Is Evaporative Cooling the Answer to High Air Conditioning Costs?

37 Comments Read/write comments


This time of year, air conditioners are running like mad to keep people cool in their homes. Here in Atlanta, we've had a couple of weeks of hot, muggy weather, with a little break on Sunday. Now we're heading back to the mid-90s with high dew points again. As a result, some people are starting to dread those air conditioning bills arriving and wondering what they can do to save energy. Is the Kickstarter-funded Mistbox the answer?

Using evaporative cooling

Evaporative cooling is great. It's how our bodies keep cool when it's hot. We sweat. The sweat evaporates and that phase change carries heat away from our bodies because it takes heat to turn liquid water into water vapor.

When you spray a mist of liquid water into the air, the tiny droplets of liquid can evaporate into individual water vapor molecules. Again, it takes heat to do that, and in this case the heat comes from the air. The result is cooler air. If you live in a dry climate, you may even use this principle to cool the air in your home with a device called a swamp cooler.

Now, let's take that principle and apply it to your air conditioner. I've explained how air conditioners work in a previous article, collecting heat from inside the house and dumping it to the air outside the house. Because the outdoor air is hot, the compressor has to make the refrigerant that carries that heat even hotter than the outdoor air because heat always flows from hot to cold, or in this case, hotter to hot.

If you can evaporate water into the air right before it gets pulled into the condensing unit, you can decrease the temperature of the air and thus increase the capacity and efficiency of your air conditioner. Then, instead of dumping that heat from hotter to hot, you're dumping it from hotter to not-quite-as-hot, and that's easier to do.

Got it?

The Mistbox

The photo at the top, from the Mistbox website, shows the setup for this new device. You connect the box to your outdoor faucet with the hose supplied. The water goes into the box and gets turned into a mist on all four sides of the condenser. As the liquid vaporizes, the air cools and helps the air conditioner run better.

It's got controls to determine when it operates, and here's what their website says about them: "Temperature. Sound. Magnetic field. Mistbox senses these to mist at exactly the right time." 

The Mistbox connects to wifi as well, and naturally there's an app so you can track how it's doing on your smartphone.

The Mistbox folks think highly of their product, as attested to by these quotes from their website:

"Mistbox is computer optimized and its algorithm continually improves to maximize savings."

"Mistbox is smart. It learns from its environment – your unique climate & AC system."

"Mistbox starts saving you money from the very first day. It usually pays for itself in the first full season of use, and you could save thousands in cooling costs over its lifetime."

But is it really all they make it out to be?

Why you should be skeptical

The principle of evaporative cooling is sound. The application is a bit tricky, though. Here are a few reasons why you may not get the kind of benefits they promise.

1. What about outdoor humidity?

The computer optimization and continually improving algorithm are all well and good, but why doesn't their website give a single hint that they look at the outdoor humidity, too? The website says the controls are based on temperature, sound, and magnetic field. In the app settings, the user chooses a temperature trigger but there's nothing for humidity.

For that mist to evaporate in the air, however, the wet bulb temperature has to be lower than the dry bulb temperature. Ideally, the two numbers are significantly different. The closer those two numbers are to each other, the more of that Mistbox water will be wasted. In other words, the Mistbox should work great in a dry climate but not as well when the outdoor humidity is high.

2. What about indoor humidity?

The Mistbox website touts the energy-saving benefit of less runtime. In a humid climate, that's not necessarily a good thing. Removing humidity from the indoor depends on how much the air conditioner runs, so when you reduce the runtime, you may end up with high indoor humidity. (Caution: Putting the AC thermostat in the fan-on position will make it worse.) How much sense would it make to save a little bit of money on your air conditioning costs only to have to install and run a dehumidifier that you didn't need before?

3. Can it really pay for itself in one season?

The cost of the Mistbox is $450. They say, "Most Mistbox owners save between 20-40% on their cooling costs." Let's say you save 30%. To pay for the Mistbox and the filter you need to change annually, you'd have to spend $1600 on air conditioning. Even with a 10 SEER, 3 ton (right-sized) air conditioner in Houston, Texas, the annual cost is only a bit over $600 (assuming $0.12/kWh). If you go up to a 5 ton AC, the annual cost is barely over $1000. You'd have to go to a place like Phoenix to spend $1600 per year on cooling with a single air conditioner.

