AC efficiency question—using the fan

Factual Questions
1

If we run a room air conditioner all day, is there going to be a significant difference in energy use if we crank the fan all the way up or leave it down around a third or so? That is, not lowering the temperature, just upping the fan speed.

If it makes any difference, it’s a two-year-old 8,000 BTU Frigidaire, with the thermostat generally set to 73 or 75 degrees. The room is about 30 X 30, but it’s a home office so there are a couple odd shapes to the walls (dormers sticking out). There are two regular windows facing due north and south (one has the AC in it), and two smalish plant-filled semicircular windows that face west. The southern windows have blinds that are drawn during the day. There are two computers constantly running and generating heat (especially the one CRT monitor), and then there are the heat engines of Mrs. Dvl and I (and the two pooches and two cats), who spend all day in it.

So, what does “cranking up the AC” really mean? Does it mean turning the fan on high? I’d imagine that on it’s own, the internal fan/air mover doesn’t draw that much energy. But if it’s moving hot outside air over the coils that much faster, the unit has to compress/cycle the gas more often (I know very little about HVAC, so forgive me if I have that wrong).

Or would high energy costs come from setting the temperature of the unit down to sixty or so and letting it run all day like that?

Would it make a difference if the AC was on a treadmill?
Ug. Summer’s here. Anyone want a pina colada?

2

[QUOTE=Rhythmdvl]
If we run a room air conditioner all day, is there going to be a significant difference in energy use if we crank the fan all the way up or leave it down around a third or so? That is, not lowering the temperature, just upping the fan speed.

If it makes any difference, it’s a two-year-old 8,000 BTU Frigidaire, with the thermostat generally set to 73 or 75 degrees. The room is about 30 X 30, but it’s a home office so there are a couple odd shapes to the walls (dormers sticking out). There are two regular windows facing due north and south (one has the AC in it), and two smalish plant-filled semicircular windows that face west. The southern windows have blinds that are drawn during the day. There are two computers constantly running and generating heat (especially the one CRT monitor), and then there are the heat engines of Mrs. Dvl and I (and the two pooches and two cats), who spend all day in it.

So, what does “cranking up the AC” really mean? Does it mean turning the fan on high? I’d imagine that on it’s own, the internal fan/air mover doesn’t draw that much energy. But if it’s moving hot outside air over the coils that much faster, the unit has to compress/cycle the gas more often (I know very little about HVAC, so forgive me if I have that wrong).

Or would high energy costs come from setting the temperature of the unit down to sixty or so and letting it run all day like that?

Would it make a difference if the AC was on a treadmill?
Ug. Summer’s here. Anyone want a pina colada?
[/QUOTE]

The 2 major components in your A/C system that use energy are the fan and the compressor. The fan uses a fraction of the energy the compressor uses.

Running the fan continuously will add about $6-9 to your monthly electric bill, I would guess.

The compressor is another matter. If you run the thermostat down—in effect lowering the temperature-----the compressor will work that much harder to remove the heat. In a typical central system, the compressor would use between 4 and 8 times the energy the fan does. If the focus is on energy usage (vs comfort) leaving the fan on auto or low, and the thermostat at the highest reasonable setting will produce the lowest energy bill, short of turning it off.

Assuming an 8’ ceiling, decent windows and insulation, that 900 sq ft room would need roughly 15,000 btus, give or take. Better windows & insulation, shade trees, orientation, etc etc and maybe you’d get that number down.

So…to answer your question, running the fan continuously—even on high speed----won’t break the bank. (although, as noted, lower speeds mean lower energy usage)

On the other hand, running the thermostat down (vs the fan selector) will cost 4-8 times the cost of running the fan alone.

3

[QUOTE=Rhythmdvl]
If we run a room air conditioner all day, is there going to be a significant difference in energy use if we crank the fan all the way up or leave it down around a third or so? That is, not lowering the temperature, just upping the fan speed.

If it makes any difference, it’s a two-year-old 8,000 BTU Frigidaire, with the thermostat generally set to 73 or 75 degrees. The room is about 30 X 30, but it’s a home office so there are a couple odd shapes to the walls (dormers sticking out). There are two regular windows facing due north and south (one has the AC in it), and two smalish plant-filled semicircular windows that face west. The southern windows have blinds that are drawn during the day. There are two computers constantly running and generating heat (especially the one CRT monitor), and then there are the heat engines of Mrs. Dvl and I (and the two pooches and two cats), who spend all day in it.

So, what does “cranking up the AC” really mean? Does it mean turning the fan on high? I’d imagine that on it’s own, the internal fan/air mover doesn’t draw that much energy. But if it’s moving hot outside air over the coils that much faster, the unit has to compress/cycle the gas more often (I know very little about HVAC, so forgive me if I have that wrong).

Or would high energy costs come from setting the temperature of the unit down to sixty or so and letting it run all day like that?

Would it make a difference if the AC was on a treadmill?
Ug. Summer’s here. Anyone want a pina colada?
[/QUOTE]

Is this a window unit? It seems hard to believe that you have a central unit for one room.

4

So if I understand you correctly, you’re saying that all things being held equal (especially a set thermostat temperature), the cost difference between running it with the fan on maximum and running it with the fan on minimum is chiefly the difference in power requirements of the fan—that moving a greater quantity of air over the coils has a negligible effect on overall energy consumption.
[on preview]Did I edit “window unit” out of the OP? I’m an idiot (must be the heat). And here I was thinking I was providing way too much detail to cover possible questions. Anyway… it is indeed a window unit. (though the house is ducted for central air, the previous owners never installed it, and we’re not ready to do that yet–though when we do, the office will be it’s own zone, keeping this question alive and in a time-travel sort of way, render my omission moot.)

5

[QUOTE=Rhythmdvl]
So if I understand you correctly, you’re saying that all things being held equal (especially a set thermostat temperature), the cost difference between running it with the fan on maximum and running it with the fan on minimum is chiefly the difference in power requirements of the fan—that moving a greater quantity of air over the coils has a negligible effect on overall energy consumption.

[/quote]
That is correct.

No, you were clear. I referenced a central system as an example, although the point is/would be the same.