Air conditioning/running the fan

mmd789July 2, 2014

I have read many comments about the argument, "leave fan on auto vs. leave fan on all the time" while air conditioner is running. I am leaning towards leaving it on auto, but my spouse believes otherwise; here's the problem we have:

The thermostat for the air conditioner is upstairs in the hallway. I noticed it was very cold one day and saw that the thermostat read 78�F; it wasn't 78�F, not even downstairs.....where it is always warmer than the upstairs when the air conditioner is on. The thermostat was set at 72� (I prefer 75�) and I figured the air conditioner kept running the entire time trying to get from what it "thought" was 78�F to the setting of 72�F. I figure this to a big waste of energy and dollars. We both know this happens with the thermostat. However, he believes that by leaving the fan on all the time, this "misreading" won't happen, so he insists on setting the fan on "on" instead of "auto". Well, I left it on the entire time and today the "misreading" occurred again.
Does someone know if the position of the fan (ON vs. AUTO) has anything to do with the occurrence of thermostat not reading the temperature properly. Spouse claims that the air conditioner serviceman said the fan would be best set on "on" to avoid this misreading. I remember the serviceman not being able to fix this problem, as he was called several times for the same reason.
Any ideas/thoughts?

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The auto/on switch has nothing to do with how the thermostat measures the temperature of the room.

I don't understand the comment about the thermostat "misreading" the temperature. It is possible the thermostat is defective and not measuring the temperature accurately. You can verify this by placing another temperature gauge as close as possible to the thermostat. Here is a link to a device I use. It is not the most accurate but it should be within a degree. It also measures humidity which is important for comfort.

Is the thermostat mounted on an outside wall? Is there a large hole behind the thermostat? If the wall is warmer than the room air temperature, or warm air can reach the thermostat through a hole, then the thermostat reading would not be accurate.

Running the fan constantly will help circulate the air throughout the house and help balance temperature differentials. Running the fan in the summer tends to raise the humidity level which could make things worse. The fan will also use quite a bit of electricity so I doubt you will save money by doing this.

Here is a link that might be useful: AcuRite 613 Indoor Humidity Monitor

    Bookmark   July 3, 2014 at 9:19AM
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ken_adrian Adrian MI cold Z5

i have a trilevel with basement ... what a nightmare ... and to be clear.. the fan we are talking about.. is the furnace fan ... not that big loud thing outside ... the inside one just moves air ... and thasts what putting the fan only on.. means ...

hot air rises ... cold air falls... its all about gravity ...

in summer... all cold air is sent to the highest level ... and as it cools upstairs.. it falls downstairs ... to the point where the basement is 65 ... and in fact.. i have no ducts open on the lower tow levels ... also we have running ceiling fans to keep heat off the ceiling ...

in winter.. its the opposite... all warmed air is sent to the lower two levels... and it rises to the top 2 ...

especially in winter .. with big south facing windows... it can be.. on a zero degree day in a wonderful MI winter... 85 upstairs... and 60 in the basement... on sunny days.. i run the furnace fan almost all day ... and the heat never really kicks on ... i just move it around the house ...

one thing i have learned.. is to forget about the actual number on the thermostat.. and just work with what is comfortable ... [learned this after arguing with mom for years.. with her telling me i was crazy to run the heat at 72 in winter ... finally i realized ... i was not living in her ranch house.. and i had to get over the guilt of how she heated her house]

i realize my example is winter related... but i hope it helps you understand ..

one option.. would be to have the thermostat moved to the level that is most important to you ... and that is .. most likely .. not the top .. presumably bedroom level ...

one other thing i did when i replaced the furnace.. was to install a cold air return in the basement ... to increase whole house air movement ...

in my MI ... its either money on gas to run the furnace ... again.. in winter... or cheaper electric to move solar heat around... i vote for the electric ...

carp .. i think i voted with hubby and the a/c guy ... lol ...

good luck


ps: shall we presume the thermostat is new and properly working???? .. maybe try hanging a dollar store thermometer next to it [attach it to a string.. and just loop the string around the thermostat] .. and find out .... wouldnt it be a kicker.. if somehow its off???

    Bookmark   July 3, 2014 at 9:39AM
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789,I suggest finding a new service man. A family squabling over finer points of subjects neither is trained on is beating dead horses at best,causing needless discontent at worst. Mike and Ken are both correct in what they are saying but it seems conterdictive until you realize they are making intirly different points,both good. Point being tech talk can be confusing even when it's correct. You may have any number of issues in addition to what you are thinking but running fan is counterproductive where low humidity and tempature is desired. Don't put much into numbers on a t-stat,they are only guides not absolutes.

    Bookmark   July 3, 2014 at 9:37PM
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I may have missed the OP's point, but I didn't take him to be suggesting that the thermostat was not properly measuring local temperature... but rather the thought was that the (stagnant) air in the hallway where the thermostat is located did not represent the overall average temperature for the upstairs. By running the inside fan continuously it would cause the air to be more evenly distributed and then the air around the thermometer would cause the thermostat temperature to better represent the upstairs temperature.

I have heard people suggest this and indeed know of neighbors that do this. I don't do so and would not recommend continuous running of the fan.

I don't think being dismissive of the handy-man's advice or of the OP's training is the issue, however.

