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I live in Phoenix where summer outdoor temperatures regularly exceed 110°F and humidity around 15% or lower is not uncommon. In this environment a mist system can easily pull the temperature down 20 to 30 or even 40 degrees in some cases. I've been toying with the idea of cooling my outdoor AC unit using mist, and did a little research.

There are several commercially-available systems to do precisely this such as this industrial unit and this residential one. But I wanted to get an idea of what other people think of these systems. In particular, I'm interested in the long-term effects of using these systems, especially with respect to mineral build-up and corrosion. The vendors of the commercial systems claim to have these factors under control, but I'd rather hear it from a neutral POV.

I'm not particularly interested in unfounded FUD or enthusiasm, and comments like, "if it works so well then we'd all be using it" are particularly unhelpful. Instead, I would like to know how these units actually fare in real-world usage. Also, bear in mind that this is Phoenix, not New Orleans; in the dry summer heat nothing stays wet for more than a few seconds. Factors like mold are an absolute non-issue.

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Interesting question. Most window units work that way. The catch is it's using water from the air, rather than a well, so isn't coming along with the mineral deposit issue. –  DA01 Jun 30 '12 at 4:53

3 Answers 3

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Mineral build-up is going to be dependent on your water. Do you have a Water Quality Report? Mine looks like this:

http://www.acwd.org/story_detail.php5?story_id=157

Hard water starts around 10 grains per gallon or 170 ppm. In other words, 1 liter of water will have 0.17 grams of calcium/magnesium. I didn't see any water usage estimates for coolnsave aside from 6 cents per day. Also, they sell a water treatment filter which could take some of the minerals out but there aren't any details provided.

The thermodynamics behind coolnsave are sound and the price is reasonable. Your best bet if no one here has any personal experience is to just try it yourself, keeping a daily inspection schedule for the first month to be sure you aren't coating your condenser with something that you can't remove.

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Yes, my neighborhood has fairly hard water, but nothing like other parts of phoenix. A common component in all of these systems is a filter or treatment step. The CoolNSafe people use a filter that looks strikingly similar to the ones at home depot for use in mist systems. One site I looked at recommended spraying CLR on the AC unit and hosing it off once every 3 mo. –  tylerl Jun 30 '12 at 7:11
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CLR is often effective at removing deposits. But a condenser is a large investment and you want to be sure your water doesn't have anything that can ruin your condenser. If the water treatment is not effective and CLR does not work or there is some way that your water is especially corrosive to your condenser, the cost could be over $1,000 to repair. That's why I recommend you inspect it every day for a few weeks. Commercial installations have water quality professionals to analyze and maintain the water chemistry. –  Philip Ngai Jun 30 '12 at 16:42
    
CLR is very corrosive, which is why they say to spray on and hose off immediately. Better than nothing, I suppose. But definitely not as good as good water treatment. –  tylerl Jun 30 '12 at 18:23

For the record, I tried it out myself. I installed the Cool-N-Save system on a 4-ton A/C unit that was scheduled to be replaced in three months.

After that three month period in the middle of the summer, the positive effects of the system were negligible at best, and probably detrimental. There were no detectible savings in cooling costs nor increase in cooling performance. And after that three-month period, despite using the manufacturer's own anti-calcium system, the calcium build-up on the outdoor unit was extreme, with a thin layer of rock uniformly covering all of the heat-exchanging surfaces, effectively destroying the unit.

So that's a strong "Do Not Buy" from me.

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The reviews on Amazon are "mixed," but with a lot of quality complaints (leaks, poor durability): http://goo.gl/EPpUCH

One commentator provided a link to another system, The Mister: http://www.themister.com/ This looks to be a higher quality product (although at 2.5X the price). It specifically avoids some of the issues raised in the cool-n-save reviews. Remove it in the winter to avoid freeze damage.

However, I think you'll find this thread on the entire concept of interest: http://goo.gl/Jy7Qdq The killer issue being water quality - and therefore mineral deposits on the condensing coil. In-line chemical filter? Panned as ineffective (they make the minerals more water soluble - so keep the mister headers clean - but the minerals remain, so they precipitate out when the water evaporates). One user posts a home brew solution using an RV water softener. One user postulated using an reverse osmosis (RO) system as the source of the water; this would work - but you'd need a pump to boost the pressure up, and may have issues with generating enough RO water for the demand (RO systems are also very inefficient - a lot of water is wasted and it adds high-salt water to the sewer system). Some industrial systems are using RO (search for "BigFogg"). Some people report using water from the condensate line - which is mineral free - and either spraying it or dripping it over the condenser coil.

I also live in Phoenix. My water hardness is 13. I used to have an outdoor patio misting system - with one of those inline filters. The filter was not effective - still got mineral build up on the misting heads. What really killed it was the sun exposure, though - the plastic tubes connecting everything got very brittle and started breaking on multiple segments.

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I liked "themister"'s web site, the information related to thermodynamics on it was accurate and correct. I'm not familiar with the filter chemistry they use. It is very interesting that it uses very little water, about 5 gallons per hour. You might be able to use DI water in that case. RO water boosted by a pump would also be possible. The waste would not be significant and you're not correct about the waste being high-salt. RO does not add minerals/chemicals, it just filters them out. –  Philip Ngai Sep 9 '13 at 18:49

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