Is Combined Chlorine ‘contagious’?

[DaveMcHbg]

Stupid sounding question, I know…but sincere.

Draining an indoor spa which needs more time to oxidize the combined chlorine, when slamming, than I care to wait. This will be following an AA treatment for imbedded rust stains.

So the question is this: Will leaving a percentage of existing water in the spa cause the combined levels to build up quicker.
Say starting full cc is 3; if we drain and refill ¾ of the water (theoretically) the cc should be 0.75?

Will leaving a small amount of old water actually cause the cc to build up quicker, or is it only the new starting point (0.75) and the levels will build up at the same speed as if all the water were replaced?
We’ll actually probably replace more like 80-90%, going any lower will cause restart drama with the pumps, but so will a rapid re-accumulation of cc cause undue grief.

Thanx, Dave

 

[jblizzle]

CC does not build up in the sense you are saying. CC are the byproduct of something in the water being broken down by chlorine. If you add more chlorine, it will break the CC down. Usually the sun takes care of a lot of the CC issues, but with indoor setups, you typically want a supplemental form of oxidation to remove the CC (ozone, UV, etc) or you can use some non-chlorine "shock".

 

[DaveMcHbg]

Thanks.
Monopersulphate has had little effect and the cc here is rather persistent- last time slamming it took about 48 hours after maintaining a 10 ppm-fc, which took 24 hrs to reach, so it was a three day process. We can’t have the spa down that long, so- drain refill seems to be the probable option.

I’ve inferred from several threads here that if the cc is not totally oxidized (during a slam) it will return quickly, basically negating any ‘shocking’ benefit.
Hence my question- If all the water containing cc is not eliminated will the results be the same as an incomplete slam. Ie. quicker buildup of cc.