I purchased a house last May.  I remember during the inspection, our
inspector told us that when we have a heat or air conditioning on, we
should keep the switch on the thermostat on "ON" instead of "AUTO." 
His rationale was that by keeping the fan running all of time time,
our HVAC system was recirculating the air that was already cooled (or
heated).  In this manner, it uses less energy to cool/heat the home,
consequently costing me less money.

I had never heard that theory before.  In fact, in every house I have
ever been to (and I have since been paying attention!), the thermostat
is set to "AUTO."  Am I in on a wonderful secret?  Is there a right or
wrong way to set the thermostat?  In the likely event that there is no
right or wrong but a better use for ON and AUTO, can you please
explain what those situations are?  As I sit here in the air
conditioning, hearing the fan blowing, it is making me curious.

---

Clarification of Question by tnsdan-ga on 07 Jul 2004 16:16 PDT

For extra credit (AKA tip):

I have also been considering purchasing on electronic/programmable
thermostat.  Any information regarding the use of those would be
helpful.

Thanks!

---

Clarification of Question by tnsdan-ga on 07 Jul 2004 17:30 PDT

Thanks to everyone who has provide comments so far.  Perhaps I can add
another wrinkle that just occured to me.  When the inspector explained
to me why I should leave it on ON, he said that all that means is that
the FAN stays on.  The HVAC may still turn on and off.  However,
because the fan is so inexpensive to run, it makes sense to keep it on
all the time to recirculate the cold air. (or hot air in teh winter)

I very much appreciate every comment that I get (keep 'em coming!). 
Nonetheless, I am oping that there is some sort of internet authority
on the subject that goes beyond anecdotal evidence.

If it makes any difference at all, I am in Lexington KY, and live in a
1700 square foot 2 story house.

THANKS!!!!

---

Clarification of Question by tnsdan-ga on 08 Jul 2004 13:31 PDT

tlspiegal-

Thanks for following up on my question.  I have the feeling that you
want to know as much for you as for me! (Which I of course have no
problem with!!!)  If there are ANY even quasi reliable internet
sources discussing this subject that you could provide, I would be
more than happy to accept your comments as an answer.

Thanks!

Hi tnsdan,

Please read the Important Disclaimer at the bottom of this page.
  
"Answers and comments provided on Google Answers are general
information, and are not intended to substitute for informed
professional advice."

Please note: I've provided information on Electronic Programmable
Thermostats near the end of my answer.

I'm more confused than ever now.  :)   But the bottom line in my
opinion is best described in the following thread posted at
GardenWeb.com - Heating and Air Conditioning Forum regarding Heat Pump
Fan.

http://ths.gardenweb.com/forums/load/hvac/msg061213103201.html

"Question:  
On our heat pump controller there is an option to keep the fan on all
the time or on auto. Is there any reason not to keep the fan always in
the ON position? I don't know if it is good or bad to do that and in
the auto postion it only comes on when either the heat or air
conditioner kicks in.

Answer:
There could be good reason in either case. If you have an electronic
air cleaner, it's advisable to run the fan 24/7 for the best cleaning
performance. Now in the cooling season, I sometimes recommend to the
customer leaving the fan in the auto position for improved humidity
control. If the fan is in continuous, the accumulated condensation on
the cooling coil will evaporate back into the room on the off cycle of
the compressor, elevating the humidity levels. This is especially true
in mild weather conditions where the compressor is not needed very
long between cycles. But in the same breath you usually get improved
temperature control with the fan running constantly.

So there are pros and cons and you must decide what's most important
to you. I say experiment to see what works out best for your
situation. Running the fan continuous does not use that much more
electricity nor is it going to prematurely wear the motor out to the
extent of worrying about it. That's my opinion and maybe some pros on
this forum will give you theirs."

*****

"I run my fan pretty much 24x7 and have for several years. I change
the filters every 15 days instead of every 30, though. Which reminds
me, it's time to change the filter.

I find that the temperature differentials between the 3 levels in my
home aren't as dramatic if I leave the fan run 24x7."

=================================================

Buildings 
http://www.advancedenergy.org/buildings/knowledge_library/air_quality/smelly_house.html

(The information relates to causes of odor coming from the
fireplace... however, part of the answer which I've posted explains
why not to leave the fan running all the time.)


"The next question to consider is this: Why does your friend want to
have the fan running all the time in the first place? In general, we
don't recommend letting the air handler fan run all the time. During
both the cooling and heating seasons, if operating the air handler fan
causes pressure imbalances in the house, it can be substantially
increasing infiltration (300% or even more), leading to higher bills,
comfort problems, and moisture problems. In addition, in the cooling
season if the air handler fan continues to run after the compressor
has shut off, the air flowing over the wet coil will simply
re-humidify the house by picking up the water and putting it back into
the air stream.

