Heat stress has always been a critical issue among firefighters. Previous studies have indicated the potential influence of water-repellent finishing on heat release properties of clothing. Nevertheless, the impact of durable water repellent in a thermal liner on firefighter heat stress has never been studied. In this work, the impact of durable water-repellent finishing inside the thermal liner on heat stress has been evaluated systematically for the first time, using a sweat-guarded hotplate, a sweating manikin, and a physiological manikin. The results showed that there was no significant difference in heat loss capabilities on the fabric level or garment level between turnout clothing with or without durable water-repellent finishing in the thermal liner. In addition, no significant differences were identified in predicted physiological responses when tested in a mild environment. However, the turnout clothing with durable water-repellent finishing in the thermal liner demonstrated significantly less weight gain after the physiological manikin test. This initial investigation lays the groundwork for understanding how durable water-repellent finishing in thermal liners may impact firefighter comfort. It acts as a starting point for further research on the potential impact of durable water-repellent finishing on thermal protection and overall comfort under various ambient conditions.