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Should I build a roof to shade my air conditioner?
July 26, 2010 10:38 AM   Subscribe

Should I build something to shade my rooftop air conditioner?

Our apartment has a split-system air conditioner compressor on the roof. It occurs to me that the compressor is sitting out in direct sun. Would it be a good idea to put a little roof over it so it was sitting in the shade? Assuming said roof had decent clearance over the compressor for air circulation?

(And on a related note: Mexican cowboys are famous for their sombreros, big straw hats that keep them out of the sun. Why don't other desert people like the Bedouin or the Sudanese wear big hats as portable shade?
posted by musofire to Home & Garden (13 answers total) 2 users marked this as a favorite
 
I get the feeling you think this will improve the efficiency of the air conditioning system, but all of the cooling action happens at the condenser coil located at fan/duct/furnace module of your central air system. I don't think shading the compressor will improve your air conditioner efficiency.

However, I think it wouldn't hurt to have a roof over it, especially to keep rain off of the compressor. It would probably last a bit longer being protected from the elements.
posted by wigner3j at 11:02 AM on July 26, 2010


What color is it? Painting it white would be a cheaper solution than building a shelter over it.
posted by nathan_teske at 11:09 AM on July 26, 2010


I get the feeling you think this will improve the efficiency of the air conditioning system, but all of the cooling action happens at the condenser coil located at fan/duct/furnace module of your central air system. I don't think shading the compressor will improve your air conditioner efficiency.

Sure it will! The best way to make a compressor more efficient is to increase the thermal differential. That is, the compressor needs to be hotter than its environment so that the heat transfers from the coils to the outdoors. The larger the temperature difference, the easier it is for the heat to transfer, the more efficient the system becomes.

Now, if you look at large, industrial systems they will almost always have water running over the coils. Why? Because water can absorb an amazing amount of energy before it heats up.

If I were going to build a roof over the airconditioner, I'd make it just that. A roof. No walls or enclosure of any kind. You want the maximum amount of airflow through there.
posted by sbutler at 11:15 AM on July 26, 2010 [4 favorites]


Sure it will! The best way to make a compressor more efficient is to increase the thermal differential.

Interesting point sbutler, I never thought of that.
posted by wigner3j at 11:38 AM on July 26, 2010


I have a friend who works on and installs HVAC systems and he says keeping it in the shade will definitely help, as will spraying water onto it (athough a water system would get more complicated, raise your water bills, and potentially cause roof leaks). I asked him this question because my heat pumps are right next to my deck and I want to one day expand the deck which will put it over the compressors. He also agreed that shelter from the elements may prolong their life as well, and of course, nothing should be done which will limit airflow.
posted by TedW at 11:45 AM on July 26, 2010


Shading your roof top AC will help.

But two points of note - First, don't build anything up there that can blow off the roof in a severe storm. Second, if you do secure your little building to the roof, make sure you get roofing tar to tightly seal any wholes you put in your roof.
posted by Flood at 11:48 AM on July 26, 2010


> Mexican cowboys are famous for their sombreros, big straw hats that keep them out of the sun. Why don't other desert people like the Bedouin or the Sudanese wear big hats as portable shade?

The wind would blow that silly hat around. Even if it was secured with a chinstrap, it would buffet severely. The headdresses that desert Arabs wear already cover the neck and forehead, so the wide brim would be redundant.
posted by Burhanistan at 12:19 PM on July 26, 2010


Shading just your condensing unit will make very, very little difference to the efficiency of your unit. Coil temperature depends on the temperature of the air passing over it and shading just the condensing unit isn't going to make a significant difference when your fan is moving thousands of CFM of air over it.

Direct sunlight at the earths surface maxes out around a kilowatt per square metre per hour under ideal conditions. A typical 3 ton A/C is giving off around 10K kilowatts per hour of heat and isn't even at the correct angle to capture maximum solar heat let alone being a selective surface.
posted by Mitheral at 2:08 PM on July 26, 2010 [1 favorite]


Shading = good
Spraying the compressor's heat sink fins with water will help too, BUT be wary of doing this very often - water contains various dissolved minerals which, when they precipitate out of the water which is evaporating out of your coil they form scale. Scale is incredibly hard to get rid of and a good thermal insulator. It will thus, over time, DECREASE the efficiency of the radiator unless you use distilled water which costs far too much to be worth it.
posted by BrooksCooper at 5:29 PM on July 26, 2010


Further to Burhanistan's comment: Bedouin and Sudanese live in much dustier climates as well, and the headdresses are a good guard against getting dust in your nose and mouth when the wind picks up.
posted by holterbarbour at 7:02 PM on July 26, 2010


We conclude that any savings produced by localized AC condenser shading are quite modest (<3>
posted by dhartung at 10:44 PM on July 27, 2010


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.
posted by dhartung at 10:45 PM on July 27, 2010


That's an awesome study dhartung.

One thing that would make a huge difference nationally would be to simple get low rise commercial refrigeration off of tar and gravel or rolled roof tops. I've seen temperatures as much as 35-50F higher on roofs compared to ground level temps at the same location doing refrigeration work. The reduction in head pressures would also lengthen the life of the compressor.
posted by Mitheral at 10:30 AM on July 28, 2010


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