PH Spike - Auto Cover to Blame?


Gold Supporter
Jul 5, 2019
Chicago Suburbs
Happy Independence Day!!!

The water chemistry on our in-ground 14,500 gallon pool has been very stable this year. Twice this year, however, the PH has spiked from a normal reading (7.5-7.8) to around 8.1-8.2.

This is our second season of pool ownership, so I'm still learning.

I can only think of two things at play here:
  1. Our pool company installed a Hayward Sense and Dispense unit this spring in order to control our SWG. The ORP readings seem to swing all over the place at times, regardless of the FC readings. It seems like our SWG has run-away at times due to low ORP readings. This seemed to be the case yesterday when our FC spiked to 21.0 and the PH spiked to 8.1 from a previous reading of 7.5. Can high FC cause a spike in PH? I haven't been able to find this anywhere in a search of the forums.
  2. We do have an auto-cover. The pool has been covered for about 2 weeks due to some perpetual rain storms we've had here in the Chicago area. Is it possible leaving the pool covered would have an impact on PH? Again, I couldn't find anything in the forums search I performed.
Test Results 13 days ago:
FC 3.5
CC 0.5
PH 7.4
TA 80
CYA 55
Salt 4000

Thanks for your help.


Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2014
Texas, San Antonio/Marion, South-Central Area
Pool Size
Salt Water Generator
SWG Type
CircuPool RJ-45 Plus
Jason, it's not the cover. The additional work from the SWG can create more aeration which can increase the pH. In addition, once your FC increases to over 10, it influences the pH level making it look higher as well until it settles again. That's a big reason here at TFP when we have owners start a SLAM to remove algae, we have them lower the pH first before increasing the FC because after the FC is over 10 the pH test isn't valid.


Silver Supporter
Bronze Supporter
Mar 5, 2020
Melbourne, Australia
Pool Size
Salt Water Generator
SWG Type
Astral Viron V35
As Pat pointed out, above FC of 10ppm, the phenol red pH-test will always show high.

But your pH will actually really be high. One reason Pat already explained, additional aeration.

But the creation of chlorine with an SWG by itself actually also increases pH. Usually, you don't notice that, because the use of chlorine for killing germs and things reduces pH again. So, as long as you are producing as much chlorine per day as you are using (i.e. FC remains constant), the chlorination cycle is pH-neutral, and only the pH-rise due to aeration will be noticed.

But when overproducing chlorine, pH will also rise. That rise will be compensated again when FC drops back down to the original level. But while FC is elevated, pH will also be elevated.
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