Saving Energy With an Evaporative Cooler

musingsheader image

Saving Energy With an Evaporative Cooler

In a dry climate, lowering the temperature of your indoor air with a swamp cooler uses much less electricity than an air conditioner

Posted on Jul 17 2015 by Martin Holladay
prime

Evaporative coolers are appliances used to cool indoor air. Evaporative coolers use much less energy than air conditioners, but they can’t cool indoor air effectively in all weather conditions.

Sometimes called swamp coolers, evaporative coolers lower the temperature of an airstream by passing the air through a moistened pad. The moving air causes water to evaporate off the pad. Evaporation requires energy (heat); in other words, the process of evaporation (a phase change process) removes heat from the air. The air exits the appliance at a lower temperature, but with more moisture, than when it entered.

To keep the pads in an evaporative cooler damp, tubing with nozzles delivers water to the top of the pads. The pads are usually made of a material called excelsior — basically, aspen wood shavings. Water trickles down the pads and drips off the bottom of the pads; the base of the metal cabinet includes a sump where the water collects. A recirculating pump pulls water from the sump and delivers it to back the top of the pads. The sump is equipped with a float valve similar to the valve in a toilet tank; the float valve opens and adds water to the sump when the water level drops due to evaporation.

These devices only work in a dry climate

Evaporative coolers work best when the outdoor relative humidity (the wet-bulb temperature) is low. If the outdoor temperature is 90ºF at 10% RH, the wet-bulb temperature is only 58ºF — good conditions for operation of an evaporative cooler. (For more information on wet-bulb temperatures, see How to Use the Psychrometric Chart.)

The bigger the difference between the dry-bulb and wet-bulb temperatures, the more effectively an evaporative cooler will operate. Most evaporative coolers can’t lower the temperature of the air below the wet-bulb temperature.

The bottom line is that evaporative coolers work well in dry climates — that is, climates where the outdoor wet-bulb temperature exceeds 70°F for no more than 1% of the cooling season (see Image #3, below). In the U.S., that generally means west of the Rocky Mountains — although a few locations east of the Rockies are dry enough for evaporative coolers, and some locations in the Far West have a few hot, humid months when evaporative coolers don’t work.

In a humid location like Florida or North Carolina, the air entering an evaporative cooler is already loaded with moisture. That means that there is no opportunity for evaporation from the damp pad. No evaporation means no potential for cooling.

Even in a favorable climate, an evaporative cooler won’t produce air as cool as the air that comes out of an air conditioner. However, most evaporative cooling system are designed for high air flow rates — 3,000 or even 8,000 cfm. The high volume of cool air produced by an evaporative cooler, and the breeze generated by the system’s operation, help make up for the fact that the air isn’t as cool as the air coming out of an air conditioner.

Evaporative coolers raise the relative humidity (RH) of the indoor air. Since these appliances are only used in dry climates, however, the increased interior RH usually doesn’t cause any problems.

If the outdoor air is dry, an evaporative cooler can lower the air temperature by between 15 F° and 40 F°; a 20 F° reduction in the air temperature is typical (see Image #4, below). Most homeowners will be satisfied if the evaporative cooler can produce air that is 70°F or colder. If the unit produces air in the low 70s, that’s better than nothing. But when the air coming out of an evaporative cooler is 75°F or warmer, few homeowners will be satisfied.

Sizing an evaporative cooler

There are three basic types of evaporative cooling systems:

  • The simplest system is a window-mounted unit that blows air into one room.
  • A more effective approach is to install a roof-mounted unit that blows air downwards through a grille mounted in the ceiling of a central hallway.
  • A third type of system distributes cool air through ducts. This type of evaporative cooler can be mounted on a roof, a wall, or the ground. Because evaporative coolers have higher cfm ratings than air conditioners, the ducts used for an evaporative cooler must (ideally) be larger than the ducts used for an air conditioner.

Evaporative coolers come in many sizes. The units are rated by their airflow rate in cfm. Various rules of thumb for sizing evaporative coolers have been proposed:

  • Choose an evaporative cooler that can provide between 20 and 40 air changes per hour. (Using this rule of thumb requires you to calculate the volume of the house. Divide the volume of the house by 60 — the number of minutes in an hour — to determine the number of cfm required for 1 air change per hour. For example, if the house volume is 12,000 cubic feet, one air change per hour requires 200 cfm; 20 air changes per hour requires 4,000 cfm; and 40 air changes per hour requires 8,000 cfm.)
  • A variation on the above rule of thumb: Take the volume of the house in cubic feet and divide by 2 to get the cfm rating of the evaporative cooler. This sizing method gives you an appliance that provides 30 air changes per hour.
  • Another variation of the above rule of thumb is based on area rather than volume: Choose an evaporative cooler rated at 2 to 3 cfm per square foot of floor space, or 3 to 4 cfm per square foot of floor space if the climate is very hot.
  • If the house already has an air conditioner, and you plan to connect the evaporative cooler to the existing ductwork — an approach that is not ideal, because the ducts are likely to be undersized — choose an evaporative cooler with a rating of 1,000 cfm per ton of air conditioning.

How much do evaporative coolers cost?

It’s possible to buy a simple 2,800 cfm window-mounted evaporative cooler for $354.

The installed cost of a large whole-house evaporative cooler is in the range of $2,500 to $4,000.

Operating tips

According to one source, it’s a good idea to wait until the outdoor dew-point temperature drops to 55°F before turning on an evaporative cooler.

Evaporative coolers blow large quantities of air — typically between 3,000 and 8,000 cfm — into your house. The cooling system won’t work well unless there is a path that allows the same quantity of air to escape.

There are two common ways to do this:

  • The homeowner opens a few windows to provide a path for the escaping air. If you take this approach, you can regulate the indoor temperature and the indoor humidity levels (and to some extent, you can can regulate the path taken by the cool breeze blowing through the house and therefore which rooms are cooled) by adjusting the windows openings.
  • A different approach is to relieve the indoor air pressure by installing special outlets (grilles attached to short vertical ducts) installed in the top-floor ceiling. These outlets allow air to escape into the attic. This approach only works if the attic has oversized ventilation openings (soffit, ridge, or gable vents). Most homeowners who take this approach install purpose-made ceiling grilles equipped with backdraft dampers; the best known brand is Up-Dux (see Image #5, below).

Most evaporative coolers have two-speed fans; obviously, the fan speed choice affects indoor temperature. Operating the unit at low speed saves energy.

Most evaporative coolers have a “vent only” option that operates the fan but not the water pump. In “vent only” mode, the evaporator pad stays dry, and the unit works like a whole-house fan. (For more information on whole-house fans, see Fans in the Attic: Do They Help or Do They Hurt?)

Why would anyone install an evaporative cooler?

Compared to an air conditioner, an evaporative cooler has several advantages:

  • An evaporative cooler usually costs less — perhaps 50% less — to install than an air conditioner.
  • An evaporative cooler almost always costs less to operate than an air conditioner — typically requiring only 15% to 35% of the energy required to run an air conditioner.
  • An evaporative cooler doesn’t have any refrigerants. (Some refrigerants can damage the atmosphere’s ozone layer, so many green builders prefer equipment that doesn’t require refrigerants.)
  • An evaporative cooler provides a high ventilation rate; some homeowners like the fact that, unlike an air conditioner, an evaporative cooler provides lots of fresh air.
  • The wet pad on an evaporative cooler is an excellent air filter, so the air introduced by an evaporative cooler is very clean.

Disadvantages of evaporative coolers

On the other hand, compared to an air conditioner, an evaporative cooler has several disadvantages:

  • Evaporative coolers only work in a dry climate. Most people are particularly desperate for cooling on hot, humid days — precisely the type of weather that incapacitates an evaporative cooler.
  • An evaporative cooler uses a lot of water, and that can be a problem in drought-prone areas of the country or in areas of the country where water bills are high. How much water do these units use? Published estimates vary; one article reports water use of 3 to 15 gallons of water per day, while another article estimates typical water use at 4 to 9 gallons of water per hour — that is, 20 to 45 gallons of water per day if the evaporative cooler is operated for 5 hours per day.
  • Evaporative coolers require a lot of cleaning and maintenance. These maintenance routines need to be implemented at least twice a year; if the evaporative cooler gets a lot of use, a monthly maintenance routine is best. If the evaporative cooler is located on your roof, you’ll need a ladder to get up there, with each maintenance visit causing wear and tear on your roofing surface. Maintenance includes regular draining and cleaning of the sump — a process that requires removal of the build-up of (a) minerals that were in the water that evaporated and (b) sediment filtered from dusty air.
  • In areas with cold winters, the plumbing pipes that serve an evaporative cooler have to be drained to avoid freeze-up problems.

