spa alkalinity nightmare

davedive

Member
Apr 17, 2017
20
Westport, CT
Hi I have a 900 gallon in-ground gunite spa attached to my pool. The pool is closed for the winter but the spa is open and I have used it several times this winter. This is a saltwater pool/spa with a pentair salt cell generator that works during the summer. The salt level in the spa has been low so on days I want to use the spa I put on the heater, add a bit of sanitizer and maybe a little muriatic acid to knock down the ph and the water has been great. A couple of weeks ago I decided to add salt to get it over 3400 so the salt generator could start working. I had to add a lot of salt but got it to 3600 and had all green lights on the generator and I just left it. Keep in mind the spa is not heated so he average temp has been about 55F. Yesterday I tested the water- Yikes!!! My ph test turned purple and chlorine test powder made pink clumps before turning clear. I could not get any readings so went to my local pool store. Their test showed F Ch=.13, T Ch=15, ph=8.5, T Alk=0 (yes 0). They told me first step I need to raise the T Alk by adding 1lb 5oz of baking soda, which I did- nothing. So I added another pound, and another, but still could not raise the T Alk!?!? Today I went back to the store and found the only thing that changed was the F Ch=15. The T Alk was still 0. Again they told me to add 1lb 5oz of baking soda and get the alkalinity up. I said it does not work and they said you have to add it slowly- only 15oz at a time with 4 hours in between. It does not really make sense to me how quickly you add it but this is what I am doing. Am I getting bad advice? Any suggestions? Thanks for your time.
 

JamesW

TFP Expert
Mar 2, 2011
24,145
Your fc is probably over 100 and it is causing all of the tests to fail.

I would drain and refill if it is safe to do so.

A spa can float just like a pool if you drain it while the ground water is high.

Consult a local professional to determine if the spa can be safely drained.

You can make a water sample of 1 part spa water and 9 parts distilled water and then test the sample for chlorine and multiply the result by 10 to see what the fc really is.
 

davedive

Member
Apr 17, 2017
20
Westport, CT
I like your idea of diluting the spa sample with distilled water. I will try that. I have a hose with fresh water going into the spa and just letting it spill over into the pool. I figured eventually it will dilute the spa water. I have some time.
 

JamesW

TFP Expert
Mar 2, 2011
24,145
You can't run a regular SWG on a spa because it will quickly create way too much chlorine.

How much did the SWG run every day?
 

davedive

Member
Apr 17, 2017
20
Westport, CT
It was running a few hours everyday but the water is cold- like 55F so I didn't think it was really generating much. Also, I forgot about it. I think I could use it if I put it very low and only put it on for a few hours before use but the best idea is probably no to use it at all- bad idea. I tried the dilution test. I didn't have distilled water but have well water. Diluted 1 part water to 19 parts well water and the Ch was 4 x 20 = 80! Wow you were pretty close. The guys at the pool place maybe should have known that.
 

davedive

Member
Apr 17, 2017
20
Westport, CT
JamesW thank you for your help. I'm just going to wait it out, keep running in the well water, keep it open and maybe by the end of the week it will be back in line. No point in adjusting anything until that chlorine is down. I should have thought of that dilution trick it's a good one.
 
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JamesW

TFP Expert
Mar 2, 2011
24,145
If the well water has a lot of iron, that can lead to iron stains.

Also, if the spa is spilling into the pool, won't the pool overflow?
 

JamesW

TFP Expert
Mar 2, 2011
24,145
Just for reference, if the cell ran at 100% for 24 hours, the fc would increase by about 180 ppm.

You could probably try 6% for 6 hours, which would produce about 2.7 ppm.
 

davedive

Member
Apr 17, 2017
20
Westport, CT
JamesW: I didn't think of the iron thing! Well- it's done. I left a hose with a low flow in there overnight and got the Ch below 4, Alk= 170, ph=8. I added 1 1/2 oz muriatic to lower alk and ph. The pool is big and partially empty so it had room for spillover. I would like try using the SWG on a limited basis- I might do that but NOT be a fool and let it just run and run (idiot). My salt is down to 800 so I would have to bring it up first. What is the other best option to keep up chlorine? Is it dry sanitizer/shock powder, plain old Clorox, bromide or something else? I have all these things in the garage.
 

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JamesW

TFP Expert
Mar 2, 2011
24,145
The SWG should be fine as long as you carefully adjust the output.

Liquid chlorine would also work.
 
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