There needs to be a pause of at least 20 minutes or so each time the
compressor shuts off during which the fan isn't running. This will
enable the water to run off the coil, into the drain pan and then into
the house drain or into the yard, next to the dead attic fan. It's
possible to install a control that will do this, but the question
remains why run the air handler all the time?

Some people do it so that the air in the house is being filtered more.
If you have allergies or similar problems, that may be worthwhile, IF
you shut the fan off while the water drips off. For most allergic
people, the worst thing you can do is raise the moisture level in the
house. This strategy also assumes that you've installed a high
efficiency filter and replace it or clean it as necessary. Running
more air through the typical fiberglass furnace filter will do little
or nothing for cleaning the air in your home."

=================================================

Thermostat Operation
http://www.energy-control.com/page13.html

"System Off - On stand by: No heating, cooling, or blower air.  Set
system switch to "Off" and fan switch to "Auto".

Fan Only - For air circulation only: Set fan switch to "On" and sytem
switch to "Off".

Cooling - To operate the heat pump as an "air conditioner": Set system
switch to "Cool", fan switch to "Auto", emergency heat switch to
"Normal" (on those thermostats that have a separate switch), and the
temperature selector level to the desired temperature.  For a cooler
temperature, set lever to lower number.  For less cooling, set lever
to higher number.

Heating - To operate the heat pump for heat: Set system switch to
"Heat", fan switch to "Auto", emergency heat switch to "Normal" (on
those thermostats that have a separate switch), and the temperature
selector level to the desired temperature.  For a higher temperature,
set lever to higher number.  For less heat, set the lever to a lower
number

Emergency Heat - Heating the home in case the outdoor units fails: Set
the system switch to "Emergency Heat" (on those thermostats that have
a separate switch), fan switch to "Auto", and the temperature selector
lever to desired temperature.  Emergency heat light will come on and
only the furnace will operate.

Continuous Air Circulation:  Regardless of whether heating or cooling
the home:  Set fan switch to "On", system switch to either "Heat" or
"Cool" and the temperature selector lever to the desired temperature.

Set your thermostat for heating or for cooling.  Then set it for the
desired temperature.  Find the temperature that is most comfortable
for you, and then leave your thermostat alone.  Manually moving the
thermostat up or down to extreme settings will not speed up
temperature changes."

=================================================

Commonly Reported HVAC Problems
http://www.hannabery.com/faq15a.shtml

"Indoor fan won't shut off unless I use the circuit breaker

Well, if you are going to have a problem with your HVAC equipment,
this is the one to have. The heating and air conditioning still works
fine. If you have allergies your fan should be running all of time
anyway to filter the air more. It really doesn't hurt the motor
running all of time. And it usually isn't an expensive repair."

=================================================

Gardenweb.com Forums: leaving fan run all the time
http://ths.gardenweb.com/forums/load/hvac/msg0712090821089.html

Question: 
Does anyone leave their A/C fan on all the time?  

Answers:
Read pros and cons

=================================================

Heating, Air Conditioning, Fridge, HVAC, Appliance Repair
http://experts.about.com/q/696/3359004.htmHeating

"Question:
I have two HVAC Chronotherm 111 programmable thermostats. 

I prefer to run the fan continuously (on) rather than on Auto for
better uniform circulation of the room air, rather than having the fan
stop when the cool air or hot air shuts off.

Is this ok? 

Answer:
It's OK your motors are rated for continuos use. 
I think in the heating mode running the blower all the time cools the
air down faster and causes the heat to come on more often.
In the cooling mode it probably doesn't make a lot of difference
because most AC units run all the time any way.
I don't know what the statistics are but I assume a motor is rated for
so many hours of running life."

=================================================

Central A/C temperature and all day settings
http://ths.gardenweb.com/forums/load/hvac/msg0610265522422.html

=================================================

More Heat, Less Money 
http://www.stretcher.com/stories/980129d.cfm

"Now a few words about layering. My scientific friends tell me that
warm air rises and cold air falls. So when we heat our homes the
warmest air is near the ceiling and the coldest air is near our feet
and our kids! If we could move that warm air down to where we actually
live we wouldn't need to keep the thermostat so high. If your room is
equipped with a ceiling fan it will do the job nicely. If you have one
with a reversing switch, that's best, but any fan will do.

No ceiling fan? There's still an answer. Use the fan from your central
heating system. Leave the fan in the `on' position. Check the owner's
manual for your heating system. Some are not made for continuous
operation. However, you may be fortunate enough to have a `slow' speed
available for the fan. If you do, use it."