To make it less hypothetical, I did some simple payback calculations for my condo. Over the last three summers, we've averaged $212 in air conditioning costs. (That's not per month. That's for the whole cooling season, May through October.) If I saved 40% with the Mistbox, it would take 5.3 years to pay for itself. At 20% savings, the payback doubles to 10.6 years.

But wait! That doesn't account for the annual filter replacements or the cost of the water used. (See next section for my discussion of water usage.) The filter costs $30. My conservative estimate for the cost of water, using my local water rate of $2.16 per 1000 gallons, is $50 per cooling season. (Yes, really!) That puts the simple payback at:

27.5 yrs
13.8 yrs

Those numbers don't look so good for a piece of equipment whose expected lifetime is 15 years.  (Yes, they’d look a little better if I used the $315 cost they quote on the website. They arrived at that cost by assuming a 30% tax credit because the device’s electronics are powered by a small photovoltaic module. I think it’s an iffy proposition to try to take that tax credit for the full $450 because the tax credit is for solar electric generation, which is certainly a small part of the cost of the Mistbox. The proof of that is that they also sell an auxiliary solar panel for $30.)

Oh, and don't forget, your payback doubles if you have to buy two Mistboxes and triples if you need three.

4. What about crud?

The Mistbox comes with a filter to help reduce the effects of scale building up on the air conditioner coil from minerals and crud in the water. It probably works fine, at least when the fitler is clean. But what about dirt picked up by the mist from the outdoor air? If someone is cutting the grass or blowing leaves near the AC while the Mistbox is running, you're gonna get crud in the coil. The more crud builds up on the coil, the lower the air flow will be and the less efficient the AC becomes.

5. How will it effect refrigerant pressure?

By adding extra cooling to the condensing coil, it's unlikely but possible that you could reduce the pressure of the refrigerant to the point where you end up with less cooling indoors, not more. The other four factors above are more likely to cause trouble, but this one could be an issue for some folks.

How much water does the Mistbox use?

So how did I come up with $50 for the cost of water over a whole cooling season? I didn't do the calculations myself. I asked an engineer who works for one of the large HVAC manufacturing companies about this device, and here's what he told me:

"Assume that we have a 3-ton AC unit with an outdoor airflow rate of about 2800 cfm, which is typical for a 14 SEER system.  Let’s also assume that it is 95 F dry-bulb and 75 F wet-bulb temperature outdoors, which is a common design condition for air conditioners.  If we look at the psych chart, we can see that if we spray water in front of the coil, we can theoretically cool that air to 75 F dry-bulb by following a constant enthalpy or wet-bulb line to the saturation curve.  If you do a water vapor mass balance on this evaporative cooling process, you can show that you will need about 32 gallons per hour of water for this process.  That is not an insignificant amount of water, but lowering the inlet temperature by 20 F is quite a benefit.  Is it worth it?"

So, under those conditions, the Mistbox would use about 32 gallons per hour. On a design day here in Atlanta, the air conditioner in my condo runs about 5 hours. Assuming the Mistbox isn't on the whole time, my conservative guess is that it would run 3 hours per day, using about 100 gallons per day. That's 3000 gallons per month or about 15,000 gallons per cooling season. At our local rate of $2.16 per 1000 gallons, the cost would be $32.40 for the season.

But any water that doesn't completely vaporize and passes through the coil is wasted. And many air conditioners have greater runtime than ours does. Also, water rates vary and are generally increasing, so I think $50 per year is a reasonable, conservative estimate for water use by the Mistbox.

It would be nice if Mistbox actually published their water use data. I know they must have some, and the fact that they don't publish it makes me think my numbers may be too low.

Is it worth it?

My first thought when Bill Spohn alerted me to this product was about Southface's attempt to do this a few years ago. David Wasserman is an engineer at Southface who oversaw the project, so I asked him for an update. "The water misters at the Eco-office," he wrote to me, "did not operate much since we could never get the controls to work properly. They have been removed."

