Advice on running pool heater for a day ahead of polar vortex

[mas985]

There are a lot of factors for when a pool will start to freeze over so it is not a simple analysis.

To cool ALL the water to 32F and assuming that it is circulating so more less even throughout, you would need to remove about 2.5m BTUs. There are 4 ways to lose heat. Evaporation, convection, radiation and conduction and these depend on a lot of factors (i.e. latitude, air temperatures, dew point temperatures, wind speed, cloud cover, pool shell construction, pool cover).

But again, water does not freeze as soon as it hits 32F. The latent heat must be removed which is another 171M BTUs but that is for the entire pool to freeze solid which does not happen all at once.

So while it theoretically could be possible to calculate when a pool would start to freeze over, practically, you would need to know a lot of information (more than just air temps) about the future weather conditions which makes it very difficult to predict.

However, I have a spreadsheet that might get you close. I put in some numbers (see lines 26-40) and came up with the first 24hr loss of around 13F to 35F . So not too long. But again, I had to make a LOT of assumptions that may not correct.

Pool Heat Transfer Tools v021 - 6XSBChamps

docs.google.com

 

[Newdude]

Wind would make a huge difference, like blowing on a giant bowl of soup.

Everyone always thinks wind chill makes it colder but it doesn't. It just cools everything (including us) that much faster that it seems colder.

 

[6XSBChamps]

Wind would make a huge difference, like blowing on a giant bowl of soup.

Everyone always thinks wind chill makes it colder but it doesn't. It just cools everything (including us) that much faster that it seems colder.

Yup. I have my pad wrapped in moving blankets. And the pad is protected from the wind by house as well.

 

[DB-Cooper]

Wow, stepped away and this thread got busy.

Well as reported by Pentair, my pool water is reporting at 39F. We've been freezing for ~33 hours now, we might hit 31F this afternoon. . We have another cold night tonight and more hopeful for just getting into the mid-30s tomorrow, another cold night and then all clear by mid-day Wednesday.

We had a warm Saturday, and basically water temp was 49F when weather was good. When freeze protection kicked on, water temps were 47 or 46, and it hung there most of yesterday, dropping a degree every several hours. I think I went to bed and it was maybe 43 and woke up to 40 and now it's 39. Below is the Intellicenter report.

I'm curious what will happen when this water inevitably gets closer and closer to 32F. Based on the similar cold snap we had late December, 2022 (where I was on vacation and thankfully everything worked fine), Pentair reported the water got to 34F before warming up. My pad is covered with a little roof, and I've wrapped it shut with tarps (pictured below) and have a 250W shop light in there.

I recall in Feb 2021 snowpocalypse, my pool I think was reporting below freezing, maybe 29, but my fountains on my booster pump basically froze over which caused the booster pump to freeze. However, we got like tons of snow and it was just dumping snow into the pot. I've since ditched the pots and have sheers and they can't collect cold water! In 2021, with the weather only getting worse, I decided to just dump everything and throw in the towel. Fortunately, everything survived except a couple cracked above ground pipes.

I also read someone on here say that they recall seeing their Pentair app reporting water temps in the 26-28 range in previous winters. I've read all the scientific threads, but I am curious how freeze protection will play out when water temps go below 32. For example, all my valves are open, and the pool switches circuits every 15 minutes. That means for 15 minutes, potentially water <32F is sitting in ~2" piping above ground (granted in the sheltered and lit tarped/pad area). I guess cold water still doesn't just instantly freeze once it stops moving and would take longer than 15 minutes? I also assume as the pool water gets colder, it'll take longer to drop than when we started in the mid 40s...

 

[JJ_Tex]

Mine is holding steady at 36. I had a little bit of surface ice in a corner, but pipes and equipment seem fine.

 

[DB-Cooper]

Mine is holding steady at 36. I had a little bit of surface ice in a corner, but pipes and equipment seem fine.

JJ_Tex,
Appreciate the update, it's always good to know I've got friends in the same battle!

 

[DB-Cooper]

Mine is holding steady at 36. I had a little bit of surface ice in a corner, but pipes and equipment seem fine.

I should've asked, what was your starting point, and how long have you been stuck at 36?

 

[JJ_Tex]

I should've asked, what was your starting point, and how long have you been stuck at 36?

It was around 50 when the front came through, low 40’s when I went to bed, then at 36 when I woke up and been there all day.

