The heat sources for heating in existing buildings are transitioning from traditional high-temperature systems to medium- and low-temperature sources. While lowering the water supply temperature can reduce heat loss during distribution and improve energy efficiency, it may also introduce potential challenges such as insufficient heat supply and lower indoor temperatures. This study investigates the factors influencing the performance of low-temperature heating and explores methods for evaluating its feasibility. Focusing on existing buildings in industrial parks located in severe cold area, combines experimental testing with numerical simulation to examine the effect of supply air parameters (i.e., temperature and volume) on heating performance. Furthermore, a dimensionless effective draft temperature is proposed as an evaluation index. The concepts of “low-temperature disturbance zones” and “heating stability zones” are introduced, enabling the assessment of heating effectiveness based on the area proportion of the former. The results indicate that when the water supply temperature is reduced from 80 °C to 60 °C, the supply air temperature decreases by 1.9 °C, and the areas of the low-temperature disturbance zones in the horizontal and vertical directions decrease by 4.75% and 19.14%, respectively. These findings provide a reference for evaluating the feasibility of low-temperature heating retrofits in existing buildings.