There is no one size fits all answer to which is better. A properly sized and installed system works fine during normal conditions. "Abnormal condition" would be during a time when several people (think party) are concentrated in an area other than where t-state is located. I normally set fan to auto but set to on during holiday gatherings. I suggest you run fan at auto then take notice whether there's wide variation in temperature between rooms. If so, your system is not properly installed. Is it possible they are telling you to set fan to on in order to mask improper installation?

Running the fan continuously will provide mixing, continuous air filtration, and may enhance occupant comfort. The issue you may have, depending on your climate zone, is control of relative humidity. American Standard recommends the fan be set to "Auto" when indoor relative humidity is 60% or greater.

Part of the comfort an air-conditioning system provides is dehumidification. Just like the moisture that condenses on the outside of a glass containing a cold drink when the air is warm and humid, indoor air passing by the cold coil of the AC system sheds humidity onto the coil while it itself is being cooled. The condensed moisture (called "condensate") drips off the coils onto a pan below and is piped outdoors or to an under-sink drain pipe).

When the A/C cycles off, the coil and the pan under it will be wet. If the fan continues to blow air past the coil (and pan), the air will pick up the humidity and redeposit it into the indoor air space.

Keep the fan off when the system is off (IE, use the "Auto" setting) during cooling season to take full advantage of the dehumidification the system can produce.

The Infinity Inverter Platinum system has a variable speed compressor and air handler. If the fan is left in the "on" position, after reaching setpoint temperature, the compressor speed will be decreased (its lower limit is 30%.) The attendant reduction in refrigerant flow will increase the evaporator coil temperature. Under some conditions, that could result in the system removing the sensible heat portion of the cooling load, but not the latent (moisture removal) portion. I suggest following the manufacturer's operating instructions over those of a salesperson whose qualifications may only be that they visited the factory. The manufacturer's recommendations are to keep the fan in "auto" mode if the relative humidity is 60% or greater.

CRHomes, that's a lot of tech talk that doesn't add useful information or understanding of those here in a casual conversation.

The Infinity Inverter system is a variable-speed system; it's inherently more complicated than a system with single-stage or two-stage compressor and a fixed-speed air handler. The concern with, and cause of lack of dehumidification with the fan in the "on" position is different from fixed-speed systems, too. If you can't follow the explanation, simply follow the manufacturer's instructions over the musings of a salesperson.

I followed it fine. I wasn't impressed. My experience is that really clever people share and explain what they know to people who may understand less than they do, using plain language. Without using any jargon or buzz words.

Yes, Elmer, I could have stated more plainly that your explanation about re-humidification doesn't apply to the Infinity Inverter air handler. The Infinity system is a variable refrigerant flow (VRF) system. The compressor doesn't cycle on/off like a fixed-speed system. Rather, its speed ramps up and down to match the cooling load. If you leave the fan in "on" the compressor speed (and refrigerant flow) may get low enough that the evaporator coil temperature is above the dew point of the indoor air. In that case you'll get no dehumidification. It's the same issue that mini split heat pumps (which are also variable speed systems) have. But it's not an issue with re-humidification as you described.

The Infinity Inverter Platinum system has a variable speed compressor and air handler.

The Infinity Inverter system is a Carrier design, not American Standard Inverter Platinum 18. (Trane)

The Infinity Inverter System is limited to 5 compressor speeds, There are no metals associated with the Infinity System (Carrier).

I would never say no to rehumidification due to constant run of the blower. You can try it, if you see increased humidity in the home, set the blower back to auto. While it's true the American Standard Inverter will run longer in AC mode, the last thing you'd want to do in my opinion is reintroduce humidity back into the dwelling.

Water left on the Evap Coil will evaporate regardless of how complicated your system is, or the brand name on the side of the box, once your compressor cycles off that's when the evaporation starts.

Yes, my apologies. I must have had a brain fart. If you'll substitute "American Standard Inverter Platinum" for "Infinity" the text is otherwise correct. We switched from Carrier to American Standard years ago. I'm easy to program, but hard to re-program.

Thereare are other negatives of keeping a fan running continuously.

In far too many homes, HVAC ducts run through unconditioned spaces. In fact, in far too many homes, the air handlers (or furnaces with blower, as applicable) are also in the attic.

In hot weather, with typical attic temps in the low 100s, it's the last place one would choose to flow in a continuous loop indoor air. Also, many ducts leak and depending where the leakage is, outdoor air can be sucked into the house.

Also, to say that the air flow "filters" indoor air is not the case in most homes. As HVAC pros continue to recommend thinner and thinner disposable filters, the filters in place do little more than keep larger size particulate matter out of the air handler passages. True filtration of the indoor air is neither the intent nor the result of what they do. Without a dedicated filtration system installed, which few have.

Lastly, continuing to run the blowers when not needed wastes electricity. In my area, at over 40 cents per kwh (and other areas following suit, price-wise), keeping it running when not needed for the HVAC function is nuts.

Dumb+dumb+dumb= even more dumb.

CRHomes, you can keep your spec sheets and lists of buzz words in front of you. I'll stick to consumer knowledge and common sense and advise others to do the same.

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Running the air handler's fan continuously provides enhanced mixing and continuous filtration-- irrespective of where the ductwork is installed or how leaky it is or the efficiency of the air filter. If the ductwork installation is inadequate then it's inadequate in all operating modes and should be repaired. If the filtration is inadequate for its intended purpose then it's inadequate in all operating modes. It should be replaced with an appropriate one. If running the fan continuously results in unacceptable indoor humidity then other alternatives should be considered.

A wise man once said "it's better to be silent and risk being viewed dumb than removing any doubt by talking too much".

CRHomes, play the ball as it lies, as they say. The condition, location, nature, yada yada of what HVAC systems people have in their homes - most of which are not newly built - are what they are. Sub-optimal, done differently today, likely improperly sized, marginally functional, all of those things. Make the best of what one has. .

klem, those are wise words. Follow them more.

"... likely improperly sized, marginally functional, all of those things. Make the best of what one has."

Making "the best of what one has" if it's a poorly-performing system is called maintenance, repair, and modification. Those are ongoing needs as systems age. If the condition of an HVAC system precludes running the fan continuously, it may not be appropriate to run it at all. Consulting a local service professional can help homeowners make a more informed decision regarding their particular system.

Your comments used to be more thoughtful and balanced, less grumpy. Too bad.

Why, oh why, do posts on this forum invariably end up in a pissing contest???

Reminds me of the political arena where fresh ideas and facts take a back seat to personal opinions on individuals.