If you have a variable speed blower in your furnace or air handler, it costs little to run the fan on a low speed 24/7 and can help even out the temperatures. It can also slightly increase humidity, especially after an A/C call since some of the moisture on the coil can re-evaporate.

As for efficiency, I don't see how running a blower versus not can be more efficient, especially when it can possibly raise the indoor relative humidity. This is more the case with conventional blowers rather than variable speed blowers.

If the efficiency claims result from the more even temperatures causing the system to have to run a bit less, then maybe, but I think that's more of a comfort thing.

If anyone else has a different take on this I'm eager to learn myself. I don't run my fan 24/7, only as needed. If I had a variable speed blower that could run at a very low speed always, I might, at least 30% of the time there is no cooling call (this is the CIRC feature on some Honeywell thermostats).

I don't see how its more "efficient". If it's running while the a/c or heat are off, its just circulating air thru out the home and drawing amps. Some people leave it on because they say it "Evens out the temperature" in the home. I leave mine on all the time but its 13 years old and a varible speed motor and frankly i'm scared to turn it off because sometimes i think it might not start up again...LOL

Even with our low electric rates, we figure leaving a conventional blower on constantly can add $20 a month to the juice bill. VS would be a fraction of that but moves less air.

The pro as mentioned is more even temps. The biggest con besides price as mentioned is the summer humidity issue.

Mine isn't variable speed so that much of a cost increase is a little scary - although it's not much more than $200 a year, and I get the advantage of moving heat or cool that's occurring naturally around the house (ie not from the furnace)

I don't see why it would add the the RH% but if it does, I'm all for it, since the climate where I am is extremely dry - my humidity meter is often off the scale dry, less than 35%.

I run the fan all the time, when I was a kid, my dad, an engineer did, and I find myself it does indeed even out the temperature, also, you get the benefit of added air filtration - we have a dog, plus, it draws fresh air that way (since there's the fresh air intake which joins to the cold air return) without having to open a window.

I must admit when the fan is off, it's beautifully quiet.

My original compromise plan was a thermostat which would cycle the fan every 15 minutes, but it didn't work properly and the one I ended up with is either on or off.

My other main reason for running 100% is my stepson was living in the basement room so I was concerned there wasn't enough fresh air for him, also in summer the upstairs gets hot (no a/c yet) but the downstairs is nice and cool.

Also, and I know this is unusual and 'against the rules' - I run the humidifier in the summer with no heat - since there's no a/c there is no danger of condensation - the duct walls stay pretty warm and there is no sign of mould, and it stops the summer months in here from being incredibly dry. But our case isn't that typical.

In an air-conditioned place that's being dehumidified by the a/c coils, I don't know how a decent humidity level is maintained, especially in this sort of climate.

running the fan is something we don't do in my high humidity area as it allows condensation to re-enter
conditioned space..as Ryan said.
in what building science calls shoulder seasons..
when a/c is not really needed (bout 3 months out of the year here..LOL) running the fan constantly is not
recommended.
lots of local hvac companies recommended leaving the fan on all the time..again in my area..RH increased to 65 & 70% in some homes. I have not heard this specific recommendation from hvac co to homeowner in a while.
guess more are installing VS.
there is no efficiency in leaving the fan ON.
best of luck.

Thank you all for such prompt replies.

My most recent home has a HP that was installed just prior to my move. I will need to look at the unit as I am not certain if variable speed is an option. My home is loft style (camelback w/open stairwell) The bedroom is upstairs. The temperature differential is approx. 15-20 deg. from ground to 2nd floor. It's difficult to regulate the temps w/o affecting one area drastically. I am in the Ohio River Valley (Louisville,KY) where the humidity can be brutal in Summer but weather has been known to vary 20-40 with 24 hours. One day 70 the next day snow, perhaps the next day 50 or maybe 15. You never know!

I'm trying to regulate things the best I can, taking into consideration the limitations of my region and floor plan design. My two main concerns are efficiency (in $) and practices that are best for my HVAC system. OH yeah, did I forget my personal comfort? LOL.

Thanks again and in advance!
-September

I just had a new Lennox G71 furnace and XC-15 AC unit installed on Wednesday. Nearly all the salesman recommended running the blower 24/7, essentially for comfort reasons. This furnace has a variable speed DC motor and it does not cost much to operate and so far it has been running on AUTO at such a low speed you can't even tell it's on.

And for now I have just left it on AUTO. Our Honeywell VisionPro IAQ has a CIRC setting that I need to read-up on. If running the blower would help even out the downstairs to upstairs temps I'm all for it as the upstairs bedrooms get a lot hotter due to the cathedral living room with adjoining stairs and the large skylight at the top of the stairs.

Guess a bit of experimentation is in order. Maybe use AUTO during the Winter and ON or CIRC in the warmer months?

Tom the CIRC feature would probably not be a bad setting to use if you want more airflow and filtration. Because I believe the IAQ will run the blower 30% of the time when there is not a call for A/C, at a low speed. This would probably (just speculating) make it less likely to re-evaporate condensation from A/C that had run as opposed to having the blower run 24/7 and immediately after a call for A/C.

running a blower full time is what we call a total comfort system. You have air in your home and you want to treat that air for health and comfort. you want to heat that air, cool it, clean it, humidify it, and mix it so you don't have stagnant
air pockets and cool or hot spots. So if your supply and returns registers are located right in every room , and you run the blower full time you can have a
total comfort system. I have heard the statement of the cost of running full
time blower. Keeping that air mixed will save on heat and cooling. They say that a motor running full time will last longer than off and on every 10 min.
I know there is a lot of different opinions on this , so i will say this is just my
opinion, and the way i was schooled. later paulbm