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Don't Set Your Air Conditioner Thermostat Like This

Posted by Allison Bailes on Mon, Jun 06, 2011
 

thermostat air conditioner setting comfort humidity hvacIt's been hot here in the Atlanta area lately. Just three weeks ago we were wearing jackets with low temperatures in the thirties Fahrenheit, and now we're getting up above our summer design temperature every day. That means that a lot of air conditioners are running as everyone tries to keep cool.

One common question I get from homeowners is: Should I put the fan in the on position? Most thermostats have two settings for the fan - auto and on - and some well-meaning HVAC techs tell their customers to put the fan in the on the position to distribute the cool air in the house better.

Let me back up a bit here. For purposes of this discussion, your air conditioner has two components: the part that cools and the part that moves the air. You adjust the part the cools by changing the temperature setting on your thermostat. You can affect the air movement by setting the fan to auto or on.

When the fan is in the auto mode, the air conditioner moves air only when the cooling part is running. When the thermostat senses that the house has reached the setpoint, it shuts off both the cooling and the fan. When the fan's in the on mode, the fan keeps running continuously, even when the cooling part is turned off.

One of those settings is better than the other. The photo below shows how my thermostat is set, and that's how I recommend you set yours, too.

thermostat air conditioner setting comfort humidity hvac fan on auto

The number one reason you don't want the fan to run continuously is that your air conditioner is probably oversized. Since your AC is responsible for doing two jobs, cooling and dehumidifying, moisture from the air condenses on the cold evaporator coil when it runs. An oversized AC will shut off after a short run time and leave a lot of water sitting on the coil. By running the fan after the coil warms up, a lot of that water will evaporate and be put right back into your home.

So, putting the fan in the on position compounds the error of having an oversized cooling system. Your house will cool down quickly without dehumidifying well. Running the fan continuously makes the dehumidification worse and your home less comfortable.

Even in a desert where dehumidification doesn't matter because there's no moisture in the air, there are other reasons not to leave the fan in the on mode. First, it costs more because you've got that fan using about 300 watts while it's running, and you're probably not getting much benefit from it. Good HVAC design in new homes will obviate the need for this if you're running the fan because of air distribution problems.

Another reason to use the auto mode is that if you have leaky ducts outside the building envelope, it can add to the air leakage of the house. Depending on the type of duct leakage, you may end up with higher infiltration (air leaking in) or higher exfiltration (air leaking out). In either case, you add to your cooling load and make the home less comfortable.

Here's another really good recommendation, too. If it's just too hot out, go find yourself a nice waterfall to cool off in, like I did this weekend.

air conditioner cooling thermostat hvac design temperature waterfall

 

Building or remodeling a house and need help with the HVAC and duct system design? We do that.

Click here to find out more about our HVAC design services.

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Comments

Allison, hope this fines well. Just want to thank you for an excellent web site that I refer prospects and customers to for second opinions. This article is a good example as I have hard time convincing a lot of customers not to run their air handler fans continuously. I also want to thank you for exposing me to Dr. Bartlett. I have always held my own thoughts about population factors and sustainability but could never figure out why it was so blatantly ignored. Listening to him has help me put more of the puzzle together. Its good to know that someone has the courage to buck PC and state the mathematical facts weather the politicians or business folks (I refuse to call them leaders) like it or not. I am not aware of his health but hopefully he will stay around a while. We need him and more like him. Thanks again. 
 
Leroy
Posted @ Monday, June 06, 2011 8:31 AM by Leroy Sloan
Leroy: I'm glad you find the EV website and blog useful. And it's always nice to hear from someone who, like me, gets that the arithmetic of growth means that nothing we do is 'sustainable' until we tackle that issue - before it tackles us. Dr. Bartlett has had a long, productive life and, last I heard, he was still out there at age 88 spreading the word.
Posted @ Monday, June 06, 2011 8:48 AM by Allison Bailes
Allison, under what circumstances, then, would I ever want to run my fan continuously (even if just for a short while)? Why have the ability to cycle-on continuously?
Posted @ Monday, June 06, 2011 9:24 AM by John Poole
Sorry, Allison,I don't fully buy your arguments for leaving the T'stat in the AUTO position. If the  
AC unit is oversized, water left on the coil still has a good chance of getting in the home, as obvious from many buildings that have humidity problems even when the stat is in AUTO position. If anything, the water will evaporate into the ductwork and then be blown into the building the next time the fan cycles on. There is more opportunity for water on the coil to flow to the drain between fan cycles; but the short cylcing of the AC unit may not allow much additional time. 
 
Speaking of moisture in the ductwork, if that ductwork is lined and has a decent layer of debris, the moisture sitting around in the duct can help generate a healthy mold growth on the debris. One thing noted by many of those who sample mold or even particle counts is an increase in counts when air handlers come out of set-back after sitting for awhile, similar to fan cycling when the stat is in AUTO position. In AUTO, long time periods may exist between fan cycles that allow mold growth. On the other hand, mold does not like to grow in breezy conditions, such as when air is flowing through the duct. Even with elevated moisture conditions, mold growth is stopped or slowed due to air movement. Before the messages start to fly, yes, the answer is tighter ducts with insulation on the outside--but we don't live in that perfect world and sometimes alternatives are needed. 
 
You also did not include the fact that the only time air filtration occurs is when the fan is running. For persons with allergies during that time of year AND a decent filtration system, running the fan constantly can drop allergen counts inside the home, making the season survivable. 
 
The HVAC system might also just be designed so that it actually pressurizes the building when it is running. In which case, it could be helping to prevent intrusion of particle pollution in areas with elevated outdoor levels, and oh yeah, those accursed allergens.  
 
I would also be a little more reflective about the argument for the air flow mixing the air. Mixing the air might be more important for contaminants and humidity control than for temperature control. In a small office, I have often heard complaints about "lingering odors". A home may not have as many issues as an office, well until someone burns something in the kitchen. 
 
I am sure that if I thought about the pro and con arguments long enough, I could come up with more for setting the stat to ON. Otherwise, I will give you the arguments about leaky ducts and "Oh my God" electric bills when the fan runs constantly. I just want to point out that having the stat set to AUTO may not always be the rosy scenario. I am not an advocate for running the fan constantly all of the time. From an IAQ viewpoint though, constantly running the fan might be the better alternative. 
 
And, I do like your waterfall idea and will put that on my to-do list.
Posted @ Monday, June 06, 2011 9:30 AM by Matt Klein
John P.: I can think of only two situations I've ever put the fan into the on position. One was when I lived in the house I built and we were using the woodstove to heat the house on really cold days. The house was tight and well insulated with the woodstove right in the center, but some of the furthest parts would get a tad chilly without the fan running. All the ducts were inside the envelope, though, and there was no moisture on the coil. 
 
The other was when my stepson forgot the pocket warmer he was recharging on the stove, boiled the pot dry, and filled the house with smoke. We opened the windows, ran all the exhaust fans, and put the air handler fan on for a while, too. 
 
If you find Matt Klein's arguments convincing, you might also want to run the fan for those reasons.
Posted @ Monday, June 06, 2011 10:11 AM by Allison Bailes
Matt K.: Yes, water on the coil can get back into the house with the fan in auto mode, but it'll be a lot slower that way. More of it, however, will still be on the coil the next time the thermostat kicks the unit on. 
 
You make an interesting point about air movement and mold growth in ducts. Do you have a reference on that? If we're talking about the imperfect world we living in, though, it seems to me that there would still be plenty of ducts with low enough airflow to promote mold growth even when the fan is running. Even in the ducts with high air flow, there would still be plenty of surfaces inside the ducts where the airflow is low enough not to prevent mold growth. I'd like to know more about the conditions of the homes where they found the mold problems in ducts. 
 
Regarding filtration, that's the last step towards achieving good indoor air quality. Filtration is a bandaid solution.  
 
Putting positive pressure in the house can be a good strategy for ventilation, if it's done right. Proper sizing of the HVAC system, which gives long runtimes, is the best way to achieve it.  
 
Thanks for the long comment, Matt! I do agree that indoor air quality is an important consideration. I think in most cases, however, that we could find better answers than putting the fan in the on position.
Posted @ Monday, June 06, 2011 10:15 AM by Allison Bailes
Christopher: Thanks for the dry climate perspective on this topic. So, you're basically setting it up so that your AC becomes a temporary swamp cooler until the fan blows the moisture off the coil. I like it! John Proctor's a smart guy. 
 
