Warming profile for a closed versus open pool (and when should I open)?

[Sampo]

I've been wondering about the insulation and warming properties of my pool cover. The combination of the solid safety cover and the underlying air-gap clearly provides significant insulation in the winter: on occasional checks, my water seems to have maintained a temp of around 40-44F despite our cold winter in Southeast Pennsylvania, well above the average air temperatures. I have only a rough understanding of the thermal physics of how this is happening (direct insulation versus protection from radiative loss to space, for example). But I suspect that a big part may be the blanket of warm air that stays trapped below the cover -- which itself is heated by sunlight on the dark plastic. But however it works, there is always a balmy jungle-like atmosphere of steam beneath whenever I peek below the cover.

As Spring starts to near, the lengthening and decreasing obliquity of daylight will naturally increase this effect: the air below the cover will get ever hotter, and that (along with rising soil temperatures) will start to pre-warm my pool. But at some point, such heating must be slower than having a springtime sun shine directly on an open pool. That is, at some point, the pool cover would impede the warming of my pool.

So I am curious: how might we estimate when this switch occurs? That is, when does the warming profile of an open pool exceed that of a closed/covered pool? Or simply, "if I want my pool to warm up as fast as possible for the season, when should I open it?". Maybe the answer is as simple as -- when the average air temperature exceeds that of the water. But I bet that there are many on these forums who have a deeper understanding of these mysteries -- please share your wisdom!

I've usually opened at the end of March. I don't have an active pool heater. But since last year, I've been using a solar cover to speed up the warming (or rather, to slow its cooling at night). Since my pool is surrounded by a significant number of trees, it's a bit of a race against time until the leaves fill out by the end of April. So when should I start that race?

"CYA later" -- and welcome back to all to another pool season!

Sampo

 

[Bperry]

I've been wondering about the insulation and warming properties of my pool cover. The combination of the solid safety cover and the underlying air-gap clearly provides significant insulation in the winter: on occasional checks, my water seems to have maintained a temp of around 40-44F despite our cold winter in Southeast Pennsylvania, well above the average air temperatures. I have only a rough understanding of the thermal physics of how this is happening (direct insulation versus protection from radiative loss to space, for example). But I suspect that a big part may be the blanket of warm air that stays trapped below the cover -- which itself is heated by sunlight on the dark plastic. But however it works, there is always a balmy jungle-like atmosphere of steam beneath whenever I peek below the cover.

As Spring starts to near, the lengthening and decreasing obliquity of daylight will naturally increase this effect: the air below the cover will get ever hotter, and that (along with rising soil temperatures) will start to pre-warm my pool. But at some point, such heating must be slower than having a springtime sun shine directly on an open pool. That is, at some point, the pool cover would impede the warming of my pool.

So I am curious: how might we estimate when this switch occurs? That is, when does the warming profile of an open pool exceed that of a closed/covered pool? Or simply, "if I want my pool to warm up as fast as possible for the season, when should I open it?". Maybe the answer is as simple as -- when the average air temperature exceeds that of the water. But I bet that there are many on these forums who have a deeper understanding of these mysteries -- please share your wisdom!

I've usually opened at the end of March. I don't have an active pool heater. But since last year, I've been using a solar cover to speed up the warming (or rather, to slow its cooling at night). Since my pool is surrounded by a significant number of trees, it's a bit of a race against time until the leaves fill out by the end of April. So when should I start that race?

"CYA later" -- and welcome back to all to another pool season!

Sampo

Having the cover on will always help prevent heat loss. But it’s hard to swim with the cover on. General recommendation is to open the pool once the water reaches 60F and don’t close it until it’s below 60F.

 

[sande005]

Go for walks near small ponds, streams, etc. Look for signs of any greenery in the water - plants, algae, etc.
If you see it there, you better open the pool no matter what, to get some Cl into it.
I'm wintering in Philly, and the temps have been crazy warm (for me and my usual winter experience), but I have no knowledge of what starts to grow when here. At home in Minn., my pool can go green when it is still in the high 40's....

