Background Workers are vulnerable to heat stress when they perform tasks in hot environments. Personal cooling systems (PCSs) are designed to reduce heat strain. A cooling vest with ventilation fans is a type of PCS that can blow air around the body to facilitate convective and evaporative cooling. Objective This study aims to evaluate the performance of a ventilation unit for a tailor-made cooling vest designed to protect construction workers from heat stress. The performance of this newly designed ventilation unit (Unit B) was compared with that of the ventilation unit used in a commercially available cooling vest (Unit A). Method The designed ventilation unit consists of a pair of ventilation fans and a portable battery pack. A hot wire anemometer was used to measure the air flow rate of the fan. The air flow rate and work duration of different ventilation units were compared. The sweating manikin test was conducted to further compare the cooling powers of the ventilation units. Result Compared with Unit A powered by the alkaline AA battery (6 V, 2122 mAh), Unit B powered by the rechargeable lithium-polymer (Li-Po) battery (7.4 V, 4400 mAh) achieved a higher air flow rate (8–22 L/s vs. 0–13 L/s) and longer work duration (7.05 h vs. 6.22 h). The average cooling power of Unit B on the sweating manikin was 68 W, which was higher than that of Unit A (51 W). Practical applications Results showed that the newly designed ventilation unit exhibited higher air flow rate and cooling power. The ventilation unit designed in this study will be incorporated into a tailor-made PCS for protecting workers in the heat.