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

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.

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!

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.)

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.

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. :wink:

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.

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.

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

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?