This post was edited by saltidawg on Sat, Jul 5, 14 at 13:01

    Bookmark   July 4, 2014 at 8:13PM
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mike_home wrote:
"Running the fan in the summer tends to raise the humidity level which could make things worse."

Whoa, I hadn't heard that before. Why would that be?

We tend to run the fan most of the time in the summer to help equalize our upstairs and downstairs temps (at least a little), but I've never heard this would have an affect on humidity (nor can I think of what the mechanism might be).

I really wish programmable thermostats would have a setting that allows me to switch between fan on/auto at different times, but alas we end up turning it on and off manually all the time.

    Bookmark   July 7, 2014 at 9:28PM
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When the AC is running and the coil is cold, water condenses into the drain pan. The condenser stops, the condensation also stops. Any water on the coil or drain pan will evaporate back into the air if the fan continues to run. This will raise the humidity level.

    Bookmark   July 7, 2014 at 9:54PM
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"When the AC is running and the coil is cold, water condenses into the drain pan. The condenser stops, the condensation also stops. Any water on the coil or drain pan will evaporate back into the air if the fan continues to run. This will raise the humidity level"

Mike, absolutely!

    Bookmark   July 7, 2014 at 9:57PM
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toolbelt68 (7)

Hmmm, I always though that the water in the drip pan was drained off. If that is the case the pan should be empty, right?

My advise is to close off any low cold air returns during the Summer and close off the high returns in the Winter.

An open cold air return in the basement will help heat the basement. Close it off during the Summer.

    Bookmark   July 7, 2014 at 10:13PM
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Well, I learned something, thanks. Does it tend to make a significant difference?

    Bookmark   July 8, 2014 at 8:05AM
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"Hmmm, I always though that the water in the drip pan was drained off. If that is the case the pan should be empty, right?"

The condensation forms on the numerous cooling surfaces in the plate finned heat exchanger and drains by gravity.

Say you are cooling the house and reach the set point to turn off the system and the whole house fan stops. The condensation through out the heat exchanger will drain to the pan and be "disposed of."

Now consider the case of reaching the set point and turning off the system EXCEPT that we leave the whole house fan running. That same condensate is still found through out the heat exchanger, except now the air flow from the running fan will now carry some of that water off in the form of droplets and some off in the vapor state. Some will also make it to the pan.

Anecdotally is has been repeatedly reported here and elsewhere that running the fan continuously has a major adverse effect on whole house humidity and thus comfort.

This post was edited by saltidawg on Tue, Jul 8, 14 at 10:22

    Bookmark   July 8, 2014 at 9:31AM
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toolbelt68 (7)

True, some of the moisture will remain on the A-coil and be blown back into the house, but isn't that moisture the same moisture that was just removed less of course what went down the drain pipe? New moisture was not added to the house, it just didn't get removed. Running the fan doesn't introduce new moisture so either Auto or Fan on works.
If you really want to cool the house down remove the hot air from up around the ceiling... simple as that.

    Bookmark   July 8, 2014 at 5:01PM
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"New moisture was not added to the house, it just didn't get removed."

And thus the humidity in the home is higher than it would have been had the fan stopped when cooling ceased.

You can't have it both ways. lol

    Bookmark   July 8, 2014 at 5:05PM
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The thermostat reads the temperature only where it is located and starts and stops the machine accordingly.

Always think of HVAC not as creating a cool environment, but as removing (or moving) heat. Think bilge pump. It is removing heat from an area via the air return and then the air handler either removes heat from that air or in winter adds heat to it (either via heat strip or heat pump) Consider, your heat pump in the winter is removing heat from the outside winter air and adding heat to the inside air via the air handler. THIS IS more efficient than an electric heat strip in the air handler.

One zone (system and thermostat) to control the entire three story environment is going to be unstable and varied. Just the way it is.

I read about opening the ducts in basement in winter and closing the uppers. that may help. Closing the ducts in basement in Summer and opening in attic may also help BUT, remember, it is about moving heat from inside to the outside. Bilge pump that heat overboard. If you have more heat entering than you can pump, you will lose the ship. Also, you should be removing the heat (air handler return) from the top. You want to remove the heat from the top of the structure. Even a window shaker in the upper rooms would assist in pumping that heat outside, thus causing your central AC to run less.

Ideally, cost not being an issue, you would have separate systems for the main floor and upstairs each with its own thermostat. The basement would get the overflow.

Where do you live, how many sq feet per floor and average ceiling height on each level? Where is the thermostat located?
Stay Cool.... like Fonzie!

    Bookmark   July 8, 2014 at 6:37PM
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Say what? lol

    Bookmark   July 8, 2014 at 7:25PM
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Several years age, before our new 2-stage system (which is supposed to run longer and cool more evenly...) we found that the upstairs was always warmer in the summer. It still is, but not as much.

We got a big floor fan that we put at the bottom of the stairs. Like this, 3 speeds and tilts....

We use it to blow the cooler downstairs air up the stairs. You can feel the difference in the upstairs hallway when it is on. If the bedroom doors are let open it helps them as well. I think it is a similar effect as running the ac fan on "on" but without the humidity issue. It would help the OP if his problem is just the temp in the hallway.

In addition, my husband now wants it on every night as white noise. We have adult (use that term loosely!) sons that come in at various hours and it helps disguise the noise. Plus, he has some tinnitus and I think the noise helps with that somehow.

    Bookmark   July 8, 2014 at 8:31PM
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