The anonymous engineer I mentioned above wrote about the maintenance aspect: "Cooling towers and evaporative condensers are common on commercial and industrial systems where trained service people are available to maintain them. Homeowners are generally not willing or able to perform or pay for this type of maintenance."

The HVAC industry is ripe for disruption, but I haven't seen anything yet that's going to shake things up. The Mistbox, like the smart-vents that allow occupants to do their own zoning, is bound to be another disappointment to most homeowners. If you've got high bills from using a central air conditioner, your best bet is to fix the problems with it, not buy some Kickstarter gadget that has its own mobile app.


Related Articles

Can You Save Money by Closing HVAC Vents in Unused Rooms?

Dew Point — A More Meaningful Measure of Humidity?

Psychrometrics, Part 2 - The Quantities in the Chart

The Magic of Cold, Part 2 - Intermediate Air Conditioning Principles


NOTE: Comments are moderated. Your comment will not appear below until approved.


Richard Ahlquist
Jun 30 2015 - 2:20pm

Allison, this touches on a idea I had at one point after watching a show on geothermal systems. Putting a fully functional geothermal system on a home is a huge retrofit. Would it be possible to increase efficiency of existing systems with an add on geothermal unit. Picture say 2 or 3 u shaped pipes put 50 feet deep close to the existing condenser, connected in series. Say spread over a 10 foot radius. The at the existing condenser you surround it on 3 sides with a radiator. Something tailored to not limit the existing condenser airflow, connected via pump to the in ground U pipes.

So now you have a closed loop system that pumps coolant around through the pipes in the ground and a radiator that that liquid passes through say 3-4" from the condenser, so that when the condenser fan is pulling air through the condenser its also pulling some of that source air through the geothermal radiator.

Would a solution like that be efficient enough to be a viable way to cut costs and boost efficiency?

Richard P
Jun 30 2015 - 2:47pm

Another major problem would be mineral build up on the outdoor unit coils. This does not happen with rainwater but would quickly become a problem with mineral laden city or well water. Add chlorine to the mix and you have the potential for corrosion of the metal enclosure, exposed contact relays and circuit boards. Commercial "chillers" are generally much less efficient than residential units but need huge btu outputs regardless of costs.

Curt Kinder
Jun 30 2015 - 3:30pm

"Commercial "chillers" are generally much less efficient than residential units"

Where do you get that? Data please...

Ray Austin
Jun 30 2015 - 2:56pm

How you figure cost to run an AC in Houston varies like the amount of rain we get on an annual basis.

In Texas the utility companies are deregulated and the amount you pay for energy depends on how much you use, when you contracted your price and so on.

The other thing that fluctuates is what temperature people actually set the thermostat at. While design temperature for Houston area is 75 indoor at 95 degree outdoor in many cases people either set indoor lower or higher. Obviously for those that run their systems lower than 75 degree set point will pay more.

If you use more than 2000 KWH (kilo watt hours) you most likely will get a volume discount from the utility company and this rate can vary from 7.5 cents to maybe 9 cents per KWH depending on when you sign up for the contract.

Those that use less than 1000 KWH will pay a higher rate. Currently anywhere from 11 cents to 15 or 16 cents per KWH depending again on when you sign up.

So determining costs on a 10 SEER 3 ton AC (properly sized) set to 75 degree design temp your cost at 11 cents per KWH would be $124 per month or $868 per season (7 months)

If you used 2000 KWH or more and got a volume rate at 7.5 cents per KWH this 3 Ton 10 SEER AC would cost $85 per month or $595 for the season (7 months).

A 5 ton 10 SEER runs $207 per month at 11 cents per KWH or $1449 a season.

A 5 Ton 10 SEER runs $141 per month at 7.5 cents per KWH or $987 a season.

These figures are based on Houston area taking into consideration current utility rates at different volumes of energy consumed by the consumer.

I have a 18 SEER 3 Ton zoned system at my house. My total light bill last month was $48.23. You can view my electric bills of years past in videos that I have created that are located on my website under the "AC Tips" tab.

I don't use any water cooling or other gimmicks. Design, Design, Design.
(No two are alike... and there you go.)

