Do these devices really boost AC unit efficiency?
An air conditioner mister is a very straightforward device. It releases a fine spray of mist around your central air conditioner condenser unit (the outdoor unit) while the compressor fan is running. This fine spray of mist evaporates into water vapor as it moves through the air.
As you may have learned in high school physics, evaporation is a cooling process – the state change from liquid to gas (water droplits to water vapor) increases the energy level in the water vapor molecules, by absorbing some of the heat energy in the surrounding air. By misting the air around your condenser unit, you cool the surrounding air. The cooler the air going into the condenser unit, the less energy the condenser requires to pump heat out of the air conditioning system, and the less you pay in electricity bills.
You may have seen a garden mist cooler on display outside your local building center. My Home Depot has one on display all summer, showering a fine mist into the air and lowering the temperature around the entranceway. You could just place one of these water misters beside your air conditioning condenser unit and let it run all the time while the unit is on. One problem with that is that you’ll waste quite a bit of water, since much of the time that the mist flows from the mister, the condenser fan is not running so the cooling effect of the evaporating water provides no energy savings. Another is that some of that water won’t evaporate, and the longer the mister runs, the more water you’ll have all over the ground around your condenser unit.
In the early 2000’s, a product called the Mist’N’Save (made by a company called Cool’N’Save) became a popular device designed to do exactly this: whenever the condenser unit fan would start running, the Mist-N-Save would let out a fine mist around the sides of the outdoor unit to cool the air that is then drawn into the unit. The Mist-N-Save would connect to an outdoor water supply by the provided hose, and its misters would connect around the top edges of the condenser unit.
The misting switch was activated by a paddle that sat atop the condenser unit. When the condenser unit fan goes on (indicating the active part of the cooling cycle), the airflow from the fan causes the paddle to rise from its horizontal resting position, which turns on the flow of water to the misters. When the fan shuts off at the end of the active part of the cooling cycle, the paddle falls back to its resting position, shutting off the supply of water.
The following video provides a good illustration of how the Mist-N-Save works. Take a look now if you like – then read on to find out whether this system is the right system for you, or whether a DIY system might be within your technical ability.
Are these devices really worth the money?
When the Mist’N’Save was available – at around $70 per unit – the equivalent parts purchased separately probably amounted to about $30-40, so you were paying for the convenience of a bundled system with clear installation instructions. For a tinkerer, building an air conditioner mister system from scratch might well be a more enjoyable and more affordable undertaking.
Reviews from customers who did buy the Air Conditioner Mister indicated a couple of quality concerns: The nozzles can get clogged (especially problematic if you have hard water), and the paddle attachment appeared to be flimsy, leading to either failure to activate, or the paddle breaking loose.
More importantly, I didn’t find any scientific measurement showing that the Mist’N’Save actually works to save you energy (if it does the savings are likely minimal), and there are risks associated with continually spraying water on your air conditioner compressor. A compressor is designed to withstand occasional rain, but not a continuous fine mist; the continuous fine mist will evaporate leaving whatever minerals and other residue were in the water. Over time this build-up may become problematic in bearings, may add to the weight of fan blades, and may shorten the life if your air conditioner. This may be why you don’t find the Mist’N’Save offered for sale any more. (As of October 2023, their website appears to be under maintenance.)
My suggestion if you’re trying to find ways to save on cooling is to use proven techniques, such as better sealing of your home, using high quality window coverings that shade out sunlight, using window fans overnight when the outdoor air is cooler, and saving up for an upgrade to a high efficiency air conditioner. It’s not worth risking thousands of dollars of investment on an air conditioner with a cheap device or home-grown contraption that might ruin your equipment, and for limited (or no measurable) benefit.
An inexpensive 24 volt automatic sprinkler system valve would be easy to attach to the 24 volt relay on the ac unit, instead of using the paddle and manual valve.