 

[dailygenesis]

To me, the scariest thing about keeping the pumps on and running is that eventually, if the air temps stay below freezing for long enough, the water temperature will eventually get to 32 degrees -- and that is 32 degrees throughout the entire pool, all of the underground pipes, the equipment, etc., etc. The process of keeping the water moving means we are keeping the pool water at a uniform temperature throughout. This keeps the water from freezing at the surface and in the pipes for a long time, but the trade off is that it is gradually bringing every bit of water in the system down towards freezing. At that point, as @mas985 there is still some latent heat that keeps everything from freezing solid for a bit longer, but it just seems that eventually, if air temps don't come back up soon enough, we now have a situation where everything can freeze and freeze solid pretty quickly.

I still take this approach because the bet is that, in my climate, temps won't stay below freezing long enough to get to this point. But this is why I have started pre-heating the pool before longer bouts with freezing weather. It just buys an extra day or two before we hit that eventual point.

Many here on this thread are south of me in Texas and so the bet for them is even easier (probably even without pre-heating) to make as bouts of consecutive days below freezing up here in Oklahoma can be a little longer. I feel like in my climate we're right on the edge of where it makes sense to take this gamble. I'm fairly new to the pool game (this is just my second winter) so I'm still learning every time but these are my impressions so far. My discussions with other pool owners in my climate have led me to believe it's reasonable to take the approach I'm taking here. So, I'm not saying it's a bad bet, I just think it's good to be aware that it is a bet -- a bet that sub-freezing temps won't last long enough to make the situations worse than it would have been if we had just shut down and drained.

 

[DB-Cooper]

To me, the scariest thing about keeping the pumps on and running is that eventually, if the air temps stay below freezing for long enough, the water temperature will eventually get to 32 degrees -- and that is 32 degrees throughout the entire pool, all of the underground pipes, the equipment, etc., etc. The process of keeping the water moving means we are keeping the pool water at a uniform temperature throughout. This keeps the water from freezing at the surface and in the pipes for a long time, but the trade off is that it is gradually bringing every bit of water in the system down towards freezing. At that point, as @mas985 there is still some latent heat that keeps everything from freezing solid for a bit longer, but it just seems that eventually, if air temps don't come back up soon enough, we now have a situation where everything can freeze and freeze solid pretty quickly.

I still take this approach because the bet is that, in my climate, temps won't stay below freezing long enough to get to this point. But this is why I have started pre-heating the pool before longer bouts with freezing weather. It just buys an extra day or two before we hit that eventual point.

Many here on this thread are south of me in Texas and so the bet for them is even easier (probably even without pre-heating) to make as bouts of consecutive days below freezing up here in Oklahoma can be a little longer. I feel like in my climate we're right on the edge of where it makes sense to take this gamble. I'm fairly new to the pool game (this is just my second winter) so I'm still learning every time but these are my impressions so far. My discussions with other pool owners in my climate have led me to believe it's reasonable to take the approach I'm taking here. So, I'm not saying it's a bad bet, I just think it's good to be aware that it is a bet -- a bet that sub-freezing temps won't last long enough to make the situations worse than it would have been if we had just shut down and drained.

I'm not sure the pool will just freeze into an entire block "quickly" once it goes below 32F. Feb 2021, my water temp was already below 32F before I dumped the pad and gave up. As you stated, the pool pump was running so the entirety of the water was circulating and it was basically that temp throughout (not warmer at bottom, etc). After I pulled the plug, I think we still had 5 days below freezing. Ultimately, my pool just froze over on the top, maybe 2-3" max and I was still able to push the block down.

It's hard to understand this stuff...

 

[JJ_Tex]

I think you will be fine as long you keep the water moving. Sure, you may get some surface ice but your pipes and equipment is what you are trying to save. Keep in mind that rivers up north will still flow well below 32 degrees because the energy from the water moving keeps it from freezing up.

 

[ajw22]

Lakes do not become solid blocks of ice and neither do pools. Read…

A Look Under the Ice: Winter Lake Ecology | Ausable River Association

During the winter many aquatic organisms that live in our lakes hunker down, but a few remain active. It is the unique properties of water that allow the plants and animals that live in lakes to survive the winter.

www.ausableriver.org

https://www.nhlakes.org/wp-content/uploads/5-Lake-Ecology-Seasonal-Winter-Lake-Freezing-NH-LAKES.pdf

 

[dailygenesis]

I'm not sure the pool will just freeze into an entire block "quickly" once it goes below 32F. Feb 2021, my water temp was already below 32F before I dumped the pad and gave up. As you stated, the pool pump was running so the entirety of the water was circulating and it was basically that temp throughout (not warmer at bottom, etc). After I pulled the plug, I think we still had 5 days below freezing. Ultimately, my pool just froze over on the top, maybe 2-3" max and I was still able to push the block down.