Mark Johnson
Jun 30 2015 - 2:56pm

This sounds a whole lot like the "spray the outdoor coil" idea which has been repeatedly discussed on HVAC-TALK. Professionals see all kinds of problems trying to do that.

Mac Sheldon
Jun 30 2015 - 2:58pm

So, years ago we worked on two projects within a few months of each other. One was in Hong Kong and the other outside Las Vegas. Both used a technology called Indirect/Direct Evaporative Cooling. Assuming both loads were the same, which would you predict to be the more cost effective?

If you've been to Hong Kong you know there are three seasons; hot, hotter and hottest, and all are muggy, so why did our system work as well there as it did in the hot, dry desert climate? Because we weren't using outside air for our scavenged air supply.

We used the system as the building ventilation exhaust fan and brought otherwise wasted 75°F/50%RH air through a water mist bank then sent it across a sensible heat exchanger. The outdoor air used for make-up of the building ventilation air crossed the heat exchanger and because we had a 15-20° temperature depression we effectively condensed water on the coil thus dehumidifying the make-up air before it was diluted into the return air stream.

Mind you we were only dealing with about 5000 cfm on a massive air handling system, but significant energy savings and equipment capacity reduction was possible due to effective building design which allowed central discharge of exhaust and simple evaporative cooling technology.

Your mister for the ODU might not be the best use of water, but there are lots of other applications where either direct or indirect evaporative cooling can be very useful. Hope the creative juices are flowing....

Maybe we could rig up a mist system for the April 1st attic AC system?

Rachel Kingsley
Jun 30 2015 - 4:49pm

Yes, I like that idea! And why not use a Thermosyphoning Air Panel (TAP) to pre-treat air that is going to an HRV for heating? True, it would have more benefits during the day, but that's still a plus, right?

Leigha Dickens
Jun 30 2015 - 3:00pm

So...not great for a humid climate, and yet in a dry climate, in conscionable to put in something that uses that much water? That's like a bathtub's worth of water an hour.

Allison A Bailes III
Jun 30 2015 - 3:45pm

Yeah, I'd say that's not a good tradeoff.

Curt Kinder
Jun 30 2015 - 3:27pm

A quick look at product data for a popular 3 ton 15 SEER heat pump reveals a 27% improvement in EER in response to reducing condenser entering drybulb temperature from 95 to 75.

The improvement arises from about a 10% increase in capacity and about a 20% reduction in power usage (rounded for discussion)

That's a best case improvement for a misting device or any other scheme to use evaporation of supplemental water to reduce inlet air temperatures into an air cooled condenser from dry bulb to wet bulb temps.

By "best case" I refer to the fact that for most east climates a 95*F drybulb is about as bad as it gets for any length of time. At other times of day during which the wet bulb / dry bulb temperature delta is lower, the benefit would be less.

Given that figure I'd expect the improvement to be more on the order of 15% on an annual basis. Using city water for this purpose would be a wash at best.

Under certain circumstances the economics could be a bit better:

1) home served by well water costing much less per gallon

2) Older lower SEER system that exhibits markedly higher efficiency improvement from reduced condenser air inlet temperature.

3) Systems in hot arid climates with a much higher delta between outdoor drybulb and outdoor wetbulb. Beware higher typical water costs in such climates.

I've tinkered a bit with this principle, though not using a $450 internet-enabled doodad... I brought Artesian 71*F well water to a 3 ton SEER 10heat pump and paralleled water flow control using a 24 Volt lawn irrigation valve with the 24 Volt compressor contactor control signal. It worked, dropping compressor power by several amps.

Eventually I did away with the condenser fan motor, defrost control, and air cooled condenser, replacing all with a McGyver-patented home built coaxial heat exchanger consisting of 50' of 1/2" copper tubing crammed into 50' of 1" OD rubber hose rigged for counterflow.

While at it I added a separate desuperheater coil for domestic hot water.

The scheme worked, halving HVAC and hot water costs, but it was nowhere near suitable for prime time. In fact it most resembled a small scrap yard, marveling onlookers when it roared to life.

