An ideal firefighter turnout system should be capable of releasing body heat to prevent heat stress, indicated by THL (total heat loss) and Ref (evaporative resistance) indexes, while maintaining high thermal protective performance (TPP). Our study found no correlations between THL and Ref, or between Ref and TPP. The results showed that, when tested in the mild condition as in the standard THL test method, turnout systems with bi-component moisture barriers exhibited an advantage in THL that did not translate to more thermally stressful hot environments. A physiological manikin was used to understand the effect of turnout clothing systems on heat strain in different environmental conditions and the value of utilizing Ref or THL to predict heat strain performance. We found no difference in heat strain performance between composites with one-layer and two-layer spunlace thermal liners in mild or hot conditions. It showed that both THL and Ref had their limitations: THL only predicted thermal burden in mild environments, while Ref was only correlated in hot conditions. Thus, the exclusive reliance on either index could increase the risk of heat stress, and we recommend incorporating the Ref heat strain index, along with THL, as dual metrics for certifying the heat strain performance of turnout suits in the NFPA 1971 standard.