=================================================
Electronic Programmable Thermostats
=================================================

Thermostats
http://www.oldhouseweb.com/stories/Detailed/10261.shtml

"To maximize your energy savings without sacrificing comfort, you can
install an automatic setback or programmable thermostat. They adjust
the temperature setting for you. While you might forget to turn down
the heat before you leave for work in the morning, a programmable
thermostat won't! By maintaining the highest or lowest required
temperatures for four or five hours a day instead of 24 hours, a
programmable thermostat can pay for itself in energy saved within four
years.

Programmable thermostats have features with which you may be
unfamiliar. The newest generation of residential thermostat
technologies is based on microprocessors and thermistor sensors. Most
of these programmable thermostats perform one or more of the following
energy control functions:

They store and repeat multiple daily settings, which you can manually
override without affecting the rest of the daily or weekly program.
They store six or more temperature settings a day. 
They adjust heating or air conditioning turn-on times as the outside
temperature changes.
Most programmable thermostats have liquid crystal temperature
displays. Some have back-up battery packs that eliminate the need to
reprogram the time or clock in case of a power failure. New
programmable thermostats can be programmed to accommodate life style
and control heating and cooling systems as needed."

[edit]

"A note for heat pump owners: When a heat pump is in its heating mode,
setting back a conventional heat pump thermostat can cause the unit to
operate inefficiently, thereby canceling out any savings achieved by
lowering the temperature setting. Maintaining a moderate setting is
the most cost-effective practice. Recently, however, some companies
have begun selling specially designed setback thermostats for heat
pumps, which make setting back the thermostat cost effective. In its
cooling mode, the heat pump operates like an air conditioner;
therefore, manually turning up the thermostat will save you money."

[edit]

"There are five basic types of automatic and programmable thermostats:

electromechanical 
digital 
hybrid 
occupancy 
light sensing"

(see explanations on all five types)

=================================================

Getting the Most out of your Programmable Thermostat
http://www.trane.com/Residential/PressRelease/Thermostat.asp

=================================================


keyword search:

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Best regards,
tlspiegel

I guess the answer is that there isn't a definitive answer!  Such is
life I guess.  Thanks for all of the help on the question!

Hi tnsdan,

I live in a very hot climate in the desert SW, and recently had to
acquire a digital thermostat.  My old mercury filled one died and I
was informed that it was a good idea to keep the fan going at all
times - ***ON*** - instead of on AUTO.

I never had operated my a/c in that way, but I found several benefits
to doing so and have become accustomed to keeping it on ON.  This is
strictly for the a/c.

First: the cool air stays cool.
Second:  if you are going to keep a VERY clean air filter at all
times, much of the dust in the air will collect there and the house
will remain cleaner.
Thirdly:  I found the a/c isn't cranking on and off as much because
the air in the house is at a more consistent temperature.

I have ceiling fans running on high all the time in the summer, so why
not also have the fan from the a/c pushing air through the house? 
Makes sense to me.

Disclaimer:  My furnace is not connected to my a/c, so I wouldn't know
if this would work in the winter.  I don't run ceiling fans in the
winter.

Best regards,
tlspiegel

Your system will cool the air to the minimum temperature achievable
with your equipment. This is almost certainly cooler than you need the
air in your house to be. Your inspector says that cooling cold air is
easier than cooling warm air, and that's true. But if your equipment
is running, you're paying money to run it, no matter what. So, when
your house is comfortable you'd save money if you turned the equipment
off - which is what a thermostat does.

A programmable thermostat is a good idea if there are long periods
(when you're at work?) when it would be okay for the house to be
warmer than you're comfortable with. That way, the AC is only running
occasionally during the day, then starts cooling the house to your
desired temperature before you get back home. If you program it right,
you won't notice that the house is ever warmer than your chosen
temperature. But you probably will notice the difference on your power
bill, especially if you live in a location where power costs more
during the day than at night.

When you're in doubt about whether something is saving you energy,
just find your power meter and figure out how to read it. Read it for
a week (or month or year) with the old thermostat and a week (or month
or year) with the new one and see if you get different results.

I started to answer this and then I decided I wanted to read what
others had to say.

First of all, I would suggest simply doing the test. Learn how to read
your meter.  Check it for a couple of days to get a standard reading. 
Then switch. Run it on auto for a day or two, then on ON and see which
saves more energy.

What everyone has said so far rings true even though they appear to be
conflicting answers.

First, on the surface, it seems obvious that your house is going to
heat up to whatever temperature it gets, and your air conditioner is
going to cool it to whatever temperature you set it.  Whether or not
the fan all the time or not is irrelevant.  Thermodynamically, you're
spending energy(and money) to suck the heat out of your house.  So to
lower the temperature inside your house to 72 from 80 is going to cost
the same amount of COOLING ENERGY.  Thus, initially, it would appear
that running the fan all the time, would in fact cost extra! Right???

