Abstract: Vehicles, environmental control systems, and methods for operating an environmental control system are provided. In one example, the environmental control system (ECS) includes an ECS refrigeration unit that is configured to receive ambient air and a first portion and a second portion of hot bleed air. The ECS refrigeration unit is operable to indirectly exchange heat between the first portion of the hot bleed air and the ambient air to form a partially cooled, hot air stream, and to compress, further indirect heat exchange, and expand the partially cooled, hot air stream to form a cooled and expanded air stream. A low limit valve control regulates a low limit valve to control a rate of introduction of the second portion of the hot bleed air to the cooled and expanded air stream to form a combined air stream that when exiting the ECS refrigeration unit is a sub-freezing air stream.