The ultrasonic welding process allows economically efficient and safe for the workers waterproof joining of textile materials and is applied in the production of various technical textiles, protective clothing, and other types of textile products. In this latest research work, a flexible, textile-based heating element is produced by the connection of waterproof laminated textiles using ultrasonic technique. This novel product allows completely new options for the design of flexible heating and cooling elements for rooms and various other applications but requires high quality seams. This work starts with the theoretical background about the heating of the textile until reaching the welding temperature within the welding zone. The heat flow through the volume of the welded textiles is investigated numerically using Finite Element Method software. The simulation is performed for different sizes and configurations of the seam allowances, so that options between overlapping only in the pressed area without side allowance (no free allowance), small allowance, and larger allowance are simulated. These simulations demonstrate different distributions of the temperature in the seam cross-section at the different configurations, because of the different amounts of textile material around the welding place.