Indirect evaporative coolers

Indirect evaporative coolers send a stream of evaporatively cooled air though an air-to-air heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.. The evaporatively cooled air is used to lower the temperature of a separate air stream to which no moisture is added. Sending air through a heat exchanger requires more fan energy, thereby lowering the total efficiency of an indirect evaporative cooler compared to a normal swamp cooler.

To help readers understand how indirect evaporative cooling works, an article in Home Energy magazine explained the process with this simplified image: “Imagine blowing air through the core of a pipe. Then sprinkle water on the outside of the pipe, and blow air across the pipe. The air inside the pipe is cooled by contact with the cool pipe, but it is not in contact with the water, so its humidity does not increase.”

Some indirect evaporative coolers are two-stage coolers. After the air is precooled by passing the air stream through a heat exchanger like the one described above, the cooled air is subsequently passed through a conventional evaporative cooler. This type of two-stage cooling can produce air that is actually colder than the outdoor wet-bulb temperature.

The Coolerado

The best known indirect evaporative cooler is the Coolerado, an indirect evaporative cooler that has been on the market since 2005.

The heat exchanger used in the Coolerado consists of stacked layers of polypropylene sheets (each of which has exposed polypropylene fibers on one side). The fibers hold moisture and establish gaps (air channels) between the stacked sheets.

Intake air entering the Coolerado is split into two airstreams, and the airstreams pass each other through alternating layers of the heat exchanger. The evaporatively cooled airstream — known as the “working” airstream — absorbs heat from the “product” airstream, without allowing the product airstream to pick up any moisture. After passing through the heat exchanger, the evaporatively cooled working airstream, now warm and saturated, is exhausted from the Coolerado unit without ever entering the house. The product airstream, now indirectly cooled, is ducted to the house.

The Coolerado’s performance has been measured by Steve Slayzak, an engineer at the National Renewable Energy Laboratory. “These coolers have a really elegant internal manifold that allows for multiple stages of indirect evaporative cooling,” says Slayzak. “It makes it possible to cool the air below the wet-bulb temperature. We have measured temperatures 20 degrees below wet bulb. It’s able to provide the same temperature air as a very good swamp cooler, but without any moisture added to the air.”

The main difference between an indirect evaporative cooler and a direct evaporative cooler is the inclusion of an air-to-air heat exchanger. This heat exchanger necessarily incurs an energy penalty. “With indirect evaporative coolers, there is always a tradeoff between fan energy and performance,” notes David Springer, president of Davis Energy Group. “Adding stages to increase the cooling requires throwing off more and more air, which usually increases fan energy use.”

The Coolerado uses a lot of water — up to 12 gallons per hour, which is significantly more than a typical swamp cooler.

According to a review of the Coolerado in Environmental Building News, “Where added humidity and constantly moving air are no problem, a high-quality direct evaporative cooler [a swamp cooler] … can provide more efficient cooling at a lower first costInitial cost of buying or building something; does not include operating costs. than the Coolerado. … But for high-efficiency cooling in dry climates with no added humidity, the Coolerado represents a unique and attractive option.”

When available, a Coolerado unit appropriate for a residential application costs about $5,000. However, according to a recorded message on the manufacturer’s phone line, residential-sized Coolerado units are not currently available.

Whatever happened to the OASys?

Many articles on evaporative coolers mention the OASys, an energy-efficient appliance that was developed by engineers at the Davis Energy Group in Davis, California. Unfortunately, the OASys is no longer available.

For a while, the OASys was being manufactured by Speakman. After a while the production of the unit was moved to a factory in India. Because of poor sales, however, manufacturing was discontinued. The remaining units were shipped to a warehouse in Nevada, where a warehouse fire destroyed the entire inventory.

The end of a historical era

A few years ago I had a conversation about evaporative coolers with John Proctor, the president of Proctor Engineering Group in San Rafael, California. Proctor, a nationally known air conditioning expert, told me that evaporative coolers deserve wider use. “The problems with direct evaporative coolers are overblown,” notes Proctor. “I lived in an evaporatively cooled home in Colorado for many years and was extremely happy with it. It worked well. I’m befuddled by the fact that more people don’t use evaporative coolers.”

Like most GBA readers, Proctor is a strong advocate for the use of energy-efficient HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment, so his love of evaporative coolers isn’t too surprising. Although energy-efficiency advocates have been singing the praises of evaporative coolers for years, the tide seems to have turned away from these devices. Evaporative coolers are fading away.

A reporter for The Arizona Republic, Ryan Randazzo, described the trend in a 2010 article titled “Once-Common Evaporative Coolers Are Disappearing from Phoenix-Area Homes.” Randazzo wrote, “Now the metal boxes atop homes are rare, done in by a combination of cheap and increasingly energy-efficient air-conditioning and the time and expense of maintaining the coolers. Arizonans steadily have moved away from using the sometimes noisy, always drippy evaporative coolers, even though they may reduce energy bills.”

According to Randazzo, almost every Phoenix home had an evaporative cooler in 1940. In 1984, nearly half of all Phoenix residents still had one. By 2010, however, less than 10% of Phoenix homeowners had an evaporative cooler. “Most residents who still use them are either extremely cost-conscious, handy at fixing the units, or both,” Randazzo reported.

Randazzo continued, “New housing developments are limiting coolers’ use on roofs. And people are just happy to use an air-conditioner that rarely needs repairs vs. a cooler that needs rooftop service at least twice annually.”

Randazzo interviewed Mike Donley, president of Donley Service Center. Donley explained that “most people just tired of climbing on the roof in the spring to clean and activate their cooler, and getting up there again in the fall to clean it and seal it off. … ‘Coolers are a do-it-yourself project,’ Donley said. … ‘I hate to say this, but if you are going to [pay to] have us service it twice a year, you are better off buying a high-efficiency AC system,’ Donley said.”

Another factor driving the decline of evaporative coolers: high water bills. “‘It used to be that evaporative cooling was half to a third the cost of AC,’ Salt River Project energy expert Jerry Thieken said. ‘With new high-efficiency air-conditioners, it’s getting to be a wash. Evaps still are less expensive, but you have to throw in the water component.’”

Problems with the “lots of airflow” approach

One other factor is worth noting — one that Randazzo didn’t mention: because evaporative coolers move a lot of air, they are hard to integrate into a very tight house. Stated bluntly, these cooling appliances are incompatible with buildings aiming for very low rates of air leakage.

Cooling a house by opening up the windows and blowing in 6,000 cfm of cool, damp air works well, and these old-fashioned cooling systems remind many older Americans of breezy summer days. But when winter comes along, the evaporative cooler’s large duct opening is hard to seal. That’s why a builder aiming for a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. result of 0.6 ach50 isn’t going to install an evaporative cooler.

Matched with PV, it’s a good solution for off-grid homes

Although the use of evaporative coolers is in decline, these units are still a good choice for homeowners willing to put up with their quirks. They are also a good choice for use in commercial spaces where lots of fresh air ventilation is required — for example, gyms and exercise studios — or commercial garages where overhead doors are opened frequently.

Perhaps their best application is in off-grid homes. Few off-grid homeowners can afford the large battery systems and photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PV) arrays required to operate a central air conditioner. However, evaporative coolers are a good match for PV. It’s even possible to connect an evaporative cooler equipped with a DC motor directly to one or more PV modules, with no intervening battery or inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements..

If you are an off-grid homeowner intrigued by this approach, contact Southwest Solar of Tucson, Arizona. The company sells several models of evaporative coolers equipped with DC motors, including the 2424XP (equipped with a 24-volt motor) and the 2412XP (equipped with a 12-volt motor). Either of these models has a watt draw of only 250 watts and is rated at 3,000 cfm. Either model can be operated from a single 300-watt PV module. The purchase price of either unit is about $1,552.

Martin Holladay’s previous blog: “Installing Windows In a Foam-Sheathed Wall.”

Click here to follow Martin Holladay on Twitter.


Tags: , , , , , , ,

Image Credits:

  1. Image #1: Public domain
  2. Image #2: Rheem
  3. Image #3: NREL
  4. Image #4: Home Energy magazine
  5. Image #5: Up-Dux

1.
Jul 21, 2015 2:27 PM ET

Minisplits will Kill the Residential Evaporative Cooler Market
by Kevin Dickson, MSME

Martin,

Maybe this article would have generated more discussion if you called it "Evaporative Cooling is Dead".

When I designed my current house in 2001, I went with radiant heat because of all its perceived benefits and minisplits were not yet very common. Since central air would require costly ductwork, I felt an evaporative cooler was the perfect solution for air conditioning.

So I'd like to add a few points about swamp coolers for dry climates:

1. For most locations, cheap coolers are a bad choice. The $354 cooler won't do the job when the temperatures rise above 95F, just as the chart above shows.

2. A better cooler CAN get the job done all year (up to about 102F) as I have proven in my own house. The secret to better performance is the new 8" and 12" cardboard media. The extra first cost of the cooler is recouped because the media doesn't need replacing nearly as often. http://www.homedepot.com/p/MasterCool-5000-CFM-230-Volt-2-Speed-Side-Dra...

Here is one report that documents how much better the thicker media works: http://www.clemson.edu/psapublishing/pages/ageng/ANREA-061.pdf

3. The "tight house problem" you cite isn't really that hard to solve... just give the cooler its own little room inside the house with a large operable window. That also solves the maintenance cost issues because nobody has to climb on the roof, and it never needs to be winterized.

4. The cost of the water needed is NOT significant: http://greenbuildingindenver.blogspot.com/2012/05/dont-worry-about-swamp...

John Proctor (the AC expert) said "I’m befuddled by the fact that more people don’t use evaporative coolers.” Once he reads your article, he should understand.


2.
Jul 21, 2015 3:06 PM ET

Edited Jul 21, 2015 5:09 PM ET.

Response to Kevin Dickson
by Martin Holladay

Kevin,
Yes, I considered the "Evaporative Cooling Is Dead" title, but I decided that it might lead to eye-rolling. And in any case, like John Proctor, I retain affection for these quirky devices, even though their time has passed.

You are I are in substantial agreement. My extensive references to Ryan Randazzo's article, “Once-Common Evaporative Coolers Are Disappearing from Phoenix-Area Homes,” is due to the fact that I think Randazzo nailed it.

I'm trying to visualize your "just give the cooler its own little room inside the house" suggestion. I'm thinking that the room is airtight; the evaporative cooler is in the middle of the room with a duct leading from the cooler to a grille mounted in your exterior wall; and that the room has an operable window in a partition, so that the window opens to an interior room (perhaps a hallway). When you want to turn on the cooler, you open the window between the hallway and the mechanical room. Is that how it works? If so -- I like it.

I appreciate your point about water usage; however, some swamp coolers use more water than others, and water restrictions are increasing in many Western states, at the same time that water rates are rising.


3.
Jul 22, 2015 12:43 PM ET

Indoor Swamp Cooler
by Kevin Dickson, MSME

Here are a couple photos of my indoor swamp cooler. It works perfectly but I've never heard of it being done before or since. The awning window is wide open all summer, and since it's on the third floor and under an overhang, rain is not a problem.

The 8" media MasterCool unit performs well with a thermostat 100% of the time. I previously had a cheaper cooler which didn't. We made it work by overcooling the high mass well-insulated house at night, then sealing the house up tight until 8pm.

Phoenix has a monsoon season each summer that has high humidity with high temperatures. That fact has also helped kill evaporative cooling there. Denver and the high mountain west remains perfect for it.

rsz_swamp_cooler_1.jpg rsz_swamp_cooler_room.jpg


4.
Jul 22, 2015 1:38 PM ET

Response to Kevin Dickson
by Martin Holladay

Kevin,
A very nice (and ingenious) installation. Well done.


5.
Jul 24, 2015 4:54 AM ET

OK, just a couple more points
by Kevin Dickson, MSME

1. This installation allows me to keep the house at 66-70F all summer for $20/month.
2. The summer air quality can't be beat, about 2000cfm of filtered, scrubbed, and humidified air.
3. Low speed is very quiet and usually is enough. Hi speed is a little noisy and windy in the room shown.
4. Cooler components are super cheap, I just replaced my 7 year old pump for $16.
5. We use it as the humidifier in the winter on a humidistat (with the window closed). The single biggest complaint about Denver's climate is the dry indoor air in the winter.


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!