In the absence of people infected in the home, there can be no COVID-19 in the air. Therefore switching on the heating and air-conditioning systems does not affect the risk of contagion in any way. If there are infected people in the house, then their presence would determine the risk. However, the risk of COVID-19 in the air would not increase due to the operation of the HVAC system. In this case, the occupants in the living space must take all the necessary precautions regarding personal protection and behaviour (e.g., social distancing). Using the system or not is therefore only a question of the comfort level regarding the temperature to be kept in the living space.  Ultimately it is the doctor who must decide what to do about the infected individual(s). In any situation involving infected individual(s), the premises must be as ventilated as much as possible. In the absence of a DOAS (dedicated outdoor air system) with adequate outdoor air flow rate, it is necessary to ventilate by keeping the windows open as much as possible. This including sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection, and avoidance of air recirculation and avoiding overcrowding.