Central AC: Fan on continuously or Auto?

tanamaJune 1, 2007

Please help resolve an arguement where neither person knows what the heck they're talking about.

If you have central AC, is it best, in terms of keeping the house cool and minimizing how often the compresser kicks on, to leave the fan running continuously, or have it set to Auto so that it only runs if the cooling unit is running?

Also, if you are doing everything to keep from having to run the AC while the humidity is still low enough to do that (close all shades and downstairs windows, vent the attic, stuff like that), is there a cooling benefit to running the central AC's fan continuously?

Finally, we have two systems, one for just the first floor, and one for the 2nd/3rd floors. Does the answer differ according to the location of the system - as in, for example, there's benefit to running the first floor fan continuously but not the 2nd/3rd floor fan?

Thanks for your input!

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There is a third option

Many of the new digital thermostats have a program where the fan runs about 1/3rd of the time, circulating air through the filtration system even though there is no call for either heat or cooling. My Honewell does this and we really like it


    Bookmark   June 1, 2007 at 10:30PM
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In a dry climate, you can run the fan "on" with little problems in the gain of humidity.
In more humid climates, running the fan "on" can cause the humidity in the home to rise to a point your uncomfortable at any temperature and/or experience a damp dungeon for a home.
The ducts can also act as a heat exchanger while the fan is running and transfer what ever heat transfers into the duct into the home. Just turn on the fan once, after the system has been idle for 30 minutes and feel the first few seconds of air from the outlets- that's heat from the attic getting into the home. If your attic isn't very hot and the ducts insulated well, you may not notice it much.

    Bookmark   June 2, 2007 at 1:21AM
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Your first floor, is it above grade?

Do you have open stairwells, high ceilings?

What you should do is buy a few cheap thermometers and place them in various locations. Try different settings on the two units and record temps (humidity too if you invest in that kind of measuring device also) over a period of time.

What kind of filters do you have/use? How old are the units? Do you have fresh air intakes on the systems?

Think of a box, 3 layers high. Seal all air entry points.
add a small motor with a filter. The only air that will circulate will be the air in the boxes but it will be filtered. Add some chemicals, dust, dirt, people, pets, etc and then products of them will enter the air and be filtered out if the filter is fine enough.

If you put the motor/filter in the upper two boxes I would guess that mostly the air up there would be recirculated and the lower box would have stagnated air. Although the natural draw and physics of heat rises and cool air falls could be a factor. (the stairwells connecting the boxes).

Running a blower motor continually in most large many floor type dwellings seems to equalize temps. Hotter upstairs, cooler basements. Since most homes are not air tight, (doors open/close, gaps in joints etc), there would be some outside air getting in. (Unlike the sealed box). An added benefit can be better filtration of the air being circulated. A drawback could be humidity but if the t'stat is set for a desired temp, then the a/c unit would come on and while running, be removing humidity.

Does this make sense?

    Bookmark   June 2, 2007 at 5:56AM
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I was also wondering about this.

I'm debating whether to keep the fan running all the time, while the air conditioner only kicks in when the temp goes above a certain point.

This is a small bungalow and the basement is cooler. It makes sense to circulate that cooler air through the house, assuming then that the A/C won't need to work as much.

My main question:
is this efficient? Does it cost more to run the fan all the time, possibly reducing the load on the A/C, than to just have it cycle on and off with the A/C?

Also, when no air is moving in the house, things can sometimes get a little basmenty-smell (60-year old house). Running the fan (even when no heat or A/C is on) seems to help circulate the air, whether or not the windows are open. This seems contradictory, because of the basement-smell but the fan does seem to make a difference in air quality. I have an electronic air cleaner on the furnace, no filters.

The replies above mention that running the fan increases humidity. Why?

    Bookmark   June 2, 2007 at 12:19PM
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If the fan draws 6 amps at 115 volts and the cost of electricity is $.085 per kWh, the cost of electricity would be $1.41 per day or $42.24 per month.

Personally, I think it is a waste of energy and money.

    Bookmark   June 5, 2007 at 1:52AM
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I run the fan from 5 PM (when I come home from work) to 6 AM (when I leave for work) weekdays in the summer time. I find that it does a few things. First, it does help a little with evening-out temps between floors. Secondly, it drowns out some of the street noise. Thirdly (and most importantly), it keeps the queen away from the thermostat since she can't hear if the compressor has cycled off.

"The replies above mention that running the fan increases humidity. Why?"
- There may still be some condensate on the coil that hasn't gotten a chance to drip down into the drain pan. This may be picked up by the air blown from the fan and re-introduced into the ductwork.

    Bookmark   June 5, 2007 at 1:55PM
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I would like to hear some more opinions on this subject. I have a 2100 square ft 2-story home with a large open foyer(wifes idea). Recently I have been running the fan in the on position from 8pm to 6am to keep our upstairs bedroom cooler.

I also wonder if this makes more sense than turning down the thermostat.

    Bookmark   July 18, 2007 at 9:19PM
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If it helps, then yes. The blower takes less power than the compressor ... altho running the blower to move the warmer upstairs air down (your thermostat is downstairs?) is kinda the same as turning the 'stat down ... but not really.

    Bookmark   July 19, 2007 at 1:19AM
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"If the fan draws 6 amps at 115 volts..."

You cannot simply multiply current and voltage for an inductive load like a motor and get power used.
The missing item is the power factor of the motor, a number always less than 1.

As already noted, in high humidity areas running the fan will put water from the evaporator coil and drain pan back into the house raising the humidity.
It will also tend to even out the interior conditions.

In low humidity areas there may not be enough water to cause a significant rise in humidity.

The cost may also be affected by having to remove the humidity again.

Try it both ways and see what you prefer.

    Bookmark   July 19, 2007 at 8:52AM
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i just wanted to chime in on this ...

i live in the hot and humid east cost of the US ... if you want a shock ... buy a relative humidity gauge and put it in the outflow air duct of your home

Then after the compressor has been running for a long time on a hot and humid day ... turn the AC off and leave the fan ON ... and watch your RH gauge.

You will see the humidity of the air coming out of your air vent EXPLODE from a pleasant 45%-55% to a humid 75-85% in about 2 minutes as all the moisture sitting on your coil re-evaporates into the air blowing though your ductwork.

One bad thing is most manufacturers of AC have a blower-off delay of 90 seconds ... just enough time to push all that humid air throughout your duct work ... before the fan finally shuts off ... they should design AC system to shut the fan down immediately after the compresser shuts off to prevent the buildup of humid air in your duct work which can lead to mold growth under some conditions.

If you live in a humid climate always leave the fan on AUTO when running the AC.

If you need more air circulation or noise buy some nice house fans

    Bookmark   August 29, 2010 at 11:35PM
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