Lost 4 FC in one day because of pool heater?

[pcm2a]

Every 4-5 days I have been testing the water and it is in between 5-7. I dump in 128oz of 10% and am good for another 4-5 days. Now that it's getting warmer I was ready for a swim. Yesterday I checked the level and it was at 4 so I added 128oz, checked 2 hours later and it was at 11. This morning the pool water was at 79 degrees, I gas heated it up to 84, took about 2-3 hours. Once it was dark I took a reading and was shocked that the FC was at 7! Would heating the water cause this kind of drop in FC?

The only thing that changed today vs other days was the pool heater.

Outside temp: 86@hottest part of the day
FC: 7 (Today)
pH: 7.6 (4/25)
TA: 110 (3/5)
CH: 250/275 (4/21)
CYA: 75 (4/17)

 

[jblizzle]

Um ... some of this does not add up ...

1 gallon of 10% bleach in your 20k pool would add 5ppm of FC to your pool. So going from 4 to 11ppm after 2 hours would not be expected
Furthermore, no way that 1 gallon of chlorine should last 4-5 days ... although maybe you were lucky prior to the weather now heating up.

A typicaly daily FC loss is 2-4ppm, since your CYA is a little higher, perhaps closer to the lower end would be normal.
Basically, I would expect you to have to add a half gallon of 10% every day to maintain adequate FC ... unless you perhaps have a cover which would lower the losses to the sun.

The heater would not have caused your observation.

 

[pcm2a]

I agree that 4 to 11 was also a surprise. The 20k gallon number was calculated a while back by taking readings, adding baking soda, then taking a reading the next day and backwards calculating it. Came pretty close to my pool measurements to try and calculate it (pool is curvy).

I'm going to take another test for sanity sake, but I wouldn't expect the DPD kit to let me down. I'll add a half gallon and check it in a couple hours. Then I can repeat tomorrow night and see what it does. My 10% chlorine comes from Walmart and is always dated pretty recently. I keep all my tests, dates and times in a spreadsheet. Probably not the best way but it works I guess.

 

[jblizzle]

Check out the new-ish PoolMath app ... allows entering chemical additions and test results.

 

[pcm2a]

Second test came up at 6.5, added 64oz of 10%. Pool math says this should get it to 9-9.5, will report back in a few hours.

Update: Tested at 12:30 am and it came in at 10. It was 99% clear at 9.5 but I'm calling it 10. Pool math seems pretty close, can't wait to see what shakes loose tomorrow.

 

[pcm2a]

The test tonight at 10:30 PM shows 7.5 FC, a drop of 2.5. Was rainy in the high 70s. Going to leave it as is and see what the drop tomorrow is.

 

[pcm2a]

Ok I've been tracking it for a few days and it seems to be more normal now. It appears to be losing between 2 and 2.5 a day.

5/3/18	9:30 PM	6.5					64oz
5/4/18	12:30 AM	10
5/4/18	10:30 PM	7.5
5/6/18	12:30 AM	5.5					64oz
5/7/18	9:30 AM	4					128oz
5/7/18	10:00 PM	6.5					64oz

If I assume that I'll be adding 64oz a day for 6 months, that's around 90 gallons of bleach. That would be a cost of $357 (10% bleach from Walmart). Is that how much everyone else is spending, seems to be pricey?

Two buckets of pucks $180. That would leave $177 for pool draining to lower the CYA at a cost of $60 each time I remove 1/3rd of the water.

 

[jblizzle]

the problem is the pucks will not likely keep your FC high enough to be safe for your Rising cya. So you might have to be supplementing with bleach anyways.

 

[pcm2a]

You know that is a good point. Last year, before TFP, "the pool guy" put in 4 pucks a week and kept my FC in the 2-4 range. There was a couple of real hot weeks that he had me throw two more in over the weekend. Crystal clear water the whole summer. Knowing what I know now I don't understand how it was working? By the end of the summer I was running 4 pucks with a CYA over 100. Is my pool magical?

 

[jblizzle]

Nope, just lucky

Perhaps your pool happens to be less prone to algae, like low phosphate levels or maybe they were adding algaecides?

Just remember that a pool with clear water (no algae) does not automatically mean the water is sanitized and safe.