Yeah, designing the filter and return plenum is an overlooked flaw in many systems, and one of the main reasons that airflow is so bad. You can't just throw a big pleated, media air cleaner in the same size return plenum, or you'll send the static pressure sky high.
Posted @ Monday, June 06, 2011 10:40 AM by Allison Bailes
The way I'm seeing it here,... it is like owning any electrical appliance the consumer makes the ultimate choice on how and when to utilize the product. I do not choose to use central air and heat and use window units. I don't cool/heat certain areas of the house like the kitchen, hallway, utility, and two bathrooms. My philosophy is if it's not a room that a human is going to be occupying for any period of time it doesn't need energy waisted on it. Same as when you leave the house/room turn it off! I do understand the allergen factor though, that is a tough reason to use the "On" setting instead of the "Auto". Now there are reasonable cheap add ons to duct systems that kill all the mold/allergen spores through-out the positive air flow system. "well worth it" for the suffering due to allergies, etc. These things pay for them selves in no time compared to the cost of running the stat on the "On" position constantly. Just something else to think about.
Posted @ Monday, June 06, 2011 10:41 AM by Ken Bailes
Glad to see this article Allison. I'd like to clear up some confusion about moisture vs. oversizing. 
 
Contrary to what was suggested by Matt, the relationship between sizing and moisture is about coil temperature, not evaporation. This is a common misconception. When the condenser turns on, it takes up to 10 minutes for the coil to get cold enough to condense moisture. If the system is oversized, the average cycle time will be shorter, therefore the initial dry-coil time will represent a larger percentage of the overall run time, thus reducing dehumidification. During shoulder seasons, the coil may never have time to reach the condensation temperature. 
 
Whatever moisture may evaporate back into the house is trivial compared the moisture that was never removed to begin with. In any case, air movement during the off cycle will only increase evaporation! 
 
Aside from moisture issues, allowing the blower to run 24/7 will unnecessarily increase power consumption, especially with a PSC blower (even 400 watts isn't uncommon). If there's a reason (such as filtration) to continuously operate the fan, make sure the blower has an ECM motor, AND make sure the fan-only circuit is set up to operate on the lowest blower speed. For example, a 2-ton Lennox CBX27 uses less than 100 watts on tap 1. But if the ducts are in the attic, even if they are tight, running the fan during off cycles will add a lot of heat back to the house. 
 
Matt's other point about moisture and mold in the ducts is also misinformed. While it's true that mold can grow on dust an debris in the air handler and duct system, this is the result of high humidity due to oversizing, or in some cases, coil blow-off (due to CFM being too high for the coil). Keeping the fan on between cycles may actually exacerbate this problem. 
 
I agree with Christopher regarding filter sizing, and also about using a long fan delay to harvest moisture and provide passive evaporative cooling in a dry climate, although I disable this feature during monsoon.
Posted @ Monday, June 06, 2011 2:26 PM by David Butler
Given the number of times I hear the condensate drain pump cycle for the first thirty minutes after the system finally shuts down at my place, I would have to say that the amount moisture sitting on the coil is pretty significant. ( the catch bin holds about a pint.) 
not looking forward to this month's electrict bill.
Posted @ Monday, June 06, 2011 6:41 PM by Tom Butler
Great article Allison! 
 
You are spot on in this one.
Posted @ Monday, June 06, 2011 7:45 PM by David Richardson
Ken B.: Window units are one way to go, but I think they have a mode where you can run the fan by itself, too. A quieter (and more expensive) way to get the local cooling you like is to use ductless mini-split heat pumps. 
 
David B.: Thanks for another excellent comment packed with more explanations of some of the subtleties here. Great idea to set the 'Fan On' mode to run on the lowest speed of EC motors. 
 
Tom B.: If there's a lot of moisture left on the coil and in the drain pan after the unit shuts off, that indicates you probably have high latent loads, a big coil, and an AC that's sized fairly close to the load. If the condensate pump cycles several times after the unit shuts off, though, it may not be pumping out completely. It's hard to imagine that you'd have enough to fill and drain the pump several times. 
 
