Welcome to the Energy Efficiency Myths series from Direct Energy! As many myths arise from incomplete knowledge, they can create seemingly possible answers that many people accept as fact. Each month, we will examine common misunderstandings about energy efficiency — whether it’s in your home or about the energy industry — and deliver real facts behind the myth (and how they might be costing you money).
Shading Your Air Conditioner is Shady
It is a very common belief that shading your air conditioner’s condenser unit, that big steel box with the fan outside, will reduce the amount of energy it uses. Energy.gov confidently states that “shading the outside unit can increase its efficiency by up to 10%.” Even house-savvy BobVila.com proclaims, “Shading your air conditioner can reduce energy costs by as much as 50 percent.”
The notion sounds like common sense. AC condensers have a harder time blowing heat from their coolant when it’s hot outside. Keeping them out of the sun should help them get rid of heat more efficiently…right? Heck, I believed it too.
But even though so many, many reliable resources say it’s a good idea, the research shows that it’s based on…well, “shady” conclusions that have since been disproved. Plus, the idea has been entangled with all the other numbers and efficiencies that come from shading your home. So, let’s shine some light on what’s really happening.
In 1992, a study published by the EPA called Cooling Our Communities: A Guidebook On Tree Planting And Light-Colored Surfacing stated:
“Preliminary measurements show that planting trees or erecting a trellis covered with vines around an air conditioner can reduce air temperature around it by 6 or 7°F. this can increase the efficiency of the air conditioner by about 10 percent during peak periods.
The EPA study goes on to state, “Field measurements have shown that through shading, trees and shrubs strategically planted next to buildings can reduce summer air conditioning costs typically by 15 to 35 percent, and by as much as 5 percent or more in certain specific situations. Simply shading the air conditioner-by using shrubs or a vine-covered trellis–can save up to 10 percent in annual cooling-energy costs.”
But, in 1996, researchers from the Florida Solar Energy Center (FSEC) published Measured Impacts of Air Conditioner Condenser Shading that showed that the benefits are actually trivial when you wade into the details and do some comparing.
First, there’s an assumption that the sun shining directly onto the unit causes the whole thing to heat up like a brick. Now, shading the condenser might reduce the sun’s heat on the casing but the condenser’s casing doesn’t conduct that much heat to the internal fins and tubing. In fact, all those bits are already under shade inside the case.
The question FSEC wanted to check out was how much of an effect shade has on cooling the condenser and does this actually do anything. The most important thing to remember is that air conditioner condensers work by dumping all their coolant’s heat by moving air through their cooling coils and fins. It’s the temperature of the ambient air that has an effect on how well the condenser can dump its heat, not whether the unit’s case is hot.
FSEC points out that a 3 ton condenser moves 170,000 cubic feet of air per hour. That’s like the air inside a 55 foot cube. So during that hour a portion of that 55 foot cube of air is being replaced from beyond the immediate area. If only the condenser is kept shaded by a trellis or bushes, or some kind of structure, then the amount of air moving through the condenser won’t have time to cool down within the shaded area.
FSEC’s researchers found when they measured the air temperature entering the condenser of shaded and unshaded condenser units, they found there was only “an increase in air conditioning efficiency of approximately 1.2% per F° reduction in condenser inlet temperature.” In terms of energy savings, that adds up to less than 3%.
A similar study was conducted by the Kuwait Institute for Scientific Research in 2010 that found that “The actual increase in efficiency due to shading is not expected to exceed a maximum of 1%.”
To sum up, then:
- Shading your air conditioning condenser alone is essentially ineffective.
- Some types of shadings and plantings around condenser units actually restrict airflow, causing heated air from the unit to recirculate and reduce the condenser’s cooling efficiency. No matter how ugly you think they are, condensers need to breathe. Hide it but don’t smother
- You should shade a room-sized air conditioner!These window unit guys are smaller, compactly constructed, and move much less air than one of those big condenser units. Direct sun on one of these can reduce their efficiency up to 10%.
- Lastly, in order to get the most out of shade for your home’s energy efficiency, shade your entire home. Adding landscaping that increases shade throughout your yard creates a cooler microclimate. This reduces your home’s cooling load and helps reduce local air temperature for your condenser.