TEKNOsim is a simple and user-friendly program for analyzing thermal indoor climate and performing annual hourly simulations. The program was originally developed to calculate heating and cooling demands, and indoor climate conditions of a single zone building for peak winter and summer design days. However, TEKNOsim version 6 has been revised to incorporate several new features including annual energy and multi-zone simulations. These features greatly enhance the functionality and the ability of the program. Their implementation in TEKNOsim 6, however, entails a validation procedure to test the accuracy of the new algorithms. This paper presents preliminary results of the ongoing testing and validation of TEKNOsim 6 according to CIBSE TM33 tests for software accreditation and verification. The test results indicate that the new database libraries implemented in TEKNOsim 6 can be easily customized to precisely represent the actual building conditions. The results also show that the program can accurately calculate the total steady-state heat losses for all test cases of CIBSE TM33. The peak cooling loads calculated by TEKNOsim 6 are also in good agreement with CIBSE TM33 for most test cases. Some discrepancies are however observed between TEKNOsim 6 and CIBSE TM33, especially for test cases involving certain building elements. Nevertheless, TEKNOsim 6 is shown to be a fast and reliable program for performing early-design stage simulations.