Pool turning green despite FC at recommended level

[GearHead36]

Our pool is in N AL, and has been open all Winter. We've been running the pump for 4 hr/day for most of the Winter. Recently, it's gotten a bit cloudy, so I upped it to 6 hr/day. The weather is warming, and the pool is turning green, despite the FC level, which we've been maintaining.

The numbers (using a TF test kit) are:
FC: 3.0 (rec:5-7)
CC: 0.5 (rec:NA)
TA: 40 (rec:50-90)
pH: 7.4 (rec:7.2-8.0)
CYA: 40 (rec:30-60)
temp: 66.7°F

Yesterday the FC was 5.0. The weather and algae are obviously using up chlorine, but I don't understand why the algae started growing with a maintained FC level.

Over the Summer, pH tends to rise, so I typically start it out on the low side. Once it gets to around 7.8, I'll adjust it.

Thanks.

 

[mknauss]

Sounds like you need to follow the SLAM Process

Have you been brushing the pool regularly all winter? Brushing is required to help keep algae from forming.

 

[PoolGate]

Are you targeting the minimum chlorine level for your CYA reading? I target and suggest you target 10% of CYA.

 

[GearHead36]

I did not know that brushing was needed. Thanks.

 

[GearHead36]

Are you targeting the minimum chlorine level for your CYA reading? I target and suggest you target 10% of CYA.

No. I'm targeting the high end of the maintenance (as opposed to SLAM) level. What do you mean by "target 10% of CYA"?

 

[PoolGate]

No. I'm targeting the high end of the maintenance (as opposed to SLAM) level. What do you mean by "target 10% of CYA"?

If CYA is 70, chlorine is 7.

 

[mguzzy]

I bet you had some residual algae in the pool over the winter since you didn't close it.. and now that the conditions are ripe for growth (Higher sun, warmer temps) they are growing faster than your FC levels are killing it. I bet an OCLT will confirm that. Its not uncommon to have to start the season off with a SLAM when the pool only really had a soft close over the winter. I fell into that trap all the time before my TFP days.

 

[ajw22]

The numbers (using a TF test kit) are:
FC: 3.0 (rec:5-7)
...
CYA: 40 (rec:30-60)

FC of 3 is the minimum FC level for CYA 40. If you are running your FC at the minimums it is more likely your FC fell below the minimum levels at some points in time and algae took hold. Once algae takes hold normal FC levels may hold it in bay but not eradicate it. That requires SLAM levels of FC.

It is better to keep your FC at the high end of the range and run it hot rather then skirt the minimum levels. Nothing bad happens a few PPM above recommended levels. Fall below the minimum FC level by 1 or 2 ppm and you can end up with a green pool.

It is time for the SLAM Process. Then run higher FC levels.