This paper has put forward the method of the functioning-fuzzy-subset inference (FFSI) for the real-time fuzzy control process based on the principle of the generating, transferring and receiving of the true value in the fuzzy logic reasoning. Then the reasoning results of the compositional rule of inference (CRI) and FFSI have been compared in order to analyze the performance of the FFSI. The FFSI will be the basis of the real-time fuzzy control in theory for the HVAC systems and equipments.