It's hard to understand this stuff...

Understood. I know my explanation is oversimplifying, but I do think there is some degree of truth in it, as evidenced by the fact that you eventually pulled the plug, presumably because you felt the risk of keeping it running was higher. And on that point -- I am curious -- what factors led you to dump the pad and shut it down back in 2021? I've always had that as a plan should I lose power or lose a pump, but I don't feel like I have a solid plan for when to take that action otherwise. What made you change course then?

 

[dailygenesis]

Lakes do not become solid blocks of ice and neither do pools. Read…

https://www.nhlakes.org/wp-content/uploads/5-Lake-Ecology-Seasonal-Winter-Lake-Freezing-NH-LAKES.pdf

Would it be different for lakes if they had filter pumps that were circulating their water and keeping it at an even temp top to bottom -- and then if sub-freezing temps lasted long enough to get that entire body of water down to freezing levels?

 

[ajw22]

Would it be different for lakes if they had filter pumps that were circulating their water and keeping it at an even temp top to bottom -- and then if sub-freezing temps lasted long enough to get that entire body of water down to freezing levels?

Pumps do not change the density characteristics of water.

And even in a pool there is typically thermal stratification.

Pools turnover the water just like lakes.

Let's back up just a bit to talk about what happens before the lake freezes. During the summer, most lakes are thermally stratified. Warm lower density water sits on top of colder higher density water. As summer transitions to fall, the upper layers cool, breaking down that density difference. Eventually, the surface of the lake cools to 4 degrees Celsius (39.2 degrees Fahrenheit), the temperature at which water is most dense. This causes the water at the surface to settle to the bottom, pushing the now relatively warmer water at the bottom back to the surface. This process continues until the surface water cools below 4-degrees Celsius, at which point it becomes less dense, and eventually freezes. Remember, water is most dense at 4 degrees Celsius. It becomes less dense above and below this temperature. If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below. Our winters aren't long or cold enough to completely freeze most local water bodies. This process of lakes turning over is crtically important to the life in the lake. It is these turnover periods that infuse and distribute oxygen throughout the entire water column.

 

[dailygenesis]

Pumps do not change the density characteristics of water.

And even in a pool there is typically thermal stratification.

Pools turnover the water just like lakes.

Let's back up just a bit to talk about what happens before the lake freezes. During the summer, most lakes are thermally stratified. Warm lower density water sits on top of colder higher density water. As summer transitions to fall, the upper layers cool, breaking down that density difference. Eventually, the surface of the lake cools to 4 degrees Celsius (39.2 degrees Fahrenheit), the temperature at which water is most dense. This causes the water at the surface to settle to the bottom, pushing the now relatively warmer water at the bottom back to the surface. This process continues until the surface water cools below 4-degrees Celsius, at which point it becomes less dense, and eventually freezes. Remember, water is most dense at 4 degrees Celsius. It becomes less dense above and below this temperature. If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below. Our winters aren't long or cold enough to completely freeze most local water bodies. This process of lakes turning over is crtically important to the life in the lake. It is these turnover periods that infuse and distribute oxygen throughout the entire water column.

This all makes sense, but I feel like it is sort of in line with my point.

The point I was making is only that in some cases we as pool owners might be better off to just drain the pad and let things sit during bouts of sub-freezing weather. Whether or not this is true is dependent on how long the sub-freezing temps last and probably a lot of other factors, and it's usually not something we can definitively know in advance, so we just make a bet. We bet that running our pumps 24/7 will be better in the long run because it will delay the surface freezing (by disrupting the natural phenomenon you describe above) and we bet that the air temps will return to above freezing before any issues arise out of this. The trade of is that if temps don't return to above freezing in time and our pool water lowers all the way to it's most dense point and below, then we might be in a situation that is worse than what we would have been in had we just drained the pad and let it go.

Do you agree with this?

 

[mas985]

This all makes sense, but I feel like it is sort of in line with my point.

