high combined chlorine

[fredgrandt]

I have a 16,000 gallon vinyl in ground pool.
Winter is over and the cover came off the pool 8 days ago. 4 gallons of liquid chlorine (12.5%) were added to the water as part of the pool opening service. The pump is running.
After a few days of filtering the water is crystal clean and the bottom of the pool is clean.
I had the water tested at my local pool store using ClearCare Expert Water Analysis System and the results are as follows:
FC 9.82
TC 12.97
CC 3.05
CYA 5
The programmed recommendation from ClearCare was to add 6 pounds of shock (calcium hypochlorite 78%) for breakpoint chlorination to reduce the level of CC.
It doesn’t make sense to me that when my free chlorine is nearly 10 that I would want top add so much shock to the pool. That amount of shock would raise my FC to over 30.
Please advise.

[Denniswiseman]

You need to Slam (Shock Level and Maintain) with relation to Chlorine / CYA Chart and Recommended Pool Levels
Pool Maths
Your CC is the used up part of chlorine and you have to maintain the FC at about 10 for your CYA
You really need to get yourself a decent FAS/DPD test kit (Taylor K2006c or TF Testkits TF100 in the states) to get accurate results as maintaining an appropiate shock level means testing quite often during the day

[fredgrandt]

Regarding ‘breakpoint chlorination’ or ‘slam shocking’ – I’ve looked at all the links and charts you’ve mentioned in your response.

What I’ve read elsewhere recommends shocking with chlorine to achieve breakpoint chlorination if your total chlorine is low. But, if TC is high, as mine is, the correct approach is non-chlorine shock.

The SLAM chart has this note: “Users with a green or cloudy pool will use this chart to increase their chlorine level to SLAM their pool.” And “SLAMing your pool is the best way to eliminate algae, sanitize your pool and get your water crystal clear.”

I do not have a green or cloudy pool. The pool is clean and the water is crystal clear. It has no odor.

Is SLAMing the correct approach to resolve my situation?

[Denniswiseman]

The fact that you have CC means that although the pool is clear the chlorine isn't as effective as it should be
I would continue to test and keep your chlorine at about 10 untill the CC diminishes