pool and monitor? Also, is there a tutorial on what to add to fresh pool water? (couldn't find in Pool School).

Not sure what to do - kids are getting anxious to jump in!

 

[mas985]

Salt is actually the largest component of TDS. No matter what chlorination method you use, salt is added to the pool water and can be significant after many years. Every 1000 ppm of salt is worth about 6F in temperature difference so you might still be ok. Worth a test at the pool store if you don't have a salt test kit.

Exposed plaster should not be an issue over a few days. However, an empty pool can float if your water table is too high. Probably not an issue in AZ but thought I would mention that.

But again, even if CYA is 200 ppm, you don't need to drain the entire pool. One way to determine actual CYA is with dilution. Dilute the pool water 2:1 or 4:1 with distilled water, then scale the results appropriately. That will give you are more accurate assessment of how much water you would actually need to drain.

 

[J Level]

Store CYA was 158 on 3/29 (had tested independently at two stores and both were w/in 10ppm) - so appears to be relatively in line with TF kit test. It has been a manually chlorinated pool. Interestingly, I removed stabilized chlorine tabs (original source) over two weeks ago, to burn off high FC upon noticing issue, and FC and CYA appear to be going up. Perhaps b/c of warmer weather/water?

I will look to dilute pool water as suggested for the CYA test. Let's say I use 1:1 pool to distilled. (1/2 pool water in measure, 1/2 distilled measure). To get accurate number I would just multiply my reading x 2, correct (so say it's 70, then actual would be 140)? If i use 1/4 pool to 3/4 distilled would multiply x 4.

If CYA comes back around 150 - 160. Would I then look to drain proportionately (remove 2/3 of water) to land at appropriate level ~50?

Guessing would help w/high CH too...but i know there will be some natural CH in tap water here.

Appreciate your input!

 

[C3Cl3N3O3]

I did something similar several years ago to reduce a CYA of 200 or so. I didn't use the pump as I was able to get a siphon going out of the pool with a hose. I filled buckets with a timer to match the flow rates of the two hoses. I had worked out the replacement rate with a spreadsheet and got pretty close.