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. 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. 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. ac consumption unit per hourThe 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.split unit air conditioners prices A typical 3 ton air conditioner condensing unit might pull in 2800 cubic feet per minute (cfm) of outdoor air. air conditioning units high efficiencyIf 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. 9 Uncommon Tips for Keeping Cool with a Struggling Air Conditioner Does a Heat Pump or Air Conditioner Condenser Need to Go Outdoors?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. By the time you have to replace an AC compressor, other parts of the system may be failing too. Grainger gives you the tools you need to spot the damage and the parts to repair it. Find corrosion and failed seams with a refrigerant leak detector, install a liquid filter dryer line with the compressor for smooth operation, and upgrade your refrigerant transport with a new refrigerant line set.

When it's hot, Grainger helps you keep your cool. Surprise, its summer and our AC broke down. We were quoted prices for installation of the current standard (SEER 14) AC units (about $6500 unit cost and installation) vs Trane's SEER 18 AC units ($10,100 unit cost/install). The sales rep for the local HVAC company said the SEER 18 AC units was significantly more efficient than even the SEER rating indicated. The reason, he said, was that the unit has a variable rate compressor. He said normal online SEER savings calculators. Like this one underestimated the impact of the variable rate compressor. This is for our upstairs in our dual zone system. We live in a rather humid area in the summer, and the unit is used for both head pump and AC. We plan to live in this home for a long time. Is it worth the $3500 bump in price to get a unit with a variable-speed compressor? Are the online calculators underestimating the bump up to a variable speed compressor? In general, when is it worth the price difference to get a variable speed compressor?

Help me (and everyone else who sees this question) be better informed when talking to their HVAC sales rep! The test procedure for determining the SEER is quite elaborate, and involves measuring the efficiency at a variety of simulated outdoor temperatures. I'm not sure what the basis for the sales rep's statement was, but without more information (i.e. specific numbers) I wouldn't put too much stock in it. The SEER is not just calculated at a single fixed situation. As for whether it makes sense, that depends on your climate, the size of the area being cooled, the local price of electricity, etc. I would just use one of those calculators like you've linked to to determine the value, but make sure you get accurate numbers for all the inputs. The cost of electricity varies widely throughout the US, so get a current number for your area in the summer and then maybe round up a bit to account for the gradual rise in prices.Browse other questions tagged hvac energy-efficiency or ask your own question.