David R.: Thanks, David. It's nice to hear that from an HVAC pro like yourself.
Posted @ Tuesday, June 07, 2011 6:56 AM by Allison Bailes
I have an AC unit that is over sized and does not remove the humidity in the house. What options do I have other than replacing the unit?  
Is there a thermostat available that will turn on the AC at maybe 2 degree intervals so that I can get the longer run times to remove the humidity?
Posted @ Friday, June 10, 2011 6:03 AM by STEVE
Steve, turning down gradually would have opposite effect. You will get a longer runtime and more moisture removal by turning it down all at once. Reducing the temperature in steps will cause a shorter cycle at each step. Think about it. If a thermostat automated the process so that it made the next gradual reduction just before the unit cycles off, then the entire cycle would be no longer than it would have been if you turn it down all at once. The exception to this would be if you have a two-stage compressor. More on that in a minute.  
 
Aside from replacing your system, you can have have a qualified tech adjust the system airflow. The lower the airflow, the colder the coil, and the more it will condense moisture from the air. However, this will only marginally reduce the initial dry-coil time. So if your system is very much oversized, it may still not run long enough to reach the condensation temperature. I recommend an NCI or NATE certified tech for this, as he must verify system airflow before and after the adjustment. If the airflow is too low, there will be other negative effects. I don't recommend less than about 320 CFM per ton. 
 
If reducing airflow doesn't help, then you have two choices - either replace the unit or add a dehumidifier. But keep in mind that dehumidifiers use a *lot* of energy, especially portables. 
 
You should also make sure you use range hood when boiling water, and bath fans for at least 15 minutes after a shower. And never open windows when outside dew point is above about 55F. 
 
If you happen to have a two-stage compressor, then it is possible that making rapid changes to the thermostat will bring on the second stage unnecessarily. However, the better thermostats use a more sophisticated staging logic that looks at the rate of change instead of using simple time-based logic (unfortunately, you won't find this mentioned in the specs!). Virtually all non-OEM zone control panels use time-based staging logic.
Posted @ Friday, June 10, 2011 11:50 AM by David Butler
In evaluating scenarios like this there is never a one size fits all answer. I highly recommend a home audit to determine how a house is performing. If you're getting mold growth in the duct system, there are better ways to address it and they should be addressed. A better route to go is keep the ductwork as a secondary or back up system and install a ductless heatpump system. This will eliminate high pressure on the duct system as well as pressurization problem inherant in the vast amount of improperly sized HVAC systems. They take you back to zonal heating and cooling and with the variable feed drive on the compressor do a much better job of maintaining the proper RH in a home.
Posted @ Saturday, June 11, 2011 11:39 AM by Vic Hubbard
deppending on the type of air conditioner you're using the moisture content might be far from pleasant. For instance, when using dx systems the moisture content of the air tends to reach low levels making the air too dry. On those situations having the fan in 'on' mode might be a good option. Also, the modern room air conditioning systems use variable speed drives wih make them more effiint and less energy consuming.
Posted @ Saturday, June 11, 2011 5:32 PM by Paulo Salgado
REPLY TO POSTING: Posted @ Friday, June 10, 2011 11:50 AM by David Butler 
Thanks for the info David. My intentions were not to stand by the thermostat and lower it a two degree increments but the thermostat I have now (Honeywell PC8900 used with a single stage heat pump)) keeps a even tighter control of the temp with variations less than tenths of a degree which results in the unit run time of about five min. 
This thermostat has endless options, one of which is to control the humidity with the AC. 
After coming home one evening to find the house in the mid fifties I no longer use this option! 
I am wanting to find a thermostat that allows wider temp fluctuations in hopes of keeping the unit running longer and therefore remove humidity. 
I do use a portable dehumidifier which put a lot of heat in the house. How is this heat handled in a fixed unit? Would it be wise to vent the heat on a portable unit? 
I think a dehumidifier will always be a part of this equation because there are many times outside temps do not call for the use of AC but the humidity in the house will be high. 
Today happens to be one of those days- current temp 68F with 86% humidity outdoors.
Posted @ Wednesday, June 15, 2011 8:20 PM by Steve
Steve, just to clarify... there are other ways to use the thermostat to control RH than overcooling the space. 
 