The point I was making is only that in some cases we as pool owners might be better off to just drain the pad and let things sit during bouts of sub-freezing weather. Whether or not this is true is dependent on how long the sub-freezing temps last and probably a lot of other factors, and it's usually not something we can definitively know in advance, so we just make a bet. We bet that running our pumps 24/7 will be better in the long run because it will delay the surface freezing (by disrupting the natural phenomenon you describe above) and we bet that the air temps will return to above freezing before any issues arise out of this. The trade of is that if temps don't return to above freezing in time and our pool water lowers all the way to it's most dense point and below, then we might be in a situation that is worse than what we would have been in had we just drained the pad and let it go.

Do you agree with this?

I have been long advocating draining the pad equipment since pretty much the beginning of this forum. But generally, only when air temperatures drop below freezing for more than 12 hours per day.

Under those conditions, I just don't see any advantage in running a pump to prevent freeze damage. When air temperatures are that low for that long, the water temps are not far behind so algae is pretty much non existent so I see no benefit in keeping the pump running. I only see more risk.

 

[DB-Cooper]

Understood. I know my explanation is oversimplifying, but I do think there is some degree of truth in it, as evidenced by the fact that you eventually pulled the plug, presumably because you felt the risk of keeping it running was higher. And on that point -- I am curious -- what factors led you to dump the pad and shut it down back in 2021? I've always had that as a plan should I lose power or lose a pump, but I don't feel like I have a solid plan for when to take that action otherwise. What made you change course then?

I mentioned it earlier, but what led me to pull the plug was that I had already lost my accessory water fountain pump. I've since gotten rid of those fountains and replaced them with sheers. At the coldest point, with significant fluffy snow dropping from the sky, I witnessed that my fountain bowls (you can see a picture of one in my avatar from afar) were basically turning into containers of very dense snow. At first it was hanging on fine, but it just ended up being snow and then water stopped flowing. Presumably at that point, the pump could no longer pump water as the fountain bowl was basically plugged. I ran outside and saw that the pump was off and under the lid looked like it was basically frozen. I removed the lid and opened the drain plug. It was like 2 or 3am, I was stressed and we were still days away from improving weather, there was snow everywhere, I decided I didn't want anything else to go wrong so I just drained everything else right then and there. At the point of this, the pool/spa and everything was running fine, no ice anywhere just the slurry of snow that was dumping into the pool. The extended forecast was colder and 4-5 more days below freezing, it just didn't seem likely I could survive any longer. It's worth noting that the 2021 incident was also preceded by a 2-3 day period where temps never got above freezing (but also never went down below 28), so my pool was already quite cold.

Ultimately, the power went out for 2-3 days due to the grid fiascos, so draining the equipment was inevitable...

Finally, despite draining the pool side when everything was still running fine, I had external pipe damage on two above ground pipes, and I think one check valve cracked and/or a shut-off manual valve. Despite having all the lids off, filter drained/vented, I think the check valves or something prevented all the water from vacating back down into the pool (my pad is several feet above pool level). If I were to drain again, I would remove all the check valve inner pieces and just leave large holes in the water. Ironically, I had no damage on my fountain setup plumbing (or pump) despite it basically freezing over.

So suffice to say, it's best if I can avoid draining all together. Also, priming my pumps is an arduous task.

 

[dailygenesis]

I have been long advocating draining the pad equipment since pretty much the beginning of this forum. But generally, only when air temperatures drop below freezing for more than 12 hours per day.

Under those conditions, I just don't see any advantage in running a pump to prevent freeze damage. When air temperatures are that low for that long, the water temps are not far behind so algae is pretty much non existent so I see no benefit in keeping the pump running. I only see more risk.

Yeah, I think my point is in line with yours, the difference being I see the point of trade off as being much further down the road than just 12 hours sub-freezing. Maybe it's just the difference in our climates, but I would consider it very low-risk to keep pumps running through sub-freezing periods of 1-2 days as I have done this several times now even in my short bout of pool ownership. This current spell we are in is going to keep us below freezing for at least 4.5 days and I think I'll make out fine on this one too -- but that is with me pre-heating the pool. Somewhere around this 3-5 day mark, though, is for me where I'm starting to feel the cut off might be. I just haven't gotten up the nerve to try draining the equipment and hoping for the best. It is still appealing to me to prevent the ice as long as possible.

 

[Newdude]

with significant fluffy snow dropping from the sky

This is another aspect of the week long calculated risk. If the weather forecast changes and heavy falling snow shows up, it may turn the top of the pool into a slurpee/slushie. It won't take much for the returns to fill the skimmers or pump basket with it where it may brick up and cause a blockage. With no flow it's game over in minutes.