CYA and brushing

[waterbear]

Good morning all. I joined last week. I have decided to make the switch to BBB as past sanitizing has put me in CYA hell.
I have a 25,000 plaster pool in SE Pennsylvania - 3 years old.
My numbers last week: TH 400 - FC 0 - pH 8 - TA 180 - CYA 150
Have partially drained the pool 2x (to a total of about 1/3 of volume). We have had heavy rains - this helped to refill. Have also been refilling with house tap (200ppm CH). I have backwashed 1x.

If you have replace 1/3 of the water in your pool with water that has no CYA then you level should have come down from 150 to 100. (1/3 of 150 is 50) so that is an expected result.

Today's #'s: CH 450 - TC 10 - FC 10 - pH 7.2 - TA 140 - CYA 100.
The problem is that I thought I would see the CH and CYA come down much more by now (CH went up???)

You lowered your pH from 8 to 7.2 This and the brushing might have dissolved some scale deposits and caused your CH to rise.
Also don't be surprised if you TA rises if you pH rises. Unless you have airated to drive off carbon dioxide gas you haven't really lowered the TA....just shifted the balance of the carbonate/bicarbonate/carbonic acid buffer system in your water that we all know and love as Total Alkalinity to less of the carbonates and bicarbonates and more of the carbonic acid which won't show on the TA test. To actually LOWER your TA read this post

http://www.poolforum.com/pf2/showthread.php?t=191

IF your TA does not rise above accepted levels then you probably don't have to mess with it but, IMHO, with a CH of 450 you might be better off getting you TA to about 100-120 ppm. Less of a chance of scaling.

Hope this helps! Seems like you're doing a great job so far!