Restricted Airflow

From John Muller
Boca Raton, Fla.

Manufacturers' instructions supplied with residential central air conditioners require, or recommend, the minimum acceptable clearance distances between the air-cooled condenser and nearby obstacles which might interfere with the flow of air into or out of the condenser. For the high-SEER models today, most manufacturers call for a separation of 12 inches between the condenser and any nearby wall, wood fence, shrub, or tree. Where several condensers are installed together on a slab, a face-to-face separation of 24 inches is called for.

When an old, worn-out compressor is replaced with a new model, the typical dealer installs the new condenser on the same slab or pad that held the old unit without giving consideration to:

  • The larger physical size of the high-SEER condenser.

  • The greater separation distances required by the new high-SEER condenser.

  • The growth of nearby shrubs and trees, which has taken place near where the old condenser was installed.

    On slabs near my home, it is not unusual to see a new, high-SEER condenser installed with one side literally up against a dense screen hedge. Nor is it unusual to see a five-inch, face-to-face separation between adjacent condensers on a slab.

    Perhaps each of these conditions causes only a small drop in actual SEER. But if one condenser is afflicted with both conditions, as is sometimes the case, the effect upon performance may be considerable. A mechanical inspector expressed the opinion that the new condenser's actual performance was probably no better than that of the 6- to 8-SEER unit it replaced. There is no hard data to indicate whether this assertion was an exaggeration or not.

    When attention is called to the discrepancy between the actual clearance at the site and the clearance called for by the manufacturer, the typical dealer's response is that restricted airflow causes little or no efficiency loss. However, no test data or technical paper are ever offered to support this assertion.

    By Daniel Kramer, P.E.
    Specialist Grade Member of RSES

    I made some tests on my own 5-ton residential air conditioner and your estimate of change in EER is not unreasonable.

    My condenser has a flat face and is 44 inches long and 24 inches high. It has two 20-inch fans, each driven by a 1/6th-hp shaded pole motor. With a 90 degrees F outside temperature, the discharge pressure was 115.5 psig (115.5 degrees F, 25.5 degrees TD), and the suction was 67 psig (39 degrees). I covered one-third of the condenser width (14.5 inches) with newspaper and the head rose to 285 psig (127 degrees F, 37 degrees TD), the suction to 72 psig (42 degrees). I used a new service gauge and an alcohol-in-glass, 6-inch-long service thermometer.

    This had to be a worst case, since pushing a three-sided unit against a wall or against a bush could not stop the airflow as effectively as my newspaper drawn tight against the condenser face.

    Under the condenser's partly blocked conditions, the entering air TD rose about 45 percent. This is somewhat more than I might have expected. The increase in TD was probably aggravated by the reduction in airflow across the condenser, caused by the shaded pole motors slowing down under the increased air resistance from the blocked coil.

    I estimate a capacity loss of only about 10 percent. The energy consumption might rise about 15 percent.

    While I encountered no rise in inlet air temperature from air recirculation under the blocked condition, I would not be surprised if placing units side-by-side, or with their air inlet sides adjacent a wall or the air inlet side of another unit, could have just such an effect from the reduced air pressure at that location, further worsening the capacity and EER reduction problem.

    I hope these tests and comments help you in persuading installers to provide extra room for air circulation on these units.

    If you have a technical question, fax it to 847-622-7266 or submit it online by visiting The News' Extra Edition page and clicking on The Hotline link in the left-hand column.

    Publication date: 02/07/2005