Will Shading Your Air Conditioner Save You Money?

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hvac-condensing-unit-airflow-landscaping

I got a question this weekend that's often asked—and, I'm sure, wondered about—by homeowners: "Will my household AC system run more efficiently (perhaps cycle on/off fewer times, or the compressor won't have to run as long when it cycles on) by shading the compressor?" I've written about the outdoor unit of air conditioners and heat pumps a few times, but I've never tackled this question directly. Let's change that now.

Why this is a natural question to ask

First, a little explanation about air conditioners. That metal noisemaker that sits out in back or on the side of your house is called the condensing unit for air conditioners. The compressor is one component in that box, but the condensing coil and a few other parts are there, too. (To understand how an air conditioner works, see my article, The Magic of Cold.)

The condensing coil's job is to dump the heat picked up inside the home to the outdoor air. (If you have a ground source heat pump, that heat gets dumped into the ground rather than the air, and you won't have an outdoor unit like the one shown above.) The hotter it is outdoors, the harder it is to dump that heat and the more you'll spend keeping your house cool.

Hence the question, can shading your air conditioner's outdoor unit provide significant savings? And the answer is yes and no. It depends on what type of shading we're talking about, but for the type of shading most people are thinking about when they ask that question, the answer is no.

Why the answer is mostly no

Shading the outdoor unit with a structure as shown below will reduce the direct solar gain from insolation but won't do a whole lot for the air temperature around the unit. That photo is from a study done the Forida Solar Energy Center on the effectiveness of shading air conditioner condensing units.

Caution: The unit below exhausts from the side so the shading structure doesn't interrupt the air flow. Most AC outdoor units exhaust from the top and that structure would reduce the air flow and perhaps cause serious damage to the unit. Don't try something like this unless you have a good understanding of air conditioners.

fsec-air-conditioner-condensing-unit-location-study-shading-pf302-7

In the Discussion section at the end of their report, the FSEC authors give the big reason why small scale shading like this doesn't work. The temperature of the surrounding air has a much bigger effect on cooling efficiency than direct solar gain, and the volume of air pulled in by an air conditioner is huge.

A typical 3 ton air conditioner condensing unit might pull in 2800 cubic feet per minute (cfm) of outdoor air. If it ran continuously for an hour, 168,000 cubic feet of outdoor air would move through the outdoor unit. Since your air conditioner is probably oversized, it won't run the full 60 minutes. My oversized AC, for example, runs about 30 minutes an hour at design conditions. Even at 30 minutes per hour, though, the outdor unit pulls in 84,000 cubic feet of outdoor air.

To put those numbers in perspective, 84,000 cubic feet is about three times the volume of a typical house being cooled by that 3 ton AC. It would take a lot of shading to cool that much air.

The FSEC study sums it up this way:

We conclude that any savings produced by localized AC condenser shading are quite modest (<3%) and that the risk of interrupting air flow to the condenser may outweigh shading considerations. The preferred strategy may be a long-term one: locating AC condensers in an unobstructed location on the shaded north side of buildings and depending on extensive site and neighborhood-level landscaping to lower localized air temperatures.

A related question to shading the condensing unit is about saving money by spraying a mist of water on the unit, especially with the recent launch of the Mistbox. I wrote about that topic a couple of weeks ago, and again, the answer is that you're better off looking elsewhere.

If you really want to improve the efficiency of your air conditioner, your opportunites are much greater if you look to your duct system. By fixing disconnected ducts, flacid flex, uninsulated boots, and more, you may be able to cut your air conditioning bill in half, depending on how bad your particular ducts are. And they most likely are bad.

 

Related Articles

9 Uncommon Tips for Keeping Cool with a Struggling Air Conditioner

Does a Heat Pump or Air Conditioner Condenser Need to Go Outdoors?

An Easy Way to Save Money — Let Your Air Conditioner Breathe!

Surprise! I'm an Air Conditioner Compressor Killer

Is Evaporative Cooling the Answer to High Air Conditioning Costs?

 

Top photo by Energy Vanguard. Bottom photo from Florida Solar Energy Center's air conditioner shading study.

 

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Comments

Richard Parker

Enclosing the outside heatpump unit would quickly freeze the outside coils. The unit has to pull tremendous amounts of heat out of the outside air to transfer to the inside unit.

Rich Backus

Very well done, Allison. Thank you for writing on this subject, and bringing attention to the FSEC study & conclusions. This makes sense, and allows us to refer others to your summary here when we - too - often get asked about this. Again, well done. Thanks very much.

Allison Bailes

You're welcome, Rich. Glad you found it helpful.

Gerald McClain

How about a water mist on the Condensing Unit during the heat of the day?

Allison Bailes

Gerald, I just wrote about that topic two weeks ago. See the last one in my list of related articles above.

John Proctor

Allison -- you mention noisy. There is a solution to noisy that also increases the flow through the condenser. A diverging cone at the outlet to the condenser fan. We have tested these and they really work. see http://www.proctoreng.com/dnld/diffuser_how_to_2.pdf

Allison Bailes

That's really cool, John. I hadn't seen this before. Do you have data? Sounds like something I should write an article about.

John Proctor

But of course -- happy to share
John

Bob Ellenberg

Allison, you can't emphasis the ducts enough. I recently looked at a attic installation that was only a few years old and recommended to the friend to have a competent AC company rework them. The bid was pretty high so I offered to do it with him one Saturday. For a little more than $100 I bought 90 degree boots and in a half day we installed them at all the crimped turns and removed 40% of the duct work by stretching it tightly. They could not believe how evenly their house (which wasn't cooling) maintained a comfortable temperature throughout the hottest days.

Allison Bailes

I agree, Bob. I don't think I can ever write too much about the importance of good duct systems.

Andrew

Better yet, avoid ducted systems altogether.

Walt Stachowicz

Allison, my condenser was unfortunately tucked into a corner, and the tremendous rainfalls we have in Florida dump gallons of water right on top of the fan. This can not be good for the equipment. Would building a shed roof over this help in any way, without blocking the air flow? It would be about 4' above the unit. Gutters usually don't work in our typical afternoon "showers".
See you at SEBC?

Allison Bailes

Hey, Walt, I'll defer to Cameron on your question. And no, I won't be at SEBC this year. Maybe I'll get back down there next year.

Cameron Taylor

For Walt Stachowicz:

Most manufacturers want the closest obstruction to the vertical discharge of the condenser fan to be at least six to eight feet away. This is to avoid recirculating air into the condenser, which would make it less efficient and could lead to early demise of the compressor.

If your condenser straddles the drip line of the house roof's eave over it, consider having the unit moved either inside or outside of the drip line. It can take being in rain but it would be better if it was not in the drip line of the roof's eave.

Cameron Taylor

An alternate to moving the unit would be installing gutters on the eave to channel water around the condenser.

David Butler

Condenser placement is one of the things I discuss with all of my clients (new construction). I estimate the impact is about 3 to 5 percent for a heat pump that receives morning sun in winter and no afternoon sun in summer, compared to the opposite siting. This is certainly worth considering when it's free, but other considerations often trump the most energy efficient siting (noise, visual impact, and lineset length).

Allison Bailes

David, where did you get the 3-5% figure? FSEC found 1-3%.

David Butler

Allison wrote: "where did you get the 3-5% figure? FSEC found 1-3%"

Keep in mind that my thumbnail estimate was for a heat pump, so it includes the positive impact of AM sun in winter (compared to a shaded unit).

I based my estimates on data collected from my own system. My condenser is fully shaded until early afternoon, so I checked condenser inlet temperatures in the hour prior and an hour after the unit becomes sunlit and then referred to the expanded performance data. Later I was able to log actual kWh, which showed about the same percentage impact (although it should be no surprise that those tables aren't quite as accurate as some folks like to believe). The hard part is taking what's essentially a couple of snapshots and extrapolating to the seasonal impact. I used BIN data and the expanded performance tables, but had to guesstimate the percentage of heating and cooling hours an unshaded condenser would otherwise be shaded by clouds and rain. That's obviously climate specific, thus the range. Fun stuff.

This was just something I did for myself to get a ballpark idea of the impact, not something I normally would share in public. But when I read your article regarding FSEC findings of up to 3% (I was not aware of that study), I realized that their research validates my own estimate, more or less.

paul scrivens

My heat pump is on the north side of my home (in Arizona) and I have a 3 foot roof overhang. My unit is in the shade all year and the air temperature difference between shade and sun exposure is quite large. So it can work if you position your unit with energy conservation in mind.

Allison Bailes

Yes, Paul, the temperature difference between shade & sun can be large, but remember that it won't take much AC runtime to for all of the air in the shade to be pulled through the condensing unit. It won't take long before the unit is pulling in air from the sunny parts of the yard.

paul scrivens

Hi Allison, in my case the shaded area is quite large based on the homes profile and as my home is LEED gold it doesn't use a lot of a/c, so it seems to work well, but I do understand your point.

P K

I was thinking the same on lines out temp of unit not outside air. #
30 degrees in phx between shade and sun, I'd think the air going through the metal would be diffence of ice vs fire

Ray Austin

Allison, I concur with those findings.

With my house near Katy, Texas (Houston suburb) My AC is an 18 SEER two stage beast. (Two compressor unit) On top of that my humble home of 1750 sq. ft. has a 4 zone set up.

The condenser is installed on the north side of the house and I'd have a hard time to remember ever seeing any sun hit it at all.

I just got my electric bill for the month total bill was $58.00 you can view videos of my previous electric bills from my website AC tips page. I track years and years of them...

Anyway my lowest electric bill is maybe around $45 or so. So I paid around $13 for AC last month. I run it 76 all day the zone I am in and 74 at night.

The other zones I only operate if it's really hot or someone is coming over. Right now it's 76 in here with 51% humidity.

My utility rate is around 11 cents per kilowatt hour.

Stay cool my friends......

Allison Bailes

Those are impressive numbers, Ray. $13 for air conditioning in June is pretty darn good.

Ray Austin

Allison, yeah I think so too. Although the AC portion of the bill in August ballooned to a whopping $32 or there about. Considering we were hitting 100 nearly every day it's still very good.

I have yet to make my annual video of what my unit delivers in SEER versus the my own cost to run my system. I expect it to be very similar to previous years that I've posted to my YouTube channel years past.

Something interesting happened in October as my system broke down. Poor thing is like 8 1/2 years old. Anyway I decided to log a test to see if there is any remarkable difference in doing away with the variable speed blower versus a standard blower motor. You know cost savings wise on the old electric bill.

Given that my system is so efficient with the use of a zoned system this might be a little interesting to see. Won't know much until the end of 2016.

I also replaced the e-coil to the system as the old one had just begun leaking. It's an R22 system that is no longer made so that leak had to be fixed.

Anyway the results of this test should be a little interesting... stay tuned.

David Butler

@Ray, as air conditioners have become more efficient, blower energy represents an increasing portion of total cooling energy. The difference between PSC and ECM motor efficiency is very significant.

That said, you can get almost the same efficiency benefit with a non-variable (multi-tap) ECM blower, without the high cost of full variable. The motor chassis is identical sans the variable drive logic.

Multi-tap ECM motors can be adapted to work with 2-stage compressors and furnaces (although they may not have an official AHRI matched rating). Moreover, using an $8 relay, they can be configured to reduce blower speed for enhanced latent capacity when RH gets too high. In fact, the only disadvantage of non-variable ECM is that they can't ramp down to 50%, for example, when continuous fan is necessary for CFIS ventilation.

Ryan Shanahan

Living and working in a heating dominated climate I've always wondered if we could improve the efficiencies of air to air heat pumps by building a little green house (that still promotes air flow) around the outdoor unit. I've since realized that the volume of air needed to make a noticeable impact wouldn't justify trying it. Nice to know someone has already tried something similar from the cooling side and confirmed the same thing.

Ray Austin

David Butler,

That's really the reason I am doing it is to see how much of a difference it is. The variable blower would have been 3 times the cost as the regular multi tap motor I put in there.

If you match a variable speed furnace with a 14 SEER AC in Katy, Texas area, the best you can hope for is 15 SEER depending on tonnage of the AC of course... a 5 ton may not get that.

Now if I had a gas furnace I probably wouldn't have tried to do this because it would have been much more difficult and likely not worth the effort. I have an Electric air handler 240v blower went in there so the amp draws on a 240v motor are quite less than 115v motor. That is my reasoning anyway.

I know that sticking another vari motor in there it's just going to fail again in 8 years or less at more than 3 times the cost. So the question is will that extra cost add up in savings in this case?

If you know me, I will supply the answer sooner or later.

David Butler

240VAC vs 120VAC won't have much impact on consumption. The reason air handlers are 240VAC is to provide support heat strips. It's a non-issue in terms of efficiency. ECM and PSC motors are readily available both voltages.

John Proctor

Ray I am surprised that you are anticipating a fan motor failure in 8 years. The good BPM aka ECM motors have better bearings (ball or roller vs. bushing). Originally the electronics were a problem, but that seems to have been solved. I am really sorry we are not producing the Concept3 motor any longer because it would be perfect for your situation. The savings are real and significant. As David mentioned they also can be run at the appropriate speed for each situation.

Ray Austin

John Proctor,

My system is 8 1/2 years young... there is no other reason it failed other than I just ran the stuffing out of it. The bearings were seized in the old one.

I do air conditioning for a living. It's not uncommon for them to fail for one reason or another in less than 5 years. I've replaced many of them for various customers of mine. All they've done with the electronics is bury them in plastic so you can get to the caps that have shorted out. As this is normally why they quit.

Nothing last for ever... good thing too because it's how I feed myself.

I replaced one just this last summer just over a year old. PSC motor can last 15? Maybe 20?

John Proctor

Ray -- I am confused by your discussion above about Amps
Remembering Amps * Volts * Power Factor = Watts, you probably have noticed that half the volts and double the amps equals (probably) the same watts.

Ray Austin

I wanted to see if I could trip you up. I knew it was unlikely, but all the same you live in an 'engineering world' and I live in the other part.

I have in the past thrown math at people and I lose them in less than 15 seconds.

But if I show them a power bill with differences they will get that pretty quick.

I have to put things in easy to understand format. You know put one bill next to the other to show the example. I can't put out fancy complicated terms that only an engineer is going to understand. That isn't my market.

If I bring out the formula I've lost the majority of my customer base within seconds.

It doesn't do much good either to produce the same power bill results year in and year out when the Richard Parker's of the world think I am up to something that is 'suspect' as he put it on another website... even though I have been demonstrating what I pay for years now in video on my You Tube channel.

Anyways, good job John for not falling for my cheap shenanigans. LOL.

Dan Meyers

yeah, "trip you up" is what you were doing. (groan)
have some humility and acknowledge the basic assumption error.
its the most basic electrical equation.

Don Frye

I worked in Solar and other energy alternatives way back in the early 1980s. It surprises me still how many homes are designed with the condensing unit on the south side in the sun. Our studies, and studies done at the Solar Energy Center at the University of Alabama - Huntsville showed that shading the condensing unit helped, but not much. And it could not be done on the cheap, where it could fall from severe weather and damage the unit. Like studies showed that a water mist on the unit did help quite a bit, but usually resulted in damage to the unit from mineral deposits or other problems. Note, with more modern and efficient units, the savings today would be much less.
However, one study that I did recently, and would like an opinion on, is shading a PTAC unit. If adequate airflow is respected, I think there are good reasons to do this; including making the unit last longer [less damage to plastics and seals from the sun.] What do you think?

Ray Austin

So far this year the AC bill is low... in fact the total bill has been less than $40 the past two months.

It was $39 and some change the month before and $38 and some change for April.

I do notice a difference though from the variable speed motor to the fixed speed blower. It's a tad noisier and it seems that I have to drop temp more frequently to combat humidity.

But operational cost so far is of none effect.

Dirk Knowles

Sorry to beat a dead horse but I just read your blog about shading the AC unit. Ray Austin mentioned placement of the unit. Ours is not quite as bad .Our house is East facing and the unit is on the back west wall which gets the sun at the hottest part of the day. We are in Rio Rancho New Mexico and it is a sunny day with an air temp of 91. Using my handy dandy Harbor Freight infrared thermometer the temp coming off the back South facing wall is an unbelievable 133.5. The AC unit is 18 inches from this wall. The previous owners had a very nice patio cover built but stopped 15 feet short or covering this wall and therefore the AC unit. I know the cost of extending the cover would far exceed any benefits in cost savings on or electric bill but all the same I wanted to ask if you thought in this case it would make a difference. I understand heat exchange, etc. but was wondering if the builder had put this unit on the shady North side which reads 93 because of the reflection off the wonderful xeriscaping (gravel)if it would have helped the unit.

Dan Meyers Meyers

Most fail to consider the substantial heating impact of direct sun on an a solid OBJECT like the a/c unit itself.
ever touch a piece of metal thats been sitting in the sun a bit? yeah, HOT!

while central a/c condensor units shaped circle/square with coils pointing equally N/S/E/W , >50% is in the the shade at any given point in time. Window a/c coils are often in direct sun virtually all cooling day long.
Therefore, shade window a/c units to increase a/c cooling an extra 500+ BTUs(200-300 BTUs per hour per sq ft of a/c exposure.
worth noting it is much easier/cheaper to shade a window a/c unit vs a central unit.

John Proctor

Dan
Many of us have looked at the theoretical results from shading. Few people have actually tested it. The tests that I know of show two things. 1) anything that causes a higher level of air recirculation (such as a shade over the top of the unit or close to the unit) reduces the efficiency of the unit. 2) Florida Solar Energy's tests showed no measurable improvement from shading.

Dan Meyers

*meh. Window a/c units have horizontal out airflow, and...
*regardless of horizontal/vertical airflow, shades parallel to out airflow can actually IMPROVE airflow and increase a/c efficiency. Proven in tests with both trellis design and conical design.

Such designs are well-known, time tested basic stuff for shading ANYTHING, including windows.
Windows can also have massive savings with shades and awnings. And as with a/c units, don't trap the heat nor redirect it backwards. Common sense really.

next!

Ray Austin

Placement of the unit is the easiest because then you don't have a bunch of crap around the unit when it comes time to service it or replace it, plus it looks better.

Sure the metal of the unit will be hot if it's in direct sun light. This doesn't mean really too much because the efficiency or inefficiency of the unit is air temperature. If the unit is well ventilated sitting in the sun will not matter all that much.

Now personal preference for me is that you put it in the shade of the north side of the house. Simply because I work on them for a living and working in the shade is lot easier on me.

Recent utility bill for me was $51, had another issue pop up with this aging beast of mine and I expect a larger bill this next month how much larger remains to be seen I might break $80 or $90... not sure if it is / was related to this problem but should know for sure by August some time.

There certainly are differences in how my system operates now versus before when I had the variable speed motor. It's not really what I expected I can tell you that much.

Dan Meyers

"working in the shade is lot easier on me".
because the direct sunlight "doesn't mean too much", right Ray?
;>/

Ray Austin

Dan, no offense but you're a little late to the party.

I've been on this board for probably at least a couple of years now, have discussed many more topics than this with Mr. Proctor and others.

It was January when I wrote that post and I was likely rather board looking for some excitement anyway I could find it.

Allison Bailes

Yes, Ray is correct, Dan. The metal housing of an AC has a low emissivity so it can get really hot without transferring much of that heat to the cooling components.

Ray Austin

Dan,

Please!---Show me where "ANY" AC manufacture warns the installer not to place the unit in direct sunlight because the unit will not perform properly.

Then realize my market is Katy, Texas (a suburb of Houston) one of the hottest climates in which one can live.

Anthony Balcerowicz jr

I am looking to put, possibly two solar powered exhaust vents in my roof to pull excess heat out of the attic or roof area. My neighbor said this helped significantly, just installing one.

Ray Austin

Anthony,

Natural venting is the better method. The reason is that a powered attic vent can put the house under negative pressure, this is bad on so many levels... but it is mostly invisible to someone who doesn't understand such things.

Realize also that any fan that you place in a hot environment such as an attic, that fan is just going to cook itself to death.

The better method long time proven is to put in a couple of strategically placed whirly bird turbines. These do not use any power at all, spin up on their own. You need to make sure the eave vents of your home are unobstructed and maybe add additional eave vents to help the attic ventilate better.

You could take it a step further and add Ridge Vents. These are little plastic extrusions that sit on top of the roof ridge of your home and have venting thru the top, due to the design of them they only let air escape and do not let rain water in provided you install them properly.

Also if the house in question is 'A' frame you could add ventilation for when cross winds blow. I have an A frame house and I don't have these, but I do have the other things I mentioned in this post.

The temperature of my attic is usually the same or a few degrees warmer than outside ambient air temperature. I do not use 'any' powered attic vent, nor will I ever.