Just for fun II

Let's say I have a closed room with insulated walls and the sun isn't shining in the windows, if there are any. It's a pretty typical room.

If I leave a fan running in this room, what will happen to the air temperature?
  • rise because the fan is stirring the air
  • stay the same because it's a fan, not a heater
  • cool, since moving air is cooler.
  • something else
Answer this evening.

Answer: Bob is right. The electrical energy the fan is using is used to do work on the air, adding energy to the air, raising its temperature.

I did a calculation. Estimating my apartment as 375 square feet with 7.5' tall ceilings, I calculated that adding 100 W for an hour would raise the temperature by 6.9°F. With the ceiling fans running and all the other things plugged in, there's easily that much power being consumed.

Also something interesting is that when they design HVAC for buildings, the average heat load for a person they use is 450 BTU/person/hour. Converting out of stupid units, each person is putting out about 130 W.

Another calculation: 130W * 3600 s/hr * 24 hr/day = 11232 kJ/day.

11232 kJ/day ÷ 4.184 J/sci-cal ÷ 1000 scientific calories/dietary calorie = 2685 dietary calories/day you'd have to eat to make 130 W of heat continuously throughout the day. It's a pretty good estimation since an average person has a metabolism of 1500-2500 Cal/day.