For example, most of the stats that have an RH sensor can be used to turn down the blower. This decrease the coil temperature and increase the condensation. All variable speed air handlers and furnaces are compatible with this type of control. But I prefer to use non-variable air handlers that have the same high-efficiency ECM motor, but without the variable drive logic, which is way overpriced IMO. These provide exact same efficiency as full variable. The ECM motor has five discrete speed taps. An $8 relay is all that's needed for the stat to adjust the blower speed on RH call. 
 
The other option is to adjust the thermostat anticipator setting to a higher number, which will increase the width of the control band (the difference between the cut-in and cut-out temperatures). I think most, if not all, digital stats have such a setting in the installer menu. My White-Rodgers allows a control band from 0.4F to 1.6F, cut-in to cut-out. 
 
A home can tolerate brief periods of high RH during shoulder seasons, but in certain areas of the country, or in a basement home, this can be a common occurrence. In that case, a dehumidifier may be necessary. And yes, they do put out heat, even central dehumidifiers. But this is only an issue if there's a sensible load. In any case, it makes much more sense to use the AC compressor to remove the moisture (because it's much more efficient than any dehumidifier compressor) and then use the (free) hot gas for reheat the air it doesn't overcool the space. Several companies sell 'heat pipes' that do this passively. Lennox sells the Humiditrol which is an active re-heat system that's an integrated add-on. I believe it costs less than a central dehumidifier but I'm not sure if it can be added to non-Lennox equipment.
Posted @ Wednesday, June 15, 2011 9:44 PM by David Butler
What happened to all of the benefits of "fan-on" operation in a well designed, properly installed, properly sized A/c and heatpump system? A couple being increased comfort, lack of cold or hot air blowing until unit compressor, heatexchanger and duct work achieve proper temp. for comfort(blowing cold or hot air on start-up) even on modern fan delay systems ? In addition your argument regarding power consumption had always been answered by starting energy of the fan as well as wear on the fan motor would offset this expense to a point at which the savings of intermittent fan would be reduced to less than the cost of comfort and a lower(heating)or higher(cooling) setting with fan-on ? 
What about indoor air quality in a properly constructed and insulated home ? Speaking from personal experience a well maintained filtration system of any type, does a much better job of eliminating odors, dust, allergens, and particulates from all of the normal operations and products used in the home and provides for some healthy air exchange ? These conditions coupled with a central vacum system exhausted to the outside NOT to the basement or CRAWLSPACE as most installers prefer to do ! These improvements all add to the desired indoor air quality. In addition I understood the reason for a drip pan and a gravity drain or properly sized condensate pump system is for the continuous removal of condensation and therefore reintroduction of humidity from the system, Why else have it ? In my particular case the air handler and all duct work is located in enclosed and auto dehumidified, gravity drained crawl space mostly below grade in a very humid, southeastern, coastal locale of NC. My water table is very high as I live surrounded by wetlands. We do get quite cold in winter as well. I personally prefer hydronic heating of all types with my favorite being radiant floor with constant circulation and outdoor reset control. A/c and heating systems should always be installed as separate stand-alone systems. Convincing any builder of this is near impossible ! Thank you in advance for your input on these comments, Sincerely, Tom
Posted @ Sunday, February 12, 2012 10:21 AM by Thomas Clark
Tom C.: How does leaving the fan in the 'on' position increase comfort? About the only time that I can think of that might occur is a house like I lived in a few years ago that has a woodstove. I'd run the blower to help distribute the heat into other parts of the home. I don't think the things you mentioned would do much. 
 
The extra energy used at startup is not a good reason to leave the fan on. In most homes, the continuous runtime energy will swamp the startup energy. 
 
Filtration is the last thing you do to improve indoor air quality. Sealing infiltration sources and duct leakage, eliminating contaminants, and providing mechanical ventilation all should come before that. 
 
The drip pan under the evaporator coil will only catch water that drips down off the coil. When the compressor shuts off, the coil will warm up above the dew point of the air. Then the water sitting on the coil will start evaporating. Blowing air over it causes it to evaporate quicker. 
 
