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July 8, 2005 8:44 AM   Subscribe

How do I cool my pool? It's 112 degrees outside these days and my pool temperature is getting up to 86 degrees, which is bathwater nasty. How can I cool the water down? I have an Airator but I don't know how well it will work. I could add 10 pound ice blocks but how many ice blocks do I have to add to bring down the temperature? Any other suggestions?
posted by propagandist to Home & Garden (23 answers total)
I've never had that problem, but I can tell you about ice, in case you go that route: If youv'e got a vinyl-walled above ground pool, DO NOT let the ice come into contact with the vinyl for an extended period of time. Vinyl can become brittle very easily under cold temperatures, particularly in small areas. It's an easy way to introduce pinkprick holes or even pressure tears in your vinyl, and you'll end up replacing the entire vinyl over one misplaced ice block.
posted by thanotopsis at 8:54 AM on July 8, 2005

Penguins. Having a penguin at your pool lowers the ambient temperature by 10 degrees F.

In all seriousness, ice blocks are probably not efficient for a project of this size. I'm also not sure about the Airator - how large is it?

You might try setting up some kind of spray system to spray water from the pool back out over its surface; the interchange with the air could cause some heat loss and so eventually chill the pool, but I imagine it would work best at night and would take a long time to make a noticable difference.
posted by staresbynight at 8:57 AM on July 8, 2005

Get a fountain (is that what the "Airator" is?) which enhances evaporation of your pool water. I wouldn't worry about ice touching the vinyl liner. In the north vinyl lined pools freeze over every winter.
posted by caddis at 9:03 AM on July 8, 2005

Assuming the math is correct, this is why ice is probably impractical.
posted by defreckled at 9:15 AM on July 8, 2005

But you generally aren't swimming in the pool when it is iced over. The vinyl would be fine when frozen until something moves/touches it.

The spray thing will work as it's essentially a big open air evaporative cooler like the swampers they use in the SW. Also you could look at shading, as it is probably the solar radiation that is heating your pool if the water is much above the average 24hr temperature. Solar radiation is good for about 100W per sq metre. If you use a pool cover remove it over night, radiant losses to the dark sky is quite effective at cooling bodies of water.

I'm going to throw out the foolish suggestion too: make sure your heater is off. Even if the pool temperature during the day is above the cut off it may be kicking in over night. Solar pool heaters are quite common and are often incorporated into the roof of pump houses. If you have one and it can't be by passed you need to cover the collector.

defreckled the math looks right.
posted by Mitheral at 9:29 AM on July 8, 2005

I worked as a lifeguard at a pool in Augusta, GA. In the dog days of summer, it would get hot. And this was a big (25 m) pool, so here's what we did: drain 6" of water out of the pool. That took about a day. And then fill the pool back up with water.

This is tremendous waste of water. But it works. And diving into a pool that's 75 instead of 88 is a pretty compelling reason to squander a lot of water. (I know, I know, it's not.)An airator will work for a smaller pool, but it will take a few days of constantly running to have a noticeable effect. But it will work, eventually. This is probably good for 5 or so degrees (depending on how wet/dry the air is)... which might be enough to do the trick.
posted by zpousman at 9:49 AM on July 8, 2005

The airator (aerator for those who insist on correct spellings) is infact a device that sprays water over the surface of the pool. It sprays water in a pattern that hits 70% of the pools surface.

The pool is 15,000 gallons, under ground, plaster bottomed.

Don't have a heater, the temperature reading of 86 is at 6 pm after the hottest part of the day, it may well be cooler in the morning.

Shade sounds like the best option, anybody know any good places to get pool covers online?
posted by propagandist at 9:49 AM on July 8, 2005

I'm curious: why don't people in warm areas install geothermal heat exchangers for their pools? You already have to pump the water through a filter, would it be that much more work to pump it through some underground piping too?
posted by sbutler at 10:01 AM on July 8, 2005

I've seen a beautiful shade cover for pools ,from texas, the shade supports inflate
posted by hortense at 10:05 AM on July 8, 2005

my hunch is that putting a standard pool-cover over the pool won't help that much. in order to actually prevent the heat transfer from the sun to the pool you'd need to allow air flow under the shade and above the water...maybe that's already what you have in mind. Something like what hortense just suggested might help. but it may be that the largest source of heat transfer is from the air itself...maybe someone can do the math, but 112 degree air gives quite a delta T.

