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My AC condenser sits on the roof of my house and as such in the summer it is in full sun. I'm wondering if shading the condenser is a good idea.

Update: First off, what a great comment stream thanks folks! In my case the roof section in question is flat, white, completely exposed during the 12 hours that matter most. The three to six hottest weeks (95-100F high, 80-85F low) come with almost no wind, the remaining four hot months (80-95F high, 60-80F low) generally have a mild or better breeze. The roof is at about 37.3478° N.

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    I'd say there is trade off between shading and not building an obstacle for wind - and my uneducated guess is that wind and air flow could be more important than sun.
    – Pere
    Commented Jun 27 at 12:31

2 Answers 2

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Yes shading will help. The heat transfer will be more efficient without sun on the condenser.

However do not expect large reductions in energy costs. Still the shading is beneficial.

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    Most outdoor units will already shade the condenser though.
    – ratchet freak
    Commented Jun 26 at 15:06
  • To the OP, please read other answers too. . . "Do not expect large reductions" probably means you'll save about $.02 per day. Yes, you're correct. It does help. But it's very very small potatoes.
    – Turbo
    Commented Jun 27 at 15:32
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    You (meaning OP) need to be careful that the shading doesn't block airflow. Look up clearances because you might save $0.02 in solar radiation per day but cost yourself $1.00 in lost ability to blow heat away.
    – Dean MacGregor
    Commented Jun 27 at 18:32
  • To all commenting, remember the OP did not mention anything about building a box or structure around the unit, or have a concern about cost savings. I just mentioned it because many people do think they will see a great monetary return with a little action.
    – RMDman
    Commented Jun 27 at 18:40
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At least in theory it will help; however, read to the end (or skip to the last paragraph)...

Why should it help? Rate of heat transfer is a function of temperature difference. Therefore, if the temperature around the condenser is lower, the rate of heat transfer should increase. (This is basic thermodynamics.) Note that this applies whether or not the condenser coils are already "shaded" by the unit as a whole; the surrounding air will still be heated, and starting from a lower temperature, the surrounding environment will still have greater capacity to carry the warmed air away.

Note that this also means putting the condenser in an area of high wind should in theory be helpful as well. However, natural convection and the action of the condenser fan will already "stir" the surrounding air to an extent.

Will the unit be more efficient? Again, in theory, yes, since a faster rate of heat exchange means it should be able to "move" more heat in the same amount of time. This should translate into faster cooling and reduced cycle times for the same indoor effect.

However... whether there is a sufficient difference for this to be meaningful is not easy to determine without complicated models or even real-world testing. Also, shade may be trumped by air exchange, and being on top of a roof will facilitate air movement (especially if there aren't a lot of tall trees nearby). Said air exchange may very well cycle warm air into the surrounding area faster than any shade you could contrive can cool it off. Therefore, I probably wouldn't spend a lot of money on this project. If you can do it cheap, it might be worthwhile... or it might not. If you can't do it cheap, there's a good chance you'd spend more money on the effort than what you might save.

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    In practice, "yes" too. AC hot sides are kept out of the sun when possible in industrial design. Air exchange impediment is a small and easily mitigated factor compared to solar heating.
    – Therac
    Commented Jun 27 at 15:46
  • @Therac, I admit to having no domain-specific knowledge, just knowledge of basic physics (thermodynamics being of specific relevance). Do you know of any studies quantifying "when possible"? That is, is this a "nice to have" feature, or does it make a measurable and significant difference? Also, is shade still relevant if their is significant air movement past the location? I'd imagine on a roof that wind and convection is going to circulate the nearby air fairly well, possibly bringing in air from outside the shaded area and thus negating much of the effect from adding shade.
    – Matthew
    Commented Jun 27 at 17:51
  • Of course, this will depend on many factors for which the OP did not give sufficient information. What's the average wind speed? Flat or sloped roof? (I'd guess sloped, but...) What effect does the proposed shade structure have on airflow? (Significantly reduced air flow may well reduce efficiency more than what adding shade gives back.) How much does the proposed shade structure cost, keeping in mind it has to withstand rain and, depending on where the OP lives, possibly hurricanes and/or snow. Is it structurally sound? Is it, or does it need to be, permitted? So many variables...
    – Matthew
    Commented Jun 27 at 17:56
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    To all those with a calculator in their hand, I live in Florida. Sun is our speciality...and our nemesis. I can say with certainty, in the real world setting and from 50 years of experience. Throw shade on an AC condenser unit and it will give lower temperature air from the service ducts or head units. It's something here we know about. However we also know that our electric bills don't reflect a great return. However that is not the goal. getting our bodies cooler when it's 95 deg. and 87% humidity outside is what we are striving for. The Physics about it we don't care about.
    – RMDman
    Commented Jun 27 at 18:46
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    @RMDman, you're missing the point. If the air is mostly stationary, sure, shade will work. If wind is blowing air across the unit that's been warmed by the sun and roof, it won't make much difference if the unit itself is in the shade, because the temperature of the surrounding air won't be affected. Comparing to solar panels is apples and oranges; solar panels are designed to collect solar energy.
    – Matthew
    Commented Jun 27 at 21:55

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