This paper discusses the specific needs for a design tool for ventilation and cooling of the deep geologic repository (DGR) facilities during construction, emplacement and potential re-entry of the access tunnels. Two examples are shown for two different designs of the DGR, in a typically cold and a hot geographic area. An advanced numerical model is used with the capabilities of simulating Airflow, Temperature, Humidity and Contaminant (ATHC) distributions in a dynamically changing environment, previously qualified for Yucca Mountain conceptual DGR ventilation studies. This paper focuses on the results, showing that the monthly, weekly and even daily temperature variations in the outside weather may significantly affect the air temperature around the workers in the tunnels of the DGR. The main conclusion of the study is that it is highly advisable to include the thermal history in the thermal model to reasonably predict the working temperature and humidity conditions in long drifts in the DGR, ventilated by the seasonally-varying intake temperature.