Dialing in Pentair Rainbow following VSP install


Bronze Supporter
May 25, 2015
Fort Mill, SC
So I mostly use liquid chlorine but switch to the chlorinator from time to time. I had a single speed pump earlier in the year which ran for around 8 hours / day. The Pentair R171218 chlorinator in the bottom feed configuration didn't quite keep up (setting was maxed out). I approximate that it was putting out 2 ppm per day where 2.5 to 3 are required.

Now I've upgraded the system to a Jandy VSP which runs for 22 hours a day and have the opposite problem. The FC rose to shock levels very quickly. I dialed the Pentair back to 20% or so, and the FC was still holding at 12 ppm after a week. I've dialed it back further, now it's only two graduations above OFF.

One observation is that the chlorinator has more water in it now, it's about 1/3 full as opposed to barely any water before.

Is any of this to be expected? I'll switch back to liquid chlorine at some point.


Mod Squad
LifeTime Supporter
Jul 10, 2012
Tallahassee, FL
The longer run time would account for the increase in the FC. The "dial" on the tube is not very good as you are seeing. Think about this. When the pump is off the tablets are just sitting in there "building" up FC in the tube until the pump is turned back on. I bet if you tested the water right where the water comes out of the tube right after you turn the pump on the FC would be very high!