The point about loss of capacity owing to decreased high side pressure is well taken, particularly with respect to systems with fixed metering devices (piston or capillary tubing). Systems with thermostatic expansion valves should be able to compensate.

It's hard to imagine what benefit the mister's expensive cartridge filter would convey. It almost certainly WON'T remove hardness molecules.

w d
Jun 30 2015 - 3:27pm


I love your real-life economic analyses. The Mistbox is based on good science but the devil (if there is one) is to be found in the details. Nice job of exposing some of those details.

We live in Climate Zone 5. We have been saving just under 1000 kwh/yr in electricity for air conditioning for several years now by reducing the radiant heating coming through our 300 ft2 of windows. There are a number of ways to achieve this and the most effective ways involve blocking the infrared light before it passes through the window. Products like solar screens and In'Filtrate come to mind. Our experience is with Solar Grates which block most of the heat while letting virtually all of the visible light in. The operating and maintenance costs have been nil. The house stays cool during the day and the a/c runs at night to return to the temperature set point and relieve any humidity issues. The over-night electric rates are lower and this creates additional savings and keeps our 'peak demand' low during the hot weather when the demand charges are assessed.

Rather than unnecessarily heating the house and then attempting to cool it off with ever more efficient air conditioners or with "free" electricity from solar panels, it's proven more effective for us to merely reduce the a/c load in the first place.

AERC of the WCMA is evaluating the effectiveness and economics of various window attachments in conjunction with LBNL. So far it's unclear to me which designs will be tested but I have confidence LBNL will see that all viable designs compete. And, they should beware that Bailes is watching.


w d

Ken Riead
Jun 30 2015 - 3:43pm

This is not a new idea by any means. I have several prototypes that tried to do this very thing by at least 3 different inventors and they all ultimately did not work. One triggered off ambient temperature, one worked with a humidistat and one used refrigerant pressure inside a sealed tube to control the water flow. We tested these in the Midwestern climates with mostly unsatisfactory results.

The biggest fear that homeowners had was for an outside pipe or hose to burst while they were away at work or vacation. A lot of water can flow through an open pipe in a very short period of time with the potential of flooding a basement or neighbor's yard. The other major problem was that the proposed savings just were not there.

Also, some heat pump and A/C manufacturers had heartburn over these devices for various reasons and a few were considering voiding warranties if these devices were installed. Outside condensers are not as waterproof as you might think, especially if they are kept soaking wet most of the cooling season. Condenser fan motors are shielded from rain at the top but are open for cooling purposes on the sides and/or bottom, which is where the mist/spray will come from these devices.

All in all, the concept seems sound but the reality can be quite different.

Cameron Taylor
Jun 30 2015 - 4:18pm

I suppose if folks want to start treating their outdoor condensing units like cooling towers, they should expect the rust, delaminating protective coatings, and scale that every cooling tower I've ever worked on, even with great water treatment, has shown over time.

Why don't we get into a frame of mind (instead of toying around with misters, "smart vents", and other half-baked ideas) that moves away from throwing lots of energy at a crappy building envelope to achieve human comfort and more toward a cooperative effort between building envelope and HVAC systems to provide EXCELLENT human comfort at much reduced carbon and financial footprint levels?

Wynn White
Jun 30 2015 - 5:13pm


Some communities (like ours) bill wastewater based upon the water usage--and the wastewater portion of the bill dwarfs the water bill. I agree with Cameron Taylor--let's fix the building enclosure!

Jun 30 2015 - 5:37pm

I already assumed the answer about any form of evaporative cooling in a humid environment. However, my wife and I recently visited our daughter in Florida and went to an open-air (roofed) Italian market for lunch. They had about a dozen or so 4-bladed caged fans with a water mister coming out the center. The water mist was completely evaporated in the fan blast so there was no "overspray". It was surprising how comfortable this environment was given the 90/90 typical Florida weather. But this was outdoors...

Bill Spohn
Jun 30 2015 - 11:07pm


Jim Gunshinan
Jul 1 2015 - 1:33pm

What a bunch of myth busters you all are. I am so proud of our community of energy geeks.

