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©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 1, January 2019.
By George Games, P.E., Member ASHRAE
About the Author
George Games, P.E., is a design engineer and owner/principal of North East HVAC Engineering and Consulting, LLC, in East Brunswick, N.J.
In 2017, I wrote an article in the June issue of ASHRAE Journal about problematic installations of split system air-cooled DX condensing units. Here, I expand a bit on the same topic and offer a few more photo examples of other problematic installations. As with the previous article, what follows is a qualitative discussion supported by photos and a figure.
The previous article focused on the adverse effect on performance and cooling airflow through the units due to poor layout configurations. The poor layout configurations mentioned in that article included those due to units mounted too close to each other or too close to walls. It spoke to maintaining minimum space clearances around the units for proper cooling airflow.
Unit Airflow Quantity
Unimpeded airflow thru a condensing unit is extremely important as the amount of airflow the unit moves across its condenser coil surfaces is one factor that determines if the unit delivers its cooling capacity or not. The units are designed at the factory to move the selected “design” air quantity across its coils as required to achieve its intended rated capacity. This is done by selecting the right size fan and motor going into the unit. Various factors including the size of fan (in overall diameter), fan blade pitch, the number of fan blades and fan rpm determine the amount of airflow required by the unit. Also, there are considerations taken at the factory of how obstructions might affect the amount of this airflow moving through the unit. At the factory, it is assumed that obstructions external to the unit that might block or impede airflow, will largely be non-existent at the installed site. Therefore, the factory always specifies that the unit be installed with minimum space clearances (around the unit and above it) to ensure any obstructions do not affect, lower or block the required airflow from entering or leaving the unit while operating. These minimum clearances are found in the equipment’s installing manuals. They must be followed.
As an example, two excerpts of installation instructions, from a major OEM follow. The first excerpt pertains to larger tonnage units and the second for smaller tonnage units.
For units sized from 10.4 to 25 tons (36.6 to 87.9 L/s):
- “1. Minimum clearance for proper operation is 36” (914.4) from walls, shrubbery, privacy fences, etc. Minimum clearance between adjacent units is 72” (1828.8). Recommended service clearance 48” (1219.2).
- 2. Top discharge area should be unrestricted for 100” (2540) minimum. Unit should be placed so roof run-off water does not pour directly on unit.
- 3. Outdoor air temperature sensor opening (do not block opening).”
For units sized from 1.5 to 10 tons (5.3 to 35 L/s):
“Top discharge area should be unrestricted for at least 1524 (5 feet) above unit. Unit should be placed so roof run-off water does not pour directly on unit, and should be at least 305 (12”) from wall, and all surrounding shrubbery on two sides, other two sides unrestricted.”
Thus, any obstruction or blockage anywhere, upstream of the unit, or downstream of it will affect the amount of airflow through the unit. It’s a simple relationship; a fan encountering a resistance greater than designed for, will ride up on its operating curve and deliver less air—greater resistance equals less flow.
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