How to Tell If You Have an Oversized Air Conditioner

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An oversized air conditioner can make it cold but doesn't dehumidify well.

It's been hot and muggy in Atlanta the past few days. Now I'm not your typical Atlantan who says that lightly. I grew up in Houston and south Louisiana and spent nine years in Florida, so I know hot and muggy. Overall, summers in Atlanta are pretty nice because of our elevation (~1000 feet), but we do have our moments.

Anyway, it's been hot enough that we've hit our summer design temperature, which is defined as the temperature exceeded only 1% of the time. In Atlanta, the summer design temperature is 92 degrees Fahrenheit. That's the temperature at which air conditioners ideally will provide exactly as much cooling as the house needs, when sized according to the Manual J cooling load calculation protocol developed by ACCA, the Air Conditioning Contractors of America. 

When outdoor conditions are at the design temperature, an air conditioner should run pretty much continuously and be able to keep the house at the ACCA recommended indoor design temperature of 75 degrees Fahrenheit. When outdoor conditions go above the design temperature, the AC should run continuously and not quite keep the house at 75 degrees.

Now, the thing is, Manual J has some built in oversizing, so even if the air conditioner were sized according to an accurately performed Manual J load calculation, it's not going to run continuously at the summer design temperature. It should run long enough to make the house comfortable, however.

Last week I wrote about the 4 factors of comfort, temperature and humidity being the first two. An air conditioner actually does two jobs - it lowers the temperature and removes moisture from the air, thus taking care of the first two factors, with HVAC design anyway.

To remove the moisture, however, the air conditioner needs long runtimes. That's because the air moving over the cold evaporator coil causes the water vapor to condense. The more air moves over the coil, the more water condenses out and gets carried away. It takes about 15 minutes of runtime before you start getting serious dehumidification of the air, so oversized systems will not dehumidify well.

If the AC comes on, runs 10 minutes or less, and then shuts off, the house may be cool, but in a humid climate, the indoor humidity levels will stay high, probably over 60%.

I know this from personal experience. Last year we replaced the AC in our condo, and I did the Manual J load calculation. The result was that we needed a 1.6 ton air conditioner for our 1500 square feet, so we had a 2 ton system installed. Knowing that Manual J has a built-in oversizing bias, I wanted to go with the 1.5 ton system, but I chickened out. 

Our AC runs maybe 15 minutes max on a hot afternoon, and our relative humidity stays around 60%. The Manual J bias is real, and it's not small. And that's with an accurate HVAC load calculation. I've seen plenty of load calculations that use incorrect inputs so the contractor can come up with a cooling load to match the AC size he wants to install.

Now, back to the point of this article - how to tell if you have an oversized air conditioner. Just get your stop watch and time how long the AC runs on a hot afternoon. Ten minutes or less, and it's definitely oversized. Twenty minutes would be OK. Thirty minutes at a time or longer, and your humidity levels should be fairly low.

So, how long does your air conditioner run on a hot day?

 

See my followup article with real data on our AC runtimes:

My Big Fat Oversized Air Conditioner

 

Related Articles

The Magic of Cold, Part 1 - How Your Air Conditioner Works

3 Reasons Your 3 Ton Air Conditioner Isn't Really 3 Tons

Why an Oversized Air Conditioner Is a Bad Idea

 

Comments

Sam Bagwell

I think most of the houses I built before getting involved with energy efficiency had over-sized A/C. However, based on my experience before they were sold, I think most/all would run over 20 minutes on a hot day. I just checked mine and it is 49.8% RH at 78.9 degrees. So, that seems pretty good.

Sam Bagwell

If 92 degrees is supposed to be exceeded only 1% of the time, I think that it is time that the design temperature be adjusted. I haven't checked the actual stats, but based on memory, I think 3.6 days is way low.

Allison Bailes

Good job, Sam. If they're indeed running 20 minutes, they'll dehumidify the air adequately, unless you have an excessive latent load.  
 
You know, everyone's first thought when they find out their design temperature is, "That's got to be too low!" It's not, however. The design temperature is averaged over 30 years, for one thing. Also, even when we have hot days, how many hours is it actually above the design temperature? Maybe 6? And then we have lots of days that we stay below the design temperature all day long.

Edith Authement

Well there must be something definitely wrong with my a/c. My a/c runs from around 12:00 until 6 or 7 on a very hot day. It runs around 20 minutes at a time during the night.

Paul McGovern

Along the same lines, an article I wrote a few months ago for the local newspaper 
 
Q. My Air Conditioning guy told me my 15 year old compressor is on its last legs and that I will need a whole new system. What should I do? 
 
 
 
A. When a major component of an older HVAC System fails, the entire system needs replacement as new equipment is not compatible with the old. Although it is a large expense, view it as an opportunity to increase the safety, health, comfort, durability and energy efficiency of your home while saving you money over the system’s lifetime. 
 
The U.S. Department of Energy tells us that average older homes experience a natural and complete air change every 35 minutes; newer homes, every hour. To combat this air leakage, HVAC contractors generally oversize units for the comfort of their clients. ACCA (Air Conditioning Contractors of America) estimates the average HVAC system in the U.S. to be 1 ½ to 2 times the necessary capacity. However, oversizing creates “short cycling” (turning on and off over short periods of time), which causes premature wear on the equipment and prevents proper dehumidification. We are left with short-lived units, high energy bills and mold. 
 