Allison, you had me questioning the device when you mentioned "magnetic field". It got me thinking of putting aluminum foil around the outside AC outside unit to guard against AC control by aliens or Big Brother. Than your analysis and the excellent comments from others just sank the idea all together. Bill, touche! Quite the "Mist-Buster".

Jul 16 2015 - 12:01pm

Easy Solution: Install a geothermal heat pump instead and worry not about the outdoor ambient conditions affecting the efficiency of your condenser coil.

Jul 28 2016 - 1:09pm

Is there a cost effective way to do that? The last time I had a company come out to price it they told me it would cost between $15-$20,000.

Harsh Dodhiaa
Feb 12 2016 - 8:00am

I do not totally agree with you as Evaporative cooling do not work well in humid climates. Evaporative cooling works best in dry climates. Too much moisture in the outside air makes the system work inefficiently. Moisture can build up, causing condensation and corrosion.

They need some attention. The moisture pads in an evaporative cooler must be kept moist with water. If they dry out the cooler will not work well. This can be a problem in desert areas where the dry air will take a toll on the pads.

Josh Todd
Jun 13 2016 - 1:11pm

IF it did provide a 30% savings for me, it actually WOULD pay for itself in a season. I live in Charleston SC and my electric bill starts to ramp up above 300 in April. At its peak, in July and August, my electric bill is $500+. I have a new-ish home built in 2009 with all the energy saving windows and all that jazz and my AC is in proper order (or so say the HVAC guys every time I call them because it's so hot in my house) but because I have a 2600 sqft house with 22 15sqft windows and the sun hits my house the ENTIRE day because of the way it faces, in the hot months it runs ALL day and can't keep it cool. My first floor is fine, but my second floor will get up to 76, even when i set the 3rd floor at 80 (it's a zoned system). I like to keep my house around 70, so bill aside, if the mistbox would even give me a few more degrees of coolness, i would be willing to pay twice what it costs. The question I have is WILL it shave a few degrees off in the humid southeast? If it does reduce my bill on top of that, great, but I doubt it would as my AC would still be running nonstop since I'd never be able to get it down to 70

Matt Kessler
Jun 15 2016 - 11:00am

I have recently installed the Mistbox, and I will hold my judgement on its effectiveness until I start looking at my energy usage. However, I can say that the App has some major flaws. If your network requires MAC authentication they cannot give you the MAC address of the Mistbox, so you have to make your network insecure just to get this thing on it. Assuming you were willing to do that, there are issues getting on the network as well. I live in a dense city area where there are many WiFi networks. The Mistbox app picks 8 to show, and you cannot scroll to find additional ones or manually type in the network name. So if your network isn't on there, you are SOL.

I've emailed tech support on both these issues, and they acknowledged there were no solutions.


Rangaswamy Jaganathan
Jun 25 2016 - 6:46pm

I live in south of India ( Coimbatore) and the mist box idea is completely new to us., in the past we were using air conditioners hardly for 2-3 months an year and now we have started using them for more than 5-6 months .

Keith in Las Vegas
Jun 29 2016 - 6:05pm

The MistBox people recently answered the question for me of how much water the MistBox system will use, since it is limited by the mister jets in the system. I was told 7 gallons per hour.

Using your estimate that the A/C runs 3 hrs per day, that's 21 gallons a day x 30 days is 630 gallons x 6 months for the season would be 3780 gallons at $2.16/1000gals. That's $8.16 for the season.

Keith in Las Vegas
Jun 29 2016 - 6:16pm

Matt, I installed a MistBox last week and went through this same issue. What I ended up doing was unplugging and moving my wireless router as close as I could get to the MistBox unit (about 10' away). It seems that it chooses the networks to display by signal strength. Once I had the router close enough, I could chose it and set the settings. Then I moved the router back and it all seems to work fine.

I don't use MAC address filtering, but once the MistBox connects to the router, you should be able to find the MAC address in the DHCP clients table. Then you have the address to add to your filter and re-enable your filtering.

Hope this helps.

John in Scottsdale, Arizona
Jul 15 2016 - 2:29pm

I installed a mistbox on one of our ac units yesterday. I also have a solaredge solar panel inverter which gives me minute-by-minute consumption data.

