What is a possible cause of lower than normal suction pressure in an SB-230 without an ETV?

The situation of lower than normal suction pressure in an SB-230 without an Electronic Throttle Valve (ETV) can indeed stem from several issues, all of which can significantly obstruct the flow of refrigerant and affect the system's efficiency.

When an evaporator coil is heavily frosted, it suggests that the coil is not able to effectively absorb heat from the air. Frost build-up typically occurs due to inadequate airflow or refrigerant flow, causing the coil to drop in temperature below freezing, leading to ice formation on its surface. This ice can insulate the evaporator, impeding the heat exchange process and resulting in lower suction pressure.

Similarly, if there is dirt or debris accumulation on the evaporator coil, it can obstruct airflow, which can also dramatically reduce the system's efficiency. Dirty coils hinder the proper transfer of heat and can further lead to suction pressure drops, as the refrigerant struggles to evaporate properly in the coil.

A stuck closed damper door would prevent adequate airflow into the evaporator. This lack of airflow means that the evaporator has little to no heat to absorb, which contributes to decreased suction pressure.

Considering all these factors together, it becomes clear that each of the potential causes mentioned has a direct impact on the