Drain and Refill vs Exchange

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sean.a.hyde

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Jun 5, 2018
143
Pittsburgh, PA
Hi Everyone,

I finally got a handle on my pool chemistry last year and realized my CYA was ~150. I managed this by maintaining a pretty high FC and that worked ok.
This year I am changing to SWG (good thing considering how hard it is to find bleach), so I think I should lower the CYA.

I live north of Pittsburgh in SW PA. The house was built on a former golf course and I live on top of a hill with the pool about 8 feet below the walk out basement. I have an in-ground vinyl pool built in 2010. I don't exactly know what the water table is like. It seems like there is not much soil and it becomes clay pretty quickly.

I have an auto-cover on the pool and I keep rainwater off of it with a submersible pump. I used to just have the hose for this out to the edge of the concrete patio (and utilized the weeping system to get rid of the water). I stopped that toward the end of last summer (after a particularly wet year) when I got a wrinkle in the vinyl on the floor of the shallow end -- the hose now runs to the edge of the property.

Obviously Exchanging water takes longer and uses more water than Drain and Refill, and is also probably harder to guestimate how long it needs to be done for, but doesn't run the risk of the liner moving or drying out or whatever.
How do I gauge the risk here? Should I just say "it is greater than zero, so just do an Exchange"?
 
I agree with mknauss. Test your current CYA.

Then simply drain and refill leaving about a foot of water in the shallow end. Repeat that until your CYA is 60-80.

If that still makes you hesitate, then exchange. It's more trouble and costs SOME more water but not as much as you might think.

You'll be all set in no time.
 
Thanks guys.

I tried to test the CYA yesterday (let the water warm up first -- it is COLD), but it isn't sunny. It still seemed close to ~140. I'll test again next time it is sunny.
Also tested salt. 1800ppm!! All from bleach.
I'll retest that after the water exchange.

I guess I'll do the exchange.
Based on the linked guide, I will put the pump in the deep end and fill in the shallow end, as I expect the fill water to essentially be the same temp as the pool, and I have 1800ppm salt and a good amount of CH (another reason to do an exchange). I also have a TDS meter I could use.

What is the reason to put the fill hose in the bucket (in the shallow end)?
 
Do not stop once you start. Also, because your water temperatures are close, you will want to drain 10-15% more than you think you need to.
 

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It is to prevent mixing. And turning off the hose for a bit but keeping the pump running is fine. In fact, you might be able to let the pump run over night (as short as possible) without the hose. It might only drop your pool level by less than a foot, and that should be no issue.
 
I remembered I had a spare pump so I dropped that in too. Now the pumps are winning, but only just. So I think I can run it overnight.
Last year, I moved the spigot I'm using is on my deduct meter (get a rebate for sewage). Since I'm not running the sprinklers this time of year, I can actually use that meter to track how much water has gone into this effort. Looks like about 4,000 gallons so far (but since the meter rounds to the nearest hundred, it is hard to be precise). That makes the hose a bit faster than 5 gallons per minute.

I ran the CYA for the sample I took this morning when the sun was high in the sky and I could stand with my back to it. Somewhere between 130 and 140. Since I'm diluting, the precision isn't great.
pH was 7.5 (got to use my new electronic pH meter!)

I'll report back in the morning. What's the right way to test CYA going forward? I obviously don't want to mix the water...
 
You stated earlier you had a TDS meter. Have you checked the inflow vs outflow TDS? Did you check the pool water TDS before you started? Might be some good data.
 
I have an electronic TDS meter and a pH meter.
TDS:
Starting pool water: 240
Discharge water: 190
Fill Water: 130

pH:
Starting Pool Water: 7.5
Discharge Water: 7.9
Fill Water: 9.0!!!

Uh oh! That fill water pH is of the scale! I guess I'm going to need a lot acid when I'm done here?
 
Do not overreact on the fill water pH. Doubt it will be that significant once in the pool.
 
Meter says 12,300 gallons. Unplugged the other pump too. Water is right at the bottom of the skimmer, so I'll let it come the rest of the way up. Then I can fish the leaves and worms out which should mix up the water and I can run some tests.
 
Hmm..
Swirled the water for a bit to fish out all the leaves. Tried to test CYA and got <20... Either something is waaay off, or I didn't mix the water enough. Unfortunately, I won't have the pump together until this weekend, so I guess I'll let it sit for a few days.
 
Ok, shut off the hose. Meter says 13,700 gallons.
I tested the sample I took earlier from the pump discharge for CYA and still measured at least 100 (no direct sunlight anymore), so maybe it just isn't mixed at all.
I threw the robot in to pick up worm guts and hopefully it will help mix the water too.
 

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