I was hoping to see good results, given our dry desert heat.

HOWEVER, instead of improving things, it seemed that the ac unit struggled last night to get down to our normal night time set temperature. All night it didn't seem to get lower than 75 degrees, even though our set point was 72. (I didn't have this problem before mistbox). Of course, this meant the unit ran continually all night.

Anyone know what could be going on? Is it the fact that our tap/hose water is relatively warm? Perhaps it's actually HEATING the coil instead of cooling? (our outside temps are 100+ through 9pm)

Or... ?


Keith in Las Vegas
Jul 15 2016 - 8:52pm

How much did your A/C run last Sunday? According to, last night (Thursday night) and last Sunday both were 88* as measured low. Compare those two nights, since they were both about the same low temp.

John in Scottsdale, Arizona
Jul 16 2016 - 7:11pm

Keith - Thanks for your note. Well, unfortunately, there was a problem with my consumption meter from late Sunday through late Monday, so I don't have data to compare.

That being said, I'm having an AC repair person come out on Monday to do a tune up of the system. My evaporator coils (inside) had ice on them last night... so I cleaned them and replaced vent filters. I also turned off mistbox. Still have the same problem with the AC - it's constantly running and unable to keep set temperature.

I'm thinking maybe there's a problem with the refrigerant in the system... and it just happened to show itself the day I installed the mistbox.

I will let everyone know what the AC repair guy says next week.

Ray Austin
Jul 16 2016 - 8:05pm


The mist box probably doesn't use all that much water as it cycles and so if there are long spans of time in which no house water is used that water sits in the pipes for a long time.

Given the heat of your climate, the heat of the water for the first so many feet in the pipe may get heated to ambient temps depending on how your plumbing comes in the house. *THIS IS JUST A GUESS* I am not there. ;-) Take a temp reading of the water.

The other issue may be that the unit has sprung a refrigerant leak just as you were hooking up the mist box or you caused the leak in how you put the mist box on your unit. (refrigerant piping is extremely fragile these days.)

PS: My light bill was $64.36 last month live near Katy, Texas (suburb of Houston) 76 all day long, 74 at night. House is zoned and I only run the zone I am using, the rest are off.

I don't use mist box or any solar type equipment. Just high efficient 2 stage 18 SEER AC, zoned with 4 zones, the equipment and duct work in the attic. Lowest bill was $38 and some change.

John in Scottsdale, Arizona
Jul 17 2016 - 7:19pm

Ray - Thanks for your note.

I've disconnected the mistbox until the AC is fixed (right now, it's putting out air that is only ~4 degrees cooler than the room). So, the problem is not the mistbox, per se. (though good point about the heated water in the hose - though when I felt the misted water, it seemed cooler)

Given that the problem started at the same time as I installed the mistbox, there's a chance I may have broken the refrigerant line. I'll ask the AC repair guy to look at it tomorrow.

This is on a 15-year-old 12 seer unit... and our monthly electric bill during July/August can be $600+. (I've installed solar, so it's half of that... but still high).

Jul 25 2016 - 2:57pm

If you put a hosepipe out in the sun, but are only flowing3-4 gallons per hour, the water that comes out of the hosepipe will be hot. There are some unstated drawbacks and technical challenges for a system like this.

Ray Austin
Jul 26 2016 - 7:23am

John, That AC is pretty old for such a hot climate as yours. It's probably an old age problem rather than something you caused.

That light bill is ridiculous, but being in a climate such as yours I really have no idea how low you could expect to get your utilities down.

What is your rate per hour from your Utility provider? And is the market deregulated over there?

Basically how I've achieved such low utility bills is by cutting waste after installing the most efficient system back in the day it was new.

I've also converted nearly all lighting to LED bulbs. I anticipate a bump up in utilities this year that so far hasn't materialized. This was due to repairs to this aging system of mine and using different parts just to see the effects on efficiency of the system.

Being that you're in Arizona I don't know what you could achieve with a system like mine over there, completely different market from which I live, but I think you should be able to 'maybe' get it below $200 a month with the right ideas toward efficiency and cutting waste.