I'm not sure why you think cooling and heating systems should always be separate. Yes, you can better tune each to the needs of the home, but the benefits may not justify the extra cost.
Posted @ Sunday, February 12, 2012 10:57 AM by Allison Bailes
Tom: Allison makes a good point about filtration being the last line of defense in IAQ. However, homeowners with exceptional IAQ requirements might benefit from continuous fan operation, as long as equipment is properly sized and RH is under control (a big if in your climate). But the additional operating cost can be substantial, especially if blower is PSC. For example, if we assume a system cycles on for 3000 hrs/yr and blower consumes 400 watts at $0.11/kWh, then continuous fan would add about $250/yr. If motor is ECM type *and* is properly set to run at low speed off-cycle, then annual cost might be below $100/yr, but still nothing to sneeze at. 
 
As to your point about hot or cold air purge at start-up, a fan delay (a feature on most thermostats) works well if ducts are inside the envelope since the temperature of the purge air will be close to room temperature. But when ducts are outside the envelope, especially in the attic, you may still get a hot or cold shot even with fan delay. However, in this case, running the blower 24/7 is like running the heat or a/c in the wrong season. In your case, with system in a closed crawl, there's at least some penalty from increased heat transfer to and from the crawl (especially if you're complaining about hot/cold shots on start-up!) 
 
The "wear and tear" argument is a red herring. That's only an issue if you have to replace a motor during life of air handler. Blower motors are designed for far more cycles than they're subjected to in HVACR applications. Fan motors do occasionally fail, but there's no reason to expect the failure rate to be substantially different if motor runs continuously.  
 
Your point about start-up (inrush) current is woefully misinformed. The additional power consumed during inrush is a fraction of a kWh per year. 
 
The comfort argument is also a red herring. Between-cycle comfort issues arise when a system is grossly oversized, and exacerbated by an inefficient, leaky envelope. A properly sized heat pump will already be running most of the time when it's hot or cold outside. During part-load conditions, in-between-cycle temperature swings are minimal. But hey, if it make you happy, go for it. It's your money.
Posted @ Sunday, February 12, 2012 12:48 PM by David Butler
I live in an older home and rent. I also work from home as a customer service agent. My older home with it's older air-conditioning unit seem to keep a constant temperature better when the fan is running. This is especially true in my home office where I work and must keep the door closed while working. It can get downright miserable in there, but running the fan circulates fresh air in to the room and keeps it much more comfortable without having to set the temperature for the whole house lower.
Posted @ Wednesday, May 30, 2012 3:41 PM by Brad
I have a question... I live in a large home and I have three geothermal ac/hearing units. The one for the entire upstairs has broken (the coil is cracked) 
Do needless to say it doesn't cool. Until we have the money to fix it... I thought I had come up with an ingenious plan to bring the summer temps upstairs from 110 down to 80 but I'm wondering if I'm causing more damage to my unit? I placed a fan in my vaulted gentry way to push cool air from the downstairs to the upper floor and have set just my exhaust fan to on. It runs continuously. I thought the flow of air... Would make temps more comfortable... By after a few days of actually being home for the weekend I notice the downstairs unit is continuously on cooling... While the upstairs I continuously suckin out hot air from upstairs. I realize we will probably have a high eclectic bill, but I also do not want to destroy another unit... What could I or shall I do or am I just over evaluating the situation.. Till we can save the $ to fix the broken unit?
Posted @ Monday, July 09, 2012 2:28 AM by Stanyelle Billie
SB, if you're concerned about damaging the d/s unit by running continuously, worry no more. As long as the unit is properly charged, running it 24/7 causes less wear and tear than normal on-off cycling. Hate to see your next electric bill, though. You may be a penny wise and a pound foolish by not repairing the u/s system.
Posted @ Monday, July 09, 2012 3:31 AM by David Butler
Thank you for the feed back David... Unfortunate for us though with school starting in four weeks... We have four children to school shop for... And $2500 Repair wasn't in the budget. I'm hoping and praying that this airflow contraption I've set up will partially offset so we can prepare for a repair next month... My fingers are crossed. Needless to say I can't wait for fall to arrive. Maybe a discount will be thrown in with the air guys on a lull in the off season....
Posted @ Monday, July 09, 2012 5:29 AM by Stanyelle billie
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