If you run the airator that might help, plus it would hasten the loss to evaporation and you could then slowly add more cold water to replace the loss which would help a bit as well. maybe a combination of all these things would be enough?
posted by jacobsee at 10:34 AM on July 8, 2005

would it be that much more work to pump it through some underground piping too?

it would be a lot of work and expense to install the underground piping but it's a good idea oveall. if you're talking about a lot of heat transfer you need to be sure that you've covering a pretty significant volume of ground so it could be pretty expensive
posted by jacobsee at 10:36 AM on July 8, 2005

I'm in the same boat, propagandist. Recently, I started using my aerator, and I've seen a 3 or 4 degree drop. Water temp is still in the 80s though.
posted by 27 at 10:38 AM on July 8, 2005

another idea: plant some trees south of the pool (or north if you're in the southern hemisphere)
posted by jacobsee at 10:38 AM on July 8, 2005

The math in defreckled's link seems correct to me.

Essentially, you need a place to dump the heat (energy) into. Heat (energy) flows from hot to cold, so in order to dump that heat somewhere, you need to find a heat sink that is colder than your pool water. Since the air is hotter that your water, you don't really have the option to dump the heat there.

If your hose water is cooler than your pool water, you could cycle the water through and let the run off go somewhere. That could be expensive though at 15,000 gallons. You can assume that the lowest the temperature would ever get is the temperature of the hose water (if the water is fully exchanged) and the temperature will be a ratio of the mix at other conditions.

For instance, if your pool is 86F and the hose water is 76F, if you pumped in 7,500 gallons, your pool would be a minimum of 81F. I say minimum, because this assumes no contribution of the sun/air to heat the pool back up.

The pump itself also adds a fair amount of heat, especially since I assume that its efficiency is fairly low (to keep costs down). Inefficiency basically becomes heat (I know that "heat" isn't a noun, but it's an easy way to talk about it).

Long term, I would suggest to get a reflective pool cover, use the pump sparingly and only uncover the pool when you swim.

Evaporative cooling could help, but the thermal mass of 15,000 gallons of water needs a significant amount of surface area to be effective.
posted by jonah at 10:44 AM on July 8, 2005

Sounds like an evaporative fountain run -at night- is the best combination, assuming it drops below pool temp at night. For during the day, given that it's an in-ground pool, how practical is a waterfall-esque setup which is built under a shade?
posted by phearlez at 10:55 AM on July 8, 2005

jacobsee writes "another idea: plant some trees south of the pool "

This is what I was meaning by shade. Convince your neighbours to build a big McMansion directly to the south of your pool :). Seriously though one of my old neighbours planted grapes along a trellis covered walkway (like a 30' arbour) to the south of their pool. The north patio along the pool was still in full sun but the pool was shaded about 50% of the time during the day and we had yummy grapes in the fall.
posted by Mitheral at 12:44 PM on July 8, 2005

I have actually used the ice method to cool down 300L baths for Science! The Slate calculation referenced by defreckled is right. You do need an assload of ice to make a difference. For a 10 C temperature drop, we were using 100 kg of ice, about 1/3 of the tank. Can you fill your swimming pool 1/3 full with ice? Also, it takes a day or two to melt, a bit faster if your have your pool boy out there stirring it all day.
posted by bonehead at 12:54 PM on July 8, 2005

Could you try a shade sail (others here)? I saw these used on Extreme Makeover Home Edition to shade a whole yard, including a pool, for a girl with a sun allergy. They are less expensive than I would have guessed.
posted by donnagirl at 1:06 PM on July 8, 2005

Sounds like an evaporative fountain run -at night- is the best combination, assuming it drops below pool temp at night.

The best part about evaporative cooling is that it works even when the temperature is higher than the pool water due to the phase change energy of evaporation. The local weather right now in Scottsdale shows a current temperature of 104 degrees F, a relative humidity of 14% and a dew point of 47 degrees. With enough evaporation you could chill water to the dew point of 47 degrees. A fountain won't get you close to that but could still help even when the air temperature is higher than the pool temperature.
posted by caddis at 4:06 PM on July 8, 2005

What's the nighttime temperature? If it's cooler than the temperature you'd like the water at then install a solar pool heater. If you run it overnight and shunt it during the day you'll radiate heat.
posted by substrate at 7:03 PM on July 8, 2005

A solar pool heater would not work well as a radiator. The coils are trapped under glass to increase solar heating and will not release much heat to the environment. You could of course implement a radiator for the night time but why bother when it looks like the low temperature for the next week is about the same if not higher than your current pool temperature.
posted by caddis at 7:15 PM on July 8, 2005

Shade is your simplest option. You can always add active cooling later.
posted by Ken McE at 8:51 PM on July 8, 2005

modern solar pool heaters aren't enclosed in glass, unless you pay for it. I just installed one today. It's a bunch of flat plastic panels with embedded tubes. During the day it heats the water, at night it radiates.
posted by substrate at 1:50 PM on July 9, 2005

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