The heat-loss number is a dimensionless group that accounts for heat losses in the evaluation of calorimetric experiments. It is obtained with steady-state measurements at various temperatures in the range of interest. It was established that this parameter is highly dependent on the direction of temperature variation, which affects the amount of condensate on the walls. Most of the condensate is formed at the highest temperature during the test and it acts as insulation. Upon heating and then cooling, the heat loss number is significantly higher when the temperature is raised than when it is lowered, if the reactor is dry prior to the test. When the reactor walls are wet or the directions are reversed, i.e., cooling and then heating, there is no significant hysteresis, because the reactor walls are saturated with condensate in either direction.