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Cooling a room with A/C (w or w/o a fan)

  1. Jul 23, 2008 #1
    Hi, I was lying in bed unable to get back to sleep and I was thinking about something, remembered this forum, and thought I'd post it up here. Hopefully this topic is suitable for this forum.

    I installed a ceiling fan in my bedroom yesterday. Its a nice 44in Hunter fan and I'm pretty happy with how it looks and the air it pushes while being very quiet. I had been using a window air conditioning unit or a portable fan (depending on temperature) to keep me cool when sleeping and was looking forward to using the ceiling fan. I figured it would replace the portable fan in most situations (though having directional air is nice). I also thought because its quiet I would try to use it with the window A/C and thats what I did last night. We were in the 90s yesterday with high humidity, dipping down mid 70s at night and thats definitely A/C weather if I want good sleep. Long story short, I wasn't comfortable. Now there could be other factors at play here. Perhaps it was warmer or more humid than normal. Maybe I didn't let the room cool down enough before bed. Or maybe the thermostat wasn't set as low. So, this could be a meaningless anecdote but it got me thinking about that ceiling fan and here I am.

    I got out MS Paint and made some pictures.

    [​IMG]

    Thats a fairly decent approximation of my bedroom, complete with ceiling fan, a/c unit, and bed. It shows airflow and what I think the temperature in the room is. The exact value isn't important, the fact that it should be fairly uniform due to the ceiling fan is what is important.

    [​IMG]

    This shows what I think would happen without a ceiling fan. The average temperature is going to be more or less the same for both situations since there is a thermostat on the A/C unit and it isn't operating beyond capacity (i.e. it is cycling). The difference I think is that without the ceiling fan, the air is much more stratified. The A/C unit is fairly low (just above the level of my bed). It pulls in hot air at the top and pushes out cooler air from the bottom. Oops, I just realized that I have that backwards. Its actually the reverse. Pretend that I've fixed my pictures. I think this would tend to keep the air towards the top of the room relatively undisturbed and therefore warmer. Now here's my point. I don't care if the top half of the room is warmer. I'm sleeping in the bottom half. And if the average temperature is the same for both situations, that means that the lower half in the second scenario must be cooler than it is in the first scenario.

    So, my theory is that a ceiling fan is actually counterproductive in this situation. I'm ignoring evaporative effects from the ceiling fan because I have a sheet over most of my body. It may be a different story otherwise. What do you guys think? Is my reasoning sound?
     
  2. jcsd
  3. Jul 23, 2008 #2

    LURCH

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    Science Advisor

    I think your assesment is essentially correct, with one exception; the average temp of the room will not be the same. The air should indeed stratify but, with the thermostat attatched to the AC unit, the unit will run untill the cooler at the bottom of the room reaches the height of the AC unit (the bottom of the window). At this point, the AC will shut off with the bottom of the room at the same temp that the whole room would reach if the cieling fan were on. The upper portion of the room would be warmer, so the overall average would be higher. This means the AC is cooling the part of the room you care about, while doing less overall work.

    But without the fan, there should be an additional temperature gradiant moving from the left side of the room to the right. In addition to these factors, it is usefull to take into account the heat generated by the fan motor.
     
  4. Jul 23, 2008 #3
    Thanks for the response. I didn't consider that.
     
  5. Jul 23, 2008 #4

    russ_watters

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    Staff: Mentor

    Another thing to consider, adding convection in the room (via the fan) will pull more heat off the surface of the ceiling than if you leave it stratified, which increases the A/C load even more.
     
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