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  1. #1
    Join Date
    Jun 2011
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    21

    relationship between C.O.P. and outside air temperature

    Since the efficiency of an air conditioner is a function of outdoor air temperature, does it follow that you will get better efficiency if you put the condenser under some sort of cover to keep the sun off of it? For example under a tree as opposed to on the roof? This seems to make sense theoretically, but I've never seen anyone build a shade cover for over top of a condenser that was placed on a roof. Is there adequate shade built in to the unit itself, or is it better to place the unit under cover if you can?

  2. #2
    Join Date
    Jul 2011
    Location
    Croatia
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    51
    You could have some small heat accumulation in sheet metal casing, but there is air gap between casing and condenser coil anyhow, and it is sheer air temperature which directly affects cycle effectiveness, while heat radiation is separate process and rather negligible in this case.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,589
    No cover needed. And often it can do more harm then good.
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  4. #4
    Join Date
    Jun 2011
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    21
    Yeah I have read that building a cover over the unit can cause hot air to accumulate around the unit rather than blow away, but what about just putting it on the north side of the building rather than on the south or the roof? It doesn't seem like many people take that in to consideration so I guess my question is why not?

  5. #5
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    Jan 2004
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    North or South side has little to no effect.
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  6. #6
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    Jun 2011
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    21
    why not?

  7. #7
    Join Date
    Jan 2004
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    Not really effected by solar gain. While the indoor unit moves around 400 CFM per ton. The outdoor unit moves 800 to 1200 CFM per ton.
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  8. #8
    Join Date
    Feb 2010
    Location
    Ontario Canada
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    I have always taken into consideration which side of the house gets more sun and tried to put the condenser in the shade. It just makes sense to have just the ambient air cool as possible without the radiant heat added. This is why we try to find a parking space in the shade as to in the sun.
    I would not build an enclosure over or around any condenser as this would just cause problems.
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  9. #9
    Join Date
    Jun 2011
    Posts
    21
    My understanding was that it was moving all that air to keep the condensing coil cool. If it didn't blow the air away it would get too hot and the difference in temperature between the evaporator and the condenser would be greater, therefore pumping less heat and requiring the unit to have to run longer to get the house to the temperature you want it at.

    I guess given the amount of air being moved over the condensing coil it isn't necessarily made up with cooler air on the north side of the building, since the air comes in from warmer places. Still air in the shade is cooler than air in direct sunlight, but if you are blowing that air away, it might not be the case that the air that replaces it is any cooler on the north side of the building vs the south. That might explain why it is negligible.

  10. #10
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    Jan 2004
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    The air is not effected by sun light. IR heaters don't heat the air. They heat objects.
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  11. #11
    Join Date
    Jun 2011
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    21
    Sunlight must heat the air, why else would it be hotter during the day than it is at night? The question is how fast does this happen? If the process is sufficiently slow, the air on the shaded side of the house would be the same temperature as the air in direct sunlight, because the air would be circulated before any heat transfer takes place.

    Air temperature is measured in the shade because a thermometer in direct sunlight will absorb the heat from the sun directly and measure the temperature of the thermometer, which is higher than the temperature of the air. This leads me to believe that the sun heats air very slowly compared the speed at which it heats objects, and it would require a condition of much less air movement than you would have near a condensing unit to see any temperature difference between the air in a shaded area compared to the air in direct sunlight. Therefore shading the unit will have no effect.

    However the US Department of Energy does not agree (see link below), and claims that a condensing unit in the shade can use up to 10% less energy than the same unit in direct sunlight. Interesting.

    http://www.energysavers.gov/tips/air_conditioners.cfm

  12. #12
    Join Date
    Jun 2011
    Posts
    21
    So the guys hvac pro forums (google it, it won't let me post a link) have convinced me that it isn't the air temperature that matters but the radiant heat transfer from the sun to the coil itself, which explains the DOE's claim that it is best to shade the unit in a way that doesn't prevent airflow.

  13. #13
    Join Date
    Jan 2004
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    Lancaster PA
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    The sun warms the objects. The ground, parking lots, roofs, etc.

    Its cooler at night because there is no radiant heat coming from the sun to heat those objects anymore, and they then in turn give off the heat they absorbed.

    If it really made a difference, the heat pumps should be set on the south/south west side.
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