At what water TEMPERATURE do you decide to close your pool (and can algae grow at that temperature without chlorine)?

[Gary Davis]

I tend to not formally close my uncovered outdoor plaster pool since it doesn't freeze where I live, but at a late-in-the-day water temperature high of around 60 degrees F (15 C) it's currently too cold for me to swim in (this temperature point occurring around November 1st each year).

If I assume I wish to close the pool, for me, that doesn't mean all that much changes other than I kind of stop adding chlorine daily (yes, I know most people add it only weekly or they have automated systems such as SWCG), but since I exclusively use HASA liquid chlorine, the chlorine doesn't get added unless I add it.

Is there a consistent daily high water temperature that is so cold that you don't need to add chlorine anymore?

If so, what is that temperature?
And do you maintain a reduced chlorine level or do you still keep it above 7.5% of the CYA levels (which for me is set at about 30 ppm) all during the (admittedly mild) winter?

The main question, I guess, for me anyway, is at what temperature does algae (and bacteria) tend to grow so slowly that the chlorine level isn't important anymore?

 

[Newdude]

Is there a consistent daily high water temperature that is so cold that you don't need to add chlorine anymore?

Algae starts to go dormant around 60 degrees. The further below you get, the more dormant it becomes. The higher you get above 60, the more it returns to exponential growth. Opening early is arguably more important than closing late, because if anything did grow over the winter, you want to fight it in the 50s, not the 70s. Early season SLAMs are typically a day or two instead of a week or two.

With the days getting shorter and the sun angle lowering, you should find the daily loss to be decreasing. Eventually it will be slim to none. Daily adds turn to weekly adds, and then you may even hold FC for a month between doses.

If I was in a climate that cooled off but didn't freeze like yours, I'd just keep doing what I was doing all summer. The pool will tell you when it needs chlorine. Maybe there's a warm spell mid winter and it needs some chlorine weekly again. It takes 5 minutes and is second nature for you, it's not like it's a struggle. I'd bring it to SLAM level anytime it got below target range and spend the next however long enjoying looking at the water. It's a small price to pay to not look at a tarp / trampoline.

 

[proavia]

Maintain the FC and other levels just like the rest of the year. Dose FC according to the FC/CYA Levels - shoot for the high side of the target range. Since the pool won't be using as much FC, you may only need to add chlorine every 7-10 days - but only your testing will let you know the frequency needed.

 

[Newdude]

Gary, I forgot the best part about winter closings. If this year's plan doesn't pan out how you prefer it, there will be another one coming next year and we have lots of time to tweak the plan.

Once they live this plan for a winter, most are happy with it.

 

[Gary Davis]

Thanks for the advice on my winter procedure, which I can hone for the next of the rest of the years!
I'll do what is advised, which is simply maintain the minimum FC level (as dictated by the CYA level).
What it means, most likely, is that I'll just add chlorine less often.

 

[Newdude]

I'll do what is advised, which is simply maintain the minimum FC level (as dictated by the CYA level)

*over minimum. How far over is up to you and how much away time you wish to buy with this dose, just like during the regular season.

 

[proavia]

You don't want to maintain the minimum FC for your CYA level.
You never want the FC to fall below the minimum for your CYA level.

Dose chlorine to attain the top end of the target level and never let FC fall below minimum.
Use the FC/CYA Levels.

 

[Gary Davis]

You don't want to maintain the minimum FC for your CYA level.
You never want the FC to fall below the minimum for your CYA level.

Dose chlorine to attain the top end of the target level and never let FC fall below minimum.
Use the FC/CYA Levels.

Thanks for reminding me of the "over the minimum" requirement.
Both of you brought up a good point, which I don't disagree with, which is that FC needs to be OVER the minimum.

Moving forward on sanitation during the inevitable further drop in temperature, the FC levels are already showing longer-lasting levels, which means the pool is already automatically increasing the intervals between HASA dosing regimens, just as your kind advice had predicted would happen.

Regarding the consultation of FC/CYA charts, in practice, especially given my CYA is rock steady at around 30 ppm anyway, I inherently know what depth of "yellow" is required for FC to be to my satisfaction so I don't need to consult a "chart" every evening (bearing in mind, surprisingly so, that pH doesn't matter all that much for sanitation in an outdoor residential pool).

I should probably admit I'm not all that strict about it - but I don't have algae and the water is clear - at least using the process I used all summer, which was:
1. Around dusk, with the sun almost down, I'd daily add the measured amount of HASA chlorine (yes, I know, most of you dose weekly).
2. At that same time I'd often check the FC (among other factors) prior to the HASA dosing (I skip some days, so that's why this is listed as #2).
3. Rarely, but periodically, I check the FC loss over night (which has never been a problem), just in case.

I check other chemical components for saturation purposes, but not for sanitation purposes.

Moving further into the winter temperatures, I appreciate your experiences, where what seems to be necessary to take in mind will be how temperature and pH greatly affect the saturation in so much as colder water (or lower pH) makes the saturation balance far more aggressive than warmer water (or higher pH) would be on the plaster surfaces. I've already added calcium chloride and further reduced the bicarbonate, carbonate and hydroxyl alkalinity with HASA muriatic acid to be in better balance for winter.

I'm getting ready also to prepare for a plunge into testing out how my alkalanity & boric acid can help me slow down the pH at both the floor and ceiling levels (see chart below, where I'm not sure I understand it fully yet.