Maybe NOT!

 Huh?

  You see, if you have a house like mine.... and you run your AC on
auto, you've probably discovered that your child's room is 20 degrees
colder than the room YOU want cold. So you're busy trying to close off
vents and shove cardboard in the stoopid holes in order to change the
distribution of air in your house while your wife is nagging at you
that she's about to call a repairman! Meanwhile you're sweating like a
pig on the 4th of july! But I digress.

The point is, that your house isn't in equillibrium, and the
efficiency of cooling and heating around your thermostat(right where
it's measuring the temperature!) might just be different than the rest
of your house.  As a result, you might just be cooling some areas
(that you don't really use) more than you want, just to get
comfortable in your favorite part of the house.  By turning the fan
on, you keep the air circulating and the overall temperature of the
house at a better equillibrium.

Depending on the electrical efficiency of cooling, vs that of having
your fan run all the time, you may or may not see a benefit in your
electric bill.

The ideal solution would be to have a wise old HVAC technitican design
or repair the duct work and air flow in your home, but all of this
changes once you put the furniture in and move it around.   If you
don't believe me, put a nice hot television set right next to your
thermostat and see what happens!

Anyway, this is the first idea that I've ever heard of leaving the fan
on, so now I'm going home to try it!

tnsdan...

A lot will depend on the construction of your house, with regard
to where the ducts run. I heard of this idea some time ago, and
tried it where I now live. I have a heat pump rather than AC, but
the concept is the same.

What I quickly found out is that my ducts are at the ceiling, and
closest to the roof which is not as well insulated as it might be.
As a result, the air which runs through the ducts quickly turns
hot very soon after the compressor shuts off. Hot air rises anyway,
so if your ducts are located high up, they will tend to be moving
air which is warmer than the ambient air, but add to that their
proximity to a roof heated by the Arizona sun, and there's no 
question - it was almost like the heat had come on in between
compressor cycles, and I found myself turning the thermostat to
a lower temperature to compensate.

If your house is very well insulated, or the ducts are not close
to the roof, your mileage may vary.

sublime1-ga

Running the central air conditioning fan continously makes your house
more humid. If higher humidity is desired, there are less costly ways
to raise the humidity, such as drying your laundry inside your house.
 I think the subline one comment applies to most houses: mine for
sure. The leaky, poorly insulated ducts act as a heat exchanger
cooling the ceiling crawl space with your air conditioning dollars
with the fan on. There is a tiny savings if you run the fan for about
one minute after the compressor stops. In the heat mode the fan
typically runs a minute or two after the heating stops.
 All fans produce waste heat which warms your house, and increases the
minutes your air conditioner runs. Some people find they are
comfortable at a more ecconomical thermostat setting with fans
running, so there could be a small savings, most of which is used up
by the electricity the fans use and the heat the fans produce which is
removed by the compressor running more minutes. Small fans are
ecconomical in the heating season, but few people want a fan blowing
on them during the heating season.
 As for rooms that are too hot or too cold either winter or summer,
put a small fan in the doorway to blow cool air near the floor into
the warmer area which will move air near the top of the door in the
opposite direction. If the fan is pointed up as well as out, it will
also reduce the difference between floor and ceiling temperatuers.
Blocking some of the vents is helpful, but you may damage the centeral
fan or make the air conditioning less efficient, if you block more
than half of your vents. Payback on a programable thermostat is more
than a century, unless everyone in the house follows the same schedule
with rare exceptions. I've often thought, two buttons to push would be
helpful. One labled; "I'm cold" and the other labled; "I'm hot" The
thermostat would go back to being in control 5 minutes after the last
button was pushed.   Neil

Well, guess what.  I've made several calls to get a definitive answer
to this age-old question.

I apologize, because apparently I misunderstood the person who told me
when I had the new thermostat intalled, to leave it on ON.

The companies I called told me, no... leave it on AUTO, but what you
can do, is leave it on ON when you are running no a/c or heat, to
circulate the air in the house.

(Probably instead of, or addition to running ceiling fans if you so desire.)

Reasons not to leave the fan running all the time:

-  It uses more electricity.
-  You are stressing out the a/c fan by running it all the time and it
will break down over time.  ($$$ to repair/replace)
-  It will make the house warmer over time.  (I hadn't noticed that
part, but the other reasoning is good enough for me!)

Please accept my apology for giving wrong information.  

Best regards,
tlspiegel

Hi tnsdan,

Thank you for the 5 star rating and tip.  Yes, It depends on so many
variables and conditions... along with personal preference that the
decision is personal.  What works for one house, might not work for
another, and what works for one person, might not work for another
person.  I guess that's why the jury is still out!  In a perfect
world... well, that's another story altogether.  :)

Best regards,
tlspiegel