Oversized Evap Coils? – Short #237

In this short podcast episode, Bryan answers a listener-submitted question about oversized evaporator coils, particularly when they’re half a ton or a full ton larger than the condenser.

The expanded performance data contains the information that’ll tell you whether the coil is right for the system. However, when it comes to the specific issue of oversizing, there is a common misconception that coil size affects the saturated suction temperature. The truth is that they’re not related; you could fully insulate different-sized coils and move the same amount of refrigerant, but they will behave similarly.

The saturated suction temperature (suction pressure, evap temperature) is driven by the heat entering the system. Coils are designed to handle refrigerant flow rates and airflows, so having a lower airflow over a large coil allows for greater cooling due to the bigger surface area, lower bypass factor, and dwell time. When more heat enters the evaporator coil, the saturated suction temperature increases, but some of that heat enters as latent heat and helps with dehumidification. However, a larger coil also takes longer to get colder, and the superheat settings become more important.

You have to be careful with lower face velocities across the evaporator coil, as laminar flow isn’t as good at transferring heat as turbulent flow. That is one of the limitations of using a larger coil with lower airflow. If the expanded performance data has a rated match for a larger coil, then that same data will also contain information about the required airflow. Testing in and testing out is also important.

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