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Air conditioning units nowadays allow you to switch it from "off" to various levels of "cool" without requiring you to "fan" setting first.

Is there any reason to use "fan" setting (such as for example, to increase lifespan)? And why is it normally situated between "off" and "cool"? Is the purpose similar to idling a car, so to speak?

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I don't have any citations, so I'm not posting this as an answer, but I believe it's because an air conditioner has two mechanisms: A fan and a cooler. The fan can be ran without the cooler, but not the other way around. Therefore, as far as design, it's simplest to have the switch proceed from off -> 1 system on -> both systems on. I don't believe there's any harm in going directly to cooling. –  kbelder Feb 11 '13 at 16:56
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migrated from skeptics.stackexchange.com Feb 12 '13 at 3:38

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2 Answers

up vote 4 down vote accepted

Typically the blower motor or fan on a forced air AC or HVAC system only run when the AC is running (this is also referred to as the "auto" mode). Turning the fan to the on position only runs the fan, but not necessarily the AC itself, giving you some air circulation, without the extra power required to run the AC compressor.

With a window unit, you also have the option to allow outside air in with the fan, which can cool a warm room if it's cooler outside, without running the AC at all. The fan is also useful for homes where some rooms get especially hot between the AC cycles, so you can constantly run the fan instead of lowering the temperature on the thermostat, but if you do this for extended periods, I'd suspect it actually increases power consumption rather than decreases it.

As for whether you would damage an AC by skipping the fan setting, that would be highly unlikely since the fan automatically turns on when the AC is running. What you can do to damage an AC is rapidly turn it on and back off, since this can result in a pressure differential in the coolant lines that could prevent the compressor from starting normally, burning out the motor, and it could be in a state where the lubricating oils in the coolant are not at the compressor motor where it's needed (this is also why you don't turn a refrigerator on immediately after it's been tilted from shipping).

As for why the fan setting is found between off and cool on some thermostats, this is likely for convenience. It's either off, you want fresh air, or you want fresh air and cooling.

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Ah, got it. For someone living in a tropical region though, I would have no reason to blow air from outside the room. –  Aldrich Co Feb 12 '13 at 4:26
    
It's just circulating air in your house - unless you have a heat exchanger, its unlikely to be pulling in air from outside –  Steven Feb 12 '13 at 15:12
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What you're describing are the modes of a thermostat and not the A/C unit itself. In some A/C's, the thermostat is part of the unit, but with central A/C and a forced air system, typically you have an external thermostat. Some of these are very basic and require you to toggle multiple switches while others are more sophisticated and can automatically toggle between heat/cold, fan modes, etc. You've probably heard of the Nest thermostat which takes this automation to another level.

With a central A/C/forced air system, in order for the cold/hot air to circulate, it requires the fan in the furnace to run; this is what the thermostats "fan" setting controls. The compressor unit also has a fan but this will run whenever the compressor is running and is not controlled by the thermostat.

Reasons to use just the fan setting include helping to circulate the air for temperature control, or to move more air through an air filter.

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Would the rationale for circulating the air in a warm climate make any sense? Given that you can have the option of insteading Cool-ing the room while doing so. –  Aldrich Co Feb 12 '13 at 4:28
    
Of course - quite often you just want to move air. Even in a tropical environment it can help evaporative cooling. –  Rory Alsop Feb 12 '13 at 9:56
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