 

[Newdude]

and welcome back to all to another pool season!

Welcome back, indeed.

if I want my pool to warm up as fast as possible for the season, when should I open it?".

The pool is pretty much the average of the last 4 or 5 days high and low temperatures.

If its cool this spring, the pool won't warm up no matter what you do, without a heater.

The combination of the solid safety cover

This all but stops evaporation, even with my 'mesh' cover that appears solid if I hold it up to sun. Evaporation accounts for about 70% of heat loss which is the steam you're finding. It condensates on the cover and falls right back into the pool, which is a few degrees warmer than it should be at any point.

I check the long range forcast and open right after the last freezing low. It could always go sideways 3 weeks later and be cold again, but it won't be stoopid cold, or for very long, so running the pump for a few days straight is fine to get by.

 

[wireform]

I'd say when you're more or less sure the freezing temps are over you can do a soft opening and get the pump to move water getting a good mix. When you're at the 60° mark start the chemistry.

 

[Sampo]

Welcome back, indeed.

For sure! Good to see you my TFP friend! We made it through the winter!

The pool is pretty much the average of the last 4 or 5 days high and low temperatures.

I've read this before -- but I assume this must be true only for the "baseline" of an uncovered pool? It doesn't feel to have been correct for my covered pool this frigid January (though admittedly I don't have detailed logs to be sure... maybe a project for next year?). Besides, if covered and uncovered pools both had identical temps that were determined exclusively by 5 days of air temperatures, then that would imply that covering is irrelevant and that solar covers also wouldn't work (whereas all covers clearly help with the big share of evaporation, as you note). So I'm still in search of a similar "rule of thumb" to determine pool temps for covered versus uncovered setups.

I check the long range forcast and open right after the last freezing low. It could always go sideways 3 weeks later and be cold again, but it won't be stoopid cold, or for very long, so running the pump for a few days straight is fine to get by.

and

I'd say when you're more or less sure the freezing temps are over you can do a soft opening and get the pump to move water getting a good mix.

Yup, I think that's what I'll do in practice. And perhaps the answer to my original question is just "open as soon as possible! (when safe from deep freezes)".

 

[Newdude]

So I'm still in search of a similar "rule of thumb" to determine pool temps for covered versus uncovered setups.

My spitball guess is to average the recent highs/lows and add 5 degrees afterwards for the cover. Obviously 2 really warm or cold days on days 4 and 5 might swing the observed temperature versus the guess.

I've done a bunch of guess calcs with 7 days instead of 5 and it didn't seem to matter much.

Also recognize the water stratifies and the top water will be several degrees more than the bottom.

And perhaps the answer to my original question is just "open as soon as possible! (when safe from deep freezes)".

I like to look at it and want it back ASAP.

 

[Sampo]

Great points @Newdude !

Maybe I'll direct some of my off-season pool-fever next winter to tracking the temps more methodically, with a wireless thermometer or something. But I like your plan -- and I'm counting down the days to the latter part of this month when I plan to open! Hooray!

 

[wireform]

Maybe I'll direct some of my off-season pool-fever next winter to tracking the temps more methodically, with a wireless thermometer or something.

That's useless as the water is stratified by not moving/circulation. The same goes for the chemistry where down below its one thing and close to the surface it a complete different story.

 

[Newdude]

That's useless as the water is stratified by not moving/circulation.

The warmer water up top would be useful to know as the high threshold IMO. We'd know it only got colder deeper.

 

[Newdude]

Maybe I'll direct some of my off-season pool-fever

March 17th is my last night below freezing so far. Granted, long range forecasts are like playing blind darts, but soon.

Last year the SWG kicked on in 3 days or so. It warms up quick in the sun. From there it worked some days and not others but still made enough on the good days to not need LC.

 

[Sampo]

March 17th is my last night below freezing so far.

What website are you using to track weather that far out??

 

[Newdude]

The AccuWeather app has a 45 day tab on the normal 'daily' page showing 2 weeks.

They're wrong as any weather at times, but seem to get it kinda right alot.