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The AC guy that fixed my AC recently said I should turn my AC fan on 24/7 and this would increase the efficiency of my AC system. I looked it up and found this same claim in several places:

Several places say to use ceiling fans to circulate air as it makes the air "feel" cooler but do not increase the efficiency of the AC unit:

Use a ceiling fan or portable fan to supplement your air conditioning. A fan can make you feel three to four degrees cooler (and only costs a half-cent per hour to operate) so you can set your thermostat a few degrees higher and save on cooling costs. Use in occupied rooms since fans cool people, not rooms.2

If you choose to operate your fan and your AC simultaneously, you will be comfortable at a higher thermostat setting because of the cooling effect of moving air. Higher thermostat settings result in energy savings.3

To clarify, I'm talking about the fan system in the AC unit running, and not the compressor running.

I'm skeptical of everyone's claim that it would increase the efficiency of the AC unit considering it's not introducing cooler air into the house if it is 105 degrees outside.

Is leaving the AC fan on 24/7 more efficient than leaving it on auto?

To further clarify, this is with regard to a central system in houses and not a window unit. Although I think this same question could probably apply to window units.

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There are lots of factors here that you may need to clarify. Does the fan suck air from inside or outside the house? Are you happy to have only some room cooled or all of them? Are you only talking about mid-summer, or the whole year? How humid is the air (high humidity will reduce effect of evaporative cooling on skin)? –  Oddthinking Jun 28 '11 at 4:22
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The Kansas State Uni link (third one) claims dehumidifying the air a second time takes more energy. I am dubious that this is significant, especially as the evaporation caused the a/c unit to be cooled down. –  Oddthinking Jun 28 '11 at 4:27
    
Should assume that this is a window unit? –  erekalper Jun 28 '11 at 12:05
    
So what if it's more efficient? It still uses tons of electricity!!!! –  Thursagen Jun 28 '11 at 12:28
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@Ham and Bacon: "tons of electricity" is not the best classification there, especially considering that the question is talking about leaving the AC fan on, not the AC pump. Just the fan feature would probably use at most 10% of the normal AC functionality. And depending on the window model you have, this could easily require less power than working on your computer all day, and certainly much less than just keeping your food cold in the fridge. –  erekalper Jun 28 '11 at 13:20
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1 Answer

up vote 16 down vote accepted

The answer seems to be "it depends". Given the commerciality and competitive nature of the HVAC industry, I had a lot of trouble finding peer reviewed academic studies that test your claims. Therefore, I took a look at the patent records for evidence, which led me to US Patent #5492273. This patent, owned by General Electric, describes a variable-speed blower that can be controlled independently of the compressor. Here are some claims made in the background section of the patent:

In general, total heating of air conditioning capacity of a system increases as the indoor air flow rate increases. However, upon reaching a particular air flow rate, the blown air will be less warm or less cool, depending on the mode of the HVAC system, than the air in the room. For air conditioning systems, the indoor coil, or evaporator, tends to "saturate". In other words, the the coil extracts incrementally less energy from the air for equal incremental changes in air flow. Further, the electric power consumed by the indoor blower motor increases rapidly as the air flow rate increases. The system dissipates electric energy as heat in the same air that is being cooled by the system thereby reducing its net cooling capacity. Due to the effects of coil saturation and blower motor power consumption, net total capacity peaks at a certain airflow rate.

Let me briefly summarize the salient points of that paragraph before continuing:

  1. For traditional, single-blower-speed air conditioning systems, the efficiency of the system does not scale linearly with air flow speed.
  2. The blower motor's electrical usage scales superlinearly with respect to air speed.
  3. The faster the air speed the more the coil heats up, thus reducing its cooling efficiency the next time the condenser turns on:

an increase in the air flow rate beyond what is required warms the evaporator coil to produce a higher load on the compressor motor.

The patent goes on to explain that the reason why most traditional, single-speed blowers don't keep the blower on after the condenser has turned off is exactly because of point #3: The airflow will heat up the coil and require the condenser to remain on longer during the next cycle. The patent also claims that some traditional systems try and avoid this by only switching off the compressor when in blower-only mode (i.e., also keeping pumping the refrigerant), however, the patent says that that uses almost as much electricity as running the entire system:

Conventional systems ... [delay] turning off the indoor blower fan after after the compressor cycles off. However, the system also continues to draw power at the same level as during the cycle and causes additional energy to be consumed in the indoor fan motor.
[Emphasis is mine.]

This claim is a bit surprising. The patent could of course be embellishing that claim in order to make its technology seem more useful.

The patent describes a variable speed blower motor that can work independently of the condenser to set the air flow to the optimal speed for cooling. The thermostat, with information from both indoor and outdoor temperature and humidity sensors, can then intelligently decide how to control both the blower and the condenser.

Therefore, I'd say that if you have one of these variable-speed blowers and intelligent thermostats (they're getting more common now; I have one in a house I recently bought), I'd say that it would be a bad idea to force the blower to stay on all the time because the system is smarter than you! If, however, you have an older, single-speed system, that question is a lot more complicated, and it might actually be detrimental to run the blower all of the time.

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This seems to make much more sense than any other answer I've read. Thank you! –  Sklivvz Jul 21 '11 at 13:57
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Running without the compressor (while maintaining the fan and coolant pump) uses almost the same amount of electricity? That astonishes me. –  Oddthinking Jul 21 '11 at 14:38
    
@Oddthinking: It surprised me too, but that's what the patent claims. The patent could of course be embellishing that claim in order to make its technology seem more useful. –  ESultanik Jul 21 '11 at 14:41
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"Conventional systems ... [delay] turning off the indoor blower fan after after the compressor cycles off. However, the system also continues to draw power at the same level as during the cycle and causes additional energy to be consumed in the indoor fan motor." [Emphasis is mine.] –  ESultanik Jul 21 '11 at 14:45
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Oh, you mean I had to follow the link to understand better? Oops! Sorry. –  Oddthinking Jul 21 '11 at 14:53
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