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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
  • you just buy a 250 gallon or 200 gallon water cube off craigslist/ebay and use a pump connected to the tank! rain water is mineral free and non corrosive folks, that is the way to go and save money. If you have acid rain (unlikely unless you live in big city) then just drop some very dilute draino or other acid neutralizer like some laundry soap. just don't over do it becuase too basic is also bad. – user35558 Apr 13 '15 at 11:59
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    An in line filter will NOT remove salts, only particulate matter. Ion exchange might do the trick but you're just subsituting NaCL for Calcium, still corrosive. RO is the only way short of using distilled water.. One way to do this is to use an Earth Tube. If you have the space (I do), run a 50 foot corrigated pipe under the ground and draw the air thru it. Not sure how long the pipe would have to be, but it would work. Cheap, just involving a backhoe and some pipe. – user39276 Jul 9 '15 at 15:36
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    @user35558, it may also be illegal. In the western United States, water rights are complicated, and you may not own the rain that falls on your land. – Mark Jun 22 '16 at 1:50
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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
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    The fins on a condenser are almost always aluminum. The maker of Calcium Lime Rust does not recommend its use on aluminum. – Philip Ngai Aug 6 '16 at 16:04
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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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I have been experimenting with using an old mosquito misting system to mist around one of my AC condensers. It does seem to make a small difference in energy consumption and reduce register temperature by around 1 degree. Not much but enough to help when the system is running constantly when we have parties at the house. The red circle on the attached energy report for the condenser shows around a 100watt drop at 5:30pm when I ran the mister for 15 minutes. The system also shut down because the house reach target temp. What I don't understand is why ambient temperature makes such a big difference to energy used. The whole chart shows a wave as the ambient temperature changes from night to day. Oh, I use condensate from the indoor unit for the mister so it shouldn't contain calcium etc. AC Power Consumption

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    Yes, an air conditioner does two things. First, it removes moisture from the indoor air. Second, it moves heat from inside your house to outside your house. Like a water pump, the amount of energy required to move the heat is directly proportionally to how high the water has to be pushed. For an AC, the energy required is the difference between the inside temperature and outside temperature. So you can see that a lower outside temperature will directly reduce your energy usage for cooling the inside air. – Philip Ngai Aug 6 '16 at 16:15
  • (my comment was too long) The energy for reducing humidity will not change with the outside air temperature except to the extent that outside air leaks into the house. Hotter outside air can potentially carry more moisture per cubic foot of air. – Philip Ngai Aug 6 '16 at 16:19
  • using condensate from your indoor A/C unit for a mister to the outdoor unit is smart. what's the flow rate like? An online HVAC Condensate Calculator estimates 11gal/day for a 3.5 ton unit in hot, high humidity climates. What is the mister's water consumption per day? – Marc Compere Jun 23 '20 at 2:23
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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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If you're worried about mineral buildup then buy a water filter. Go to Lowe's or Home Depot and get the 5 year inline water filter cartridge.

I've thought about this too and living in the Panhandle of Oklahoma the weather is about the same conditions. We used a swamp cooler back in the day and I thought it worked just fine, so I thought about building a swamp cooler around the AC unit, no mist. It activates when the AC unit comes on and soaks vertical pads around the sides of the unit kind of like a water radiator around a condenser radiator.

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Rheem used to have a water sprayed condenser coil on their Coral air units. They had a float that looked like it came out of a toilet tank. They work of course however to alter one will mess with the designed heat rejection and pressures its designed to operate by. The condensing unit must increase the refrigerant vapor to well above the outdoor airs temp to allow heat rejection to outdoors and condensing of the refrigerant to a liquid , if you mess with this who knows how lucky you may get or for how long and under what circumstance. Old AC condensing units in shopping malls had water cooled coils that were a spiraled coil and water ran in a tube in a tube setup that simply drained to a sewer total loss system and regulated the head pressure by throttling the waters flow to the drain. today they would never allow such a total loss system.

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Many people who live in dry, hot climates may find that evaporative coolers combined with the central air conditioners many already have helps to reduce cooling costs while increasing overall cooling.

Evaporative coolers were around since before central air conditioning. These vary in size from portable units used to cool one or two rooms to units insalled as part of a central cooling system.

Evaporative coolers can work in conjunction with central air:

  1. The evaporative cooler can be used so long as the humidity is low.
  2. Water is piped or poured into the unit which circulates it over mesh plates. These vary in construction and materials but do the job os providng a wet filter-like surface through which air is fanned. This can cool the air temperature between 10 and 40 degrees F.
  3. The cost of operating an evaporative cooler is 25%-30% of central air. Electric consumption is limited to a fan(s).
  4. The downside is higher maintenance: the unit must be cleaned up to several times per year. However, most are designed for consumers to easily do this.

This works by aiding refrigerative air conditioners by 1) reducing the load. 2) By reaising the humidity. Although humidity is often viewed as something you want to get rid of, the evaporation of water into the air helps to improve the efficiency of the central air conditioner. Humidty that condenses to water transfers a higher modulus of energy than the air-to-metal transfer.

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    You explained how it works but the question is actually about "long-term effects of using these systems, especially with respect to mineral build-up and corrosion." - which you didn't address. – RedGrittyBrick Oct 15 '17 at 19:41
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I bought the MistBox last year. This is a unit that takes all the guesswork out of Mist cooling your AC. I live in Las Vegas, NV. We enjoy humidity as low as 5 to 15% all summer. With normal around 100+ the whole summer. I installed it in minutes and it worked well. I could actually tell the difference immediately. The only negatives I read while researching this was some said it will corrode the cooling Fins. This unit comes with a filter and I hooked it to my Softener. Also my Trane I3 AC has a webbing pad that protects the fins and acts like a swamp cooler evaporative pad. I had purchased this while it was still on KickStarter. It now has a solar screen that recharges the battery. The WiFi app is free and pretty cool. It gives savings # that are close to 30%.

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