Reduction in Acid Demand vs Prior Years

I’m on my third season with an AquaRite SWG. The first two seasons I had to add about a quart of muriatic acid every 2 – 3 weeks. I would add it when the pH was approaching 7.8, which would take it down to about 7.4. This spring I opened with of pH of 6.8 which climbed to 7.5 after a week or so, and has stayed there all summer. I haven’t had to add a drop of acid yet. It’s a vinyl liner pool (also in it’s third season), so I wouldn’t think the age of the pool would make a difference.

One thing that has changed this year is that I put in a 2-speed pump. I was running the old pump for 7 hours a day at 40-60% on the AquaRite (depending on temp). Now I’m running the new pump on low speed for 15 hours a day at 20-25%. I’m wondering if the lower rate and longer time of chlorine generation can somehow be causing less of a pH increase. Has anybody else experienced this? Could this be another benefit of a 2-speed pump?
Here are my current numbers, which are typical of all three seasons:

FC – 3.5
CC – 0
pH – 7.5
Alk – 90
Cya – 70
Na – 3200 (on the AquaRite display)
Temp – 83
26,000 gallon vinyl pool
2-speed 2HP Superpump (previous pump was 1-speed 2HP Superpump)

Since I always do my testing in the early morning, it may be possible that the Cl was getting a lot higher during the day than it is now. My old pump used to run from 9am to 4pm, new pump runs from 7am to 10pm. We've also had less rain and slightly cooler temps compared to last year, so maybe it's weather related and has nothing to do with the pump, though I think the rain usually lowered the pH. Either way, I've been happy to not have to touch the acid yet this year!

What is most likely is happening is that the total time the cell is running is less. (The AquaRite uses a 100 minute cycle for the time the cell operates so you are running it about 20-25 minutes for each 100 minutes of pump time or a total of 180-225 minutes total while under your old setup you were running it for 168- 252 minutes of total pump run time. I suspect you were running it at 50%-60% more often than at 40%) The new setup probably has your cell producing chlorine for less time which would mean less hydrogen gas produced in the cell which would mean less outgassing of carbon dioxide and thus less of a pH rise. Also, was your TA any higher in previous years? Keeping the TA lower with a SWG helps reduce the acid consumption also.

You may also have been aerating the water more on high speed--and that pushes pH up as well.