The other factor is that since the warm air goes up, and here the heater first heats the object (human), the energy loss will be much lower in comparison with the standard convective heater. Far infrared heater does NOT carry up air dust currents that creates breathing problems for people with breathing diseases (like asthma, bronchitis, allergy etc.) due to its characteristic of having no contact of the heating element with the air (it is hermetically covered with film), that is why, it does not need oxygen to heat (as many standard heaters do). So, you have clean air in the room and do not have to open the windows often in order to let in the oxygen, which, of course, will tend to the heat loss.