The first step in a “complete change out” is to address the ductwork and building envelope air leakage issue: 
 
Ducts - Older ductwork connections used zip ties, duct tape, pop rivets or screws to secure. ALL ductwork must be secured and sealed with mastic (seal the ducts themselves, not just the insulation around them), as should all supply and return plenums and boots including the air handler itself (these are just bent pieces of metal and are very leaky). All boots should be caulked to the adjacent wall, ceiling or floor surface. The larger ducts (trunk lines) should be smooth and either oval or round in shape. Flex-duct should be used sparingly (the undulations inside restrict airflow) and should be run straight, tight and be supported frequently to prevent sagging; turns require elbows to prevent pinching or collapsing. Studies show that poorly sealed ducts can account for 40% of your wasted energy dollar. A Ductblaster test will calculate the extent of duct leakage. 
 
Building Envelope – basically, the barrier surrounding the area we heat & cool is the building envelope. For maximum efficiency, it should be comprised of two complete and continuous air barriers with a thermal barrier sandwiched inside. ANY unsealed penetrations (sheathing joints, plumbing, electrical fixtures & service panels, cable & telephone lines, disappearing attic stairs, HVAC lines, etc.) compromise the integrity of that envelope and allow air to infiltrate or exfiltrate our living space. Outside conditions, occupant activities, stack effect, exhaust fans and HVAC systems are just a few of the factors creating air pressure differentials that cause air changes. Attics, gable knee walls, vented crawl spaces usually lack the exterior air barrier and allow air, which seeks the path of least resistance, to move at will. With a Blower Door, a HERS (Home Energy Rating Systems) Rater can identify and calculate the amount of air leakage to the outside. 
 
HVAC – (many books have been written on this subject alone) now that the ducts and envelope are tighter, load calculations (the amount of heating & cooling capacity necessary for comfort) must be figured. Accurate Manual J Load Calculations are necessary to arrive at the correct unit capacity. Correct sizing increases efficiency, lowers maintenance and operating costs, encourages proper dehumidification, and reduces expenses up-front and over time. DO NOT just replace size for size or allow a “Rule of Thumb” calculation. Ask for a copy of the ACCA Manual J Calculations (required by the 2006 International Energy Efficiency Code and by many Building Codes); if there is resistance, get a 2nd or 3rd opinion. It is that important to the bottom line.  
 
Generally speaking, by following this outline, unit capacity should be reduced by 25% to 45%, which should more than pay the cost of reducing leakage, and will substantially lower your monthly fuel/ electric bill. 
 
Various tax incentives are currently available for HVAC upgrades; consult your tax professional. 
 
 
 
Two caveats: 
 
If your home has any combustion appliances, it is necessary to conduct a CAZ (Combustion Appliance Zone) Test prior to and after the envelope tightening operation. Increasing tightness decreases natural ventilation and CAZ testing helps identify spillage or backdrafting; 
 
If you’ve tightened your home to below .35 natural air changes per hour (a blower door calculation), it may be necessary to bring in make-up air from a known source, with a relatively inexpensive air cycler, to provide adequate ventilation. 
 

Jamie Kaye

Good article Allison and good comment/article Paul. 
 
My air conditioning has been running like crazy lately for long run times, way longer than 20 minutes and sometimes it seem almost continuously throughout the day. I also have a variable speed unit, so that is helping. The relative humidity in my house is averaging around 53%.  
 
I had a Manual J done on my house when we remodeled as well as a Manual D, so we could properly size the unit and the duct design. Per the HERS rating, my unit is still oversized by a ~1/2 ton or so. My house is 2560 sq. ft. and I have a 3 ton unit. The cost savings from a tight envelope and sealed ducts has definitely saved me money and increased the comfort of my home.  
 
I completely agree with the fact that you will save money in the long run by addressing the ducts and envelope prior to just replacing with the same size as you had before. Your comfort is something that is hard to quantify monetarily, as well as the fact that high humidity will just cause numerous problems, many of which can not be seen until you open the walls. 
 
Best, 
Jamie

Allison Bailes

Edith, it sounds to me like your AC is sized properly as long as it's keeping your house cool. If the refrigerant charge is low, it can have long run times as well, but then it doesn't keep the house cool or dehumidify the air because the coil won't be cold.  
 
When it's running for a long time on those hot days, is the house cool? If so, I'll bet the relative humidity inside is also really low.

Allison Bailes

Good article, Paul. You pretty much covered the fundamentals there. 
 
Congratulations, Jamie. You're in the minority. Not only did you get a high performance building envelope in your renovation, but you also got an accurate Manual J as well as a duct design that works. I've looked at a lot of Manual J results in the past few years, and it's easy for contractors to bend the rules to get the result they want (e.g., 600 square feet per ton). I'm glad to hear that yours worked out.

Katherine

We purchased a new home in Houston TX in 2010 and the relative humidity in our new home is in the upper 50's and 60's - feels very uncomfortable. The builder told us to leave the temperature at 74 but we are so cold at that temperature??? Any suggestions??? 
Thank you.

John

My run times even during the hottest part of the day are running about 7 minutes and shutting off for 4 or 5 and does that all day...a manual J was done and according to a climate zone chart mine are close to undersizing if anything. plus my indoor humidity stays between 42% -50%. I live in N. Alabama so it is very humid..Is this still considered short cycling?

Allison Bailes

Hello, John. Yes, your system is definitely short cycling. What causes the most wear and tear on mechanical equipment is the startups and shutdowns, not continuous running, so even though your home's humidity is low, I recommend getting this checked out.  
 
Since the unit isn't off for very long between runs, it's able to pick up with the dehumidification process without difficulty. Because it's off for such a short time, though, it could be a problem with the thermostat.

John

Not necessarily a tstat problem. My particular thermostat just doesnt have a differential setting where for instance it would have to cool a full 1,2 or 3 degrees before shutting off. It is set for like .6 or .8 tenