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