Waste cutting is correlated to reducing or minimizing the amount of conditioned air to areas of the home that are unoccupied. In other words the amount you can cut really depends on how full your home is with people that want that conditioned air.

Josh Todd
Jul 26 2016 - 9:57am

76 all day??? I'd die. I keep my house at 70 during the day and 67 at night (sometimes when it's super hot out, I leave it at 69 at night). I digress. What I wanted to ask about was your zones. I've been told that when a zone is shut off, it puts extra pressure (like physical air pressure) on the other zone(s) and can damage the unit. I've also been told that zones shouldn't be more than 5 degrees apart, though if you're keeping the zone you're active in at 76, that may not be a problem for you. Do you actually shut your other zones off, or just set them high? Do you know if what I've heard is true?

Matt Kessler
Jul 26 2016 - 10:02am


These are good ideas for sure.

Now that I've been running this system for over a month, I have some preliminary results. The avg temp this monthly billing cycle last year was 71 vs. 74 this year. Nonetheless, my kwH dropped 10%. It should be noted that I added a medium sized wine refrigerator as well. If the temp had been the same, and I had not added the 'fridge, a 30% reduction that they claim doesn't seem unreasonable.

I'll also add that the mister is a bit finicky. Sometimes it starts spraying for no apparent reason and won't spray sometimes when the AC is on. It may be due to the proximity to another unit that is confusing it.


John in Scottsdale
Jul 26 2016 - 11:03am

All - Here's an update to my situation:

Well, I *think* it was a coincidence (see note below), but my 15 year old 12 seer unit died the same day I hooked up a mistbox. The AC repair guy said that I had a Freon leak in my evaporative coil and that it was unfixable (they don't make Freon/R-22 equipment any more, and it's illegal to add Freon to a system with a known leak).

I have since replaced it with a Trane XV20i (variable speed), so I expect much better efficiency (I'm already seeing it in my consumption monitoring - about 40-50KwH less per day = 220 down to 170 KwH per day).

So, now I'm trying to figure out what to do with the mistbox. I could add it to my older 13 seer unit used in the front of the house... Or I may simply sell it.

Note: A theory I've seen is that the mistbox actually cooled and slowed the (already low) Freon in my system, which caused the evaporator coil to freeze. That's why it happened all at the same time. I don't think there's any validity to the mistbox CAUSING the leak...

Ray Austin
Jul 26 2016 - 2:01pm


You have to realize this board serves many different climates and many different tastes as to what is comfortable. Design temperature for my area is 75 with 95 OAT.

Now if you put 76 at 70% humidity it wouldn't be very comfortable. It can be easily upwards of 80% humidity where I live outdoors. My system operates the zone I am in at 76 all day with humidity readings of 50-55% typically, with swings to closer to 50% during the night time hours when the system drops to 74 degree setting.

Zone systems are a nightmare if not done properly. This is what I do for a living so what I have done isn't necessarily what you will be able to do. Let alone find someone capable of doing it properly. Zone systems are complicated beast... and many that have come across this board rarely agree with what I say about these things. (That's ok with me.) I'm just here to show what is possible, but most people only want to consider just one thing rather than looking at the whole system. When the budget for HVAC starts hitting the Teen's of thousands people become weak in the knees and somehow believe there is some silver lining out there that will cure all their ills for virtually nothing.

The points you raise of air pressure are valid ones. This comes down to design of the system and in some cases the design of the home may decide if it's a good idea or not. Zone systems are typically not a system made for all. The design as well as how you intend to use it can tell someone what they want to know.

The problem is many times when you're shopping for such systems you are met with a salesman and not a true servicer of the equipment. The salesman gets paid to sell and after that they are long gone. So 'most' of them don't care whether it really works out for you or even the next guy. They are only there to sell and any way they can do it.

It does work, when designed properly. What is a design? The invisible parts of your HVAC system that can determine efficiency, performance and the ability to work properly under the load in which it was designed.

The problem is when people look at an HVAC system they see a box on the outside and a box or two on the inside. Realistically 20 years ago this is about what it amounted to. But now that energy is so expensive you're either forced to change your thinking or become a slave to your electricity provider.