Should an air conditioner fan be set to auto or on?

All right, it might not qualify as a “great” debate, but there seems to be considerable controversy. Including, at this point, from my wife.

We’re talking here about a single family ranch style home with a central a/c system. I contend that ON provides superior air filtration, while avoiding temperature gradients in different areas of the house, as well as those blasts of cold air in certain places when the AUTO fan kicks in. I recognize that there is some cost, that being the electricity needed to operate the fan motor. However, I believe that this cost is a fraction of the cost of operating the a/c compressor, and with the more even cooling achieved by the ON setting, I can increase the thermostat setting by a couple of degrees and still achieve equal comfort. This means that the power hog compressor runs less, countering the increased consumption of the fan.

But my wife had a conversation with someone in the HVAC business, who told her that AUTO was proper, and the ON setting had dire consequences regarding humidity or something. So this isn’t primarily about economy, anyway.

I’ve done some searching, and find several links that seem to support my position.

But I also find mentions like this stating

And there’s that unexplained humidity thing. What does it mean? How does that work? How can a fan, circulating conditioned air in a sealed system, increase humidity?

(Well, it’s sealed as long as no doors or windows are opened. I know the new buzz word for “central air and heat” is “HVAC” but the only humidity control in this system is collecting and draining the condensation water on/from the unit’s evaporator coil, and the only ventilation comes from the aforesaid door openings.)

Let’s have some debate on this matter before my wife and I rip that thermostat controller off the wall.

WAG: constant flow of not-necessarily-cool air speeds the evaporation of condensation, standing water by the bathtub, etc., increasing the amount of moisture in the air (though decreasing the amount on the bathroom floor and the windows).

This may be less of a problem with a central system, but in room air conditioners, the thermostat won’t be as responsive to temperature changes if the air isn’t circulating near it. So an auto setting would result in wider temperature swings, and my informal tests years ago seemed to confirm that.

And the sound of a fan going off and on might be more annoying than a constant drone. At least it is to me.

I can’t say that I keep that much water on my bathroom floor… :slight_smile:

A course I took in undergraduate school specified that the fan be set on instead of auto, basically for the reasons described in the OP.

What course was that?

I think I have heard that the additional humidity is coming from the evaporator coil and drip pan. Anything that does not make it down the drain during the cooling cycle will be blown back out into the conditioned space.

Another factor to consider is the insulation on your ducts. You avoid blasts of cold/hot air when the systems kicks on, but you are wasting energy keeping those ducts warm/cold between cycles. If the ducts run through non-conditioned space and have the normal wimpy insulation than can waste a good bit.

It was a residential solar heating course. Heat loss and required air flow were calculated from “The Arkansas Story House” a document designed by Arkansas Power & Light.

Moved to IMHO from Great Debates.

The difference has got to be neglible. Just do what your wife wants. A happy wife = a happy life.

With a properly-sized and efficient AC, the air handler is going to consume around 10% of the total system power. Since you will be running it 24/7, and a normal duty cycle is between 30-50%, you will be increasing your electric bill substantially (20%-30%).

Yeah I’m not aware of the physics of the thing or whatever, but I can’t imagine running my AC/heater fan all the time. It would be a huuuuuge waste of power.

One of the reasons for undersizing an A/C unit is that the fan doesn’t turn on and off as much, saving the start up current.

My cousin is a fireman and claims to have some knowledge about central heating/air conditioning units from classes he has taken.
They all told him that setting the fan to “on” actually saves money. Keeps the air flowing so that cool/warm air is constantly circulating, thus making the actual heater/cooler not turn on as often.
He went on to say the fan is designed to be run 24/7 and the cost should be negligible.
Just letting you know what he had to say - take it for what it is worth.

OK, I’m willing to accept serious questions of cost. I am inclined to think that power consumption by the blower would not be terribly significant, but could be persuaded otherwise. I don’t think the additional cost would be nearly what beowulff contends if we factor in a higher thermostat temperature setting, but perhaps I’m wrong.

Regardless, it’s the claim about humidity that intrigues me. Baracus, please follow me here:

The house contains some amount of water, as vapor, this being the initial humidity. When the compressor kicks on some of this vapor will condense on the cold evaporator coils, and some of that now-liquid water will run out the pipe and be permanently removed from the closed system. Thus there will be a reduction in the total amount of water (total of vapor and liquid) in the house. When the compressor shuts down and the coils are no longer cold, but the fan continues to blow, some of that liquid water remaining in the pan may vaporize and be returned to the house as humidity. But the total water in the house will still be less than the amount initially. And so the humidity inside, while higher than the maximum dehumidification when some water was liquid and in the pan, will still be lower than the initial humidity. Lower by whatever amount of liquid water ran out the drain tube. Right?

It’s not like the fan running is actually bringing in any more humidity than was there initially.

So what exactly is the catastrophic humidity effect?

If the ducts are really such crappy insulators, then when the fan is off the air inside the ducts is going to equalize with the (rather hot) air in my attic. Then, when the fan does come on, I’ll get a blast of really hot air, adding to the instant heat load in the house, followed by air of decreasing temperature as the ductwork cools off.

I’m not sure if this wastes as much or more than running constantly and having some continual loss through the insulation.

I don’t think that the suggestion is that having the fan on ON causes the humidity to continually increase, but that it results in a higher humidity than if you left the fan on AUTO. The less humidity, the higher the thermostat setting you can use and still feel comfortable.

The continual loss would be more. The key thing is that word “equalize” in the first sentence. Once the temperatures have equalized there will be no more heat transfer. If you leave the fan running the temperature difference between the air in the duct and the air in the attic will never equalize and the heat gain (per unit time) will be high and continuous.

That makes sense.

So I guess the actual effect depends on how much liquid water remains in the pan between compressor cyclings. If it is a substantial amount, then returning it as humidity would reduce comfort and presumably result in somebody lowering the temperature setting.

If though the pan holds little water, then the effect may be inconsequential.

The opposite will occur during the heating season.
I’d leave the fan on for a billing period and compare the outside temperature and fuel cost to the previous billing period.
But then, I’m an engineer. :slight_smile: