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Old 12-03-2010, 08:58 AM
CannyDan CannyDan is offline
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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
Quote:
a constant fan operation can also raise the humidity in the house during the cooling season unless a separate dehumidifier is used
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.
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  #2  
Old 12-03-2010, 09:06 AM
Really Not All That Bright Really Not All That Bright is offline
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Quote:
Originally Posted by CannyDan View Post
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?
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).
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Old 12-03-2010, 09:15 AM
Musicat Musicat is offline
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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.
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  #4  
Old 12-03-2010, 09:19 AM
CannyDan CannyDan is offline
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Quote:
Originally Posted by Really Not All That Bright View Post
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).
I can't say that I keep that much water on my bathroom floor....
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  #5  
Old 12-03-2010, 09:33 AM
carnivorousplant carnivorousplant is offline
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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.
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  #6  
Old 12-03-2010, 09:37 AM
Really Not All That Bright Really Not All That Bright is offline
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What course was that?
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  #7  
Old 12-03-2010, 09:46 AM
Baracus Baracus is offline
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Quote:
Originally Posted by CannyDan View Post
(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.)
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.

Last edited by Baracus; 12-03-2010 at 09:48 AM..
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  #8  
Old 12-03-2010, 09:48 AM
carnivorousplant carnivorousplant is offline
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Quote:
Originally Posted by Really Not All That Bright View Post
What course was that?
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.
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  #9  
Old 12-03-2010, 11:06 AM
Marley23 Marley23 is offline
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Modding

Moved to IMHO from Great Debates.
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  #10  
Old 12-03-2010, 11:11 AM
Omar Little Omar Little is offline
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The difference has got to be neglible. Just do what your wife wants. A happy wife = a happy life.
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  #11  
Old 12-03-2010, 11:55 AM
beowulff beowulff is online now
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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%).
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  #12  
Old 12-03-2010, 12:07 PM
ZipperJJ ZipperJJ is online now
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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.
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  #13  
Old 12-03-2010, 12:17 PM
carnivorousplant carnivorousplant is offline
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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.
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  #14  
Old 12-03-2010, 12:18 PM
DMark DMark is offline
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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.
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  #15  
Old 12-03-2010, 12:38 PM
CannyDan CannyDan is offline
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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?
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  #16  
Old 12-03-2010, 12:49 PM
CannyDan CannyDan is offline
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Quote:
Originally Posted by Baracus View Post
<snip> 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.
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.
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  #17  
Old 12-03-2010, 12:59 PM
Baracus Baracus is offline
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Quote:
Originally Posted by CannyDan View Post
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?
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.
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  #18  
Old 12-03-2010, 01:12 PM
Baracus Baracus is offline
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Quote:
Originally Posted by CannyDan View Post
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.
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.
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  #19  
Old 12-03-2010, 01:15 PM
CannyDan CannyDan is offline
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Quote:
Originally Posted by Baracus View Post
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.
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.
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  #20  
Old 12-03-2010, 01:27 PM
carnivorousplant carnivorousplant is offline
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Quote:
Originally Posted by Baracus View Post
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 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.
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  #21  
Old 12-03-2010, 01:29 PM
Balthisar Balthisar is offline
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I have one of those super-high efficiency heat pumps with a forced air system. It replaced my old gas forced air system. Some general observations:

Its duty cycle for both heating and AC is considerably longer than the old system.
Its much, much, much, much (did I say much?) more comfortable as a result. Because the air is circulating so much more, there are no more warm or cool spots; the air stays homogeneously mixed.
The current system is much quieter, and so your fan noise may impact on your decision to leave the fan running at all times.
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  #22  
Old 12-03-2010, 02:03 PM
CannyDan CannyDan is offline
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Balthisar, I'm with you. "Much more comfortable" is my observation too.

But remember, my wife had an offhand and only partially understood conversation with an "expert". She is adamant that there will be dire consequences related to humidity. Not to efficiency, or cost, or anything. Just humidity. And dire.

So, other than the "less than maximum humidity removal" and its effect on shall we call it "maximum comfort" described by Baracus above, is there anything genuinely dire about the humidity issue that we all seem to be missing? Or dire enough to at least make my wife's understanding more comprehensible?

Oh, and forgive me in advance for ducking out on this thread later today. I'm having computer issues at home, and will have no access to the Dope until I return to work tomorrow. But I'll be looking forward to seeing additional discussion!
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  #23  
Old 12-03-2010, 02:09 PM
Baracus Baracus is offline
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If your wife's primary concern is the humidity, then you could always buy a hygrometer for like $20 and monitor the humidity. From what I have seen it should preferably be less than 50% to minimize potential mold issues.
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  #24  
Old 12-03-2010, 02:33 PM
CannyDan CannyDan is offline
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Quote:
Originally Posted by Baracus View Post
If your wife's primary concern is the humidity, then you could always buy a hygrometer for like $20 and monitor the humidity. From what I have seen it should preferably be less than 50% to minimize potential mold issues.
No, sorry if I wasn't clear. My wife's major concern is some catastrophic but unnamed effect related to humidity, predicted by someone she spoke with who is "an HVAC expert".

I'm trying to find out what such an effect might be.

Knowing what this unnamed catastrophe is will make it easier to overcome. (Or not-- if there really is such an effect.)
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  #25  
Old 12-03-2010, 06:24 PM
slitterst slitterst is offline
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I'm no expert, (I know that admitting that might cause my Dope account to be terminated) but when we got all new HVAC a few years back, we were offered the choice between a continual system, or the standard cycling model. The continual airflow system was significantly more expensive at the time, but promised lower operating costs.

We were told not to run our standard system continually, so I suppose it depends on the age of your unit and whether it was designed for constant airflow.
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  #26  
Old 12-03-2010, 07:56 PM
Balthisar Balthisar is offline
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This really got me curious, and I'm assuming the OP did his Google research, too; however I've not found any cites indicating that there'd be humidity problems with leaving the fan on in AC mode. I've seen a few references indicating that doing so would actually help control humidity.

Several scites (that should be a new word, "site" and "cite") did raise this caution, though: you should check with the manufacturer to ensure that the blower is rated for 100% duty cycle, lest you suffer premature fan failure. Also, be sure to check your filters more often.

Oh, and slitterst, you better thank God that I'm not a mod. That ban-hammer would have fallen so fast, you'd not know what had hit you.

Last edited by Balthisar; 12-03-2010 at 07:57 PM..
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  #27  
Old 12-03-2010, 09:17 PM
Baracus Baracus is offline
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Quote:
Originally Posted by Balthisar View Post
This really got me curious, and I'm assuming the OP did his Google research, too; however I've not found any cites indicating that there'd be humidity problems with leaving the fan on in AC mode. I've seen a few references indicating that doing so would actually help control humidity

This
cite speaks of possibly increased humidity and the issue of circulating air through hot duct work. Of course, it is a electric company website, so it could be part of a nefarious plan to jack up people's power usage.
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  #28  
Old 12-04-2010, 01:17 AM
BigT BigT is online now
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You mean the duct work isn't insulated? No wonder I can't seem to cool my house down in the summer or warm it up in the winter. That's just stupid.
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  #29  
Old 12-04-2010, 08:19 AM
Baracus Baracus is offline
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Originally Posted by BigT View Post
You mean the duct work isn't insulated? No wonder I can't seem to cool my house down in the summer or warm it up in the winter. That's just stupid.
Any newer installation (like the last 40 years) will be insulated, but the insulation will usually have a R-value in only the 4 to 8 range. Better than nothing, but usually much less than what you have in your ceiling or walls.
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  #30  
Old 12-04-2010, 01:06 PM
CannyDan CannyDan is offline
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OK, I'm back briefly. Thanks for the additional comments. Now I'll be without teh intar-tubes until Monday.

All you above have confirmed that there are economic implications, and the "try both ways and see which costs more" is probably the only way to actually resolve the matter. I'll make that experiment some time in the future, perhaps.

And there is the "total dehumidification will be less than maximum dehumidification" because of re-evaporation from the drip pan. But if the drip pan retains very little liquid, this factor could be quite small.

The duct work is fairly new (less than 5 years) and while I cannot find an R value marking, it seems robust and well executed. So I'm not too concerned about excessive thermal loss through the ducts themselves.

And although my experience is limited (I worked in A/C service as summer jobs during college, and occasionally help friends with minor issues today) I've never seen a residential system having an ON/AUTO switched thermostat that didn't have a continuous-rated blower. Maybe that's common elsewhere, but not in SE Florida. So even though I haven't pulled the inspection plate to be certain, I'm pretty comfortable about the fan motor. And this is still not my real issue.

The OP contains a quote and a cite for the contention that
Quote:
a constant fan operation can also raise the humidity in the house during the cooling season unless a separate dehumidifier is used
This is my real concern. It seems to be what my wife got from her friend the HVAC "expert". I have found a number of similar contentions in my research. But nowhere do I find an actual mechanism supporting the assertion, including Baracus's cite in post 27. And no one has offered one here. Indeed, it seems that the opposite assertion dominates the references available.

So again, may we conclude that, economic issues aside, there does not seem to be any potential catastrophic humidity-related event to be expected from continuous fan operation? Is this just marketing to turn a simple cool/heat system into an actual HVAC system (by adding specific humidity control, instead of humidity buffering as a side effect of cooling.)?

Or have I/we overlooked something?
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