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We have a two story house, with a small ceiling fan mounted above the main stairway.

The house has separate thermostats for heating/cooling the upstairs and downstairs.

During the day, we spend 90% of the time downstairs. At night, everyone sleeps upstairs.

Given this setup, can we use the ceiling fan to save energy and/or make the house more comfortable throughout the day? How should we use the fan differently during the day and night? During summer and winter?

I'd love to be able to turn down the thermostats to save a few bucks, or to make the house feel more comfortable.

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I know this is not the question you are asking, but with your setup, a programmable thermostat will help you greatly with energy cost. –  Kellenjb Mar 26 '12 at 16:49
    
@Kellenjb I agree. Our downstairs thermostat is programmable; I should replace the upstairs one so it is also. –  The Other Steven Mar 26 '12 at 16:52
    
Have you tried turning the fan on your furnace so that it circulates the air (turn it to ON instead of AUTO)? I run my furnace fan 24/7 in the summer to keep the air circulating and also to keep dust/pollen down since the air passes through the filter more often –  Steven Mar 26 '12 at 18:38
    
@Steven That's probably good advice, depending on where one's registers and air-return are. We have completely separate systems and ducts upstairs and down, so I don't know if it will help our specific case. –  The Other Steven Mar 27 '12 at 17:04
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2 Answers

up vote 5 down vote accepted

Much like DA01 said, a ceiling fan generally acts to circulate air within a particular space of the home. This evens out the temperature of the air in the room by preventing "stratification" (where the air settles into noticeable "layers" so it's warmer at head height than at the floor), and also provides an illusion that the air is cooler by constantly blowing away the "bubble" of insulating warm air that your warm-blooded body will maintain around itself (also speeding evaporation of sweat).

Unlike your forced-air ventilation, the ability of a ceiling fan, or any non-ducted fan, to "push" a column of air into a different area is limited by the buffering effect that the standing air will have on the air pushed by the fan. A forced-air system uses ducts, within which air ideally only moves in one direction; supply vents carry conditioned (heated/cooled) air out to the home, while one or more returns draw in the air that will be heated/cooled. This generally forces the air to move in a large loop through your home from supply ducts to returns through the rooms.

A ceiling fan or other "open-air" ventilation doesn't have these ducts; the air that is pushed forward by the fan must be replaced by air moving within the same space as the pushed air. This conflicting movement of air creates eddies that reduce the effect of the fan as you move further from it. Within an enclosed space like a room, this effect isn't so bad, but trying to move air through a hallway, or up or down a staircase, will not be effective.

Your dual-zone HVAC is your best tool for achieving the proper temperature on each floor of your home. You'll just have to remember that warm air rises, cool air descends, so in warm weather your upstairs A/C will have to work harder, because it will be losing some of the cool air it produces to the downstairs area and will receive the warm air from both floors. The opposite is true in winter; the downstairs heater will effectively be heating both floors, and will have to work harder to heat the downstairs as all the cool air will work its way down.

Instead of trying to mix the air between floors, you might actually have better luck trying to prevent the two zones from exchanging air, using a door, curtain or other insulating break at the top of the upstairs staircase. By doing this, you reduce the amount of cool air that the upstairs will lose and the amount of warm air it will gain. This will allow the upstairs zone to cool its own space without also having to cool the downstairs, and vice-versa for heating in winter. Of course, this will only work if you have separate air returns for upstairs and down; if you only have one air return, then air will be actively pulled toward that return, wherever it is, and regardless of what's in the way.

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A ceiling fan does two things:

  • provide breeze
  • circulate air

It's primary used for the latter.

In your case, it may help by evening the temperature out, though to what extent is hard to say given the size of the fan and the size of the space.

Ideally, you'd handle the upstairs/downstairs with a split zone HVAC system. You'd then add programmable thermostats to both.

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