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Our house has two A/C systems; one for the bedroom level, one for the remainder of the house. The compressor/consdensor units are outside the south side of the house, and in direct sun all day, now that a nearby tree has been removed.

Would shading the units, perhaps with a small fence, improve their efficiency?

I'm not (very) concerned with the look, as this side of the house is not visible from anywhere aside from our veggie garden.

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3 Answers 3

up vote 10 down vote accepted

It helps for sure. There have been a ton of studies done on this and I have read at least 10-11. At one point in time I was going into a partnership for a "greening" business.

Your variance is somewhere between 1-15%. There are a ton of variables.

Here is an OK study I read a few weeks ago from FSEC. I personally don't like this study because they are first in a climate that doesn't cool well at night and then they used young trees...

Variables

  1. The shading of the air coming into condensor is super helpful. But if you have a 9 foot tree over the condenser in an open field then there is a very small volume of shaded air. Once your condenser sucks in the shaded air then it gets the hot stuff and returns diminish. So the first variable is shade volume. With huge oak trees being optimal and anything less than that losing value all the way down to a bush.
  2. Air temperature in shaded area. This depends on what is under your tree (grass, rocks, dirt, water...), it depends on how many hours a day you get the shade, air flow, how close you are to house...
  3. Air flow is huge. The more wind you can push to the unit the more efficient it will be. One of the things that people do that inhibits this is bushes right next to unit. Obviously decreases air flow.

So the bigger tree you put there the better but you also want to configure the trees so that the unit gets good air flow. The price of a tree and your work will be returned for sure and could potentially have a major change on your bills - I could see $300-600 a year in warmer climates. Also I think a small fence would have no to little impact (restricting air flow).

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+1. I think an interesting point is understanding how the shade creates a tradeoff between reduced radiative heating from the sun and convective cooling, which is impacted by air flow from the fan. –  user558 Aug 20 '13 at 10:56
    
+1 Also see science discussion below. –  bib Aug 20 '13 at 12:01
    
I didn't even get into the "misting" options since that wasn't in the question. But basically putting a hose on a timer to mist your unit is very effective (another 10% possibly) with almost no cost other than setting up the mister. –  DMoore Aug 20 '13 at 17:39
    
Thanks, pretty much what my intuition was telling me. A tree is not an option at present, but I will experiment with some other ideas, keeping airflow in mind. –  TomG Aug 22 '13 at 1:09

DMoore gives a good explanation and practical advice. This is just a summary of the science side.

Heat is bad for an AC unit. More heat, more work

Heat gets transferred three ways

  • radiation
  • convection
  • conduction

Radiation occurs when sunlight directly hits something (like the body of the unit) - shade reduces this.

Convection occurs when warm air flows over and through the unit - shade reduces this, but as DMoore points out, you need a lot of shaded air to make a major difference, not just a bit of shade directly over the unit.

Conduction occurs when something hotter (other than air) touches something cooler - this is not a sgnificant factor in the AC analysis.

Of the three, convection is probably the most important. See DMoore's discussion about how to handle this.

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I've always thought so. The goal of a/c is to warm up refrigerant (with heat from inside), pump it outside and release the heat. The cooler the ambient outside, the faster the transfer will occur (as long as ambient is above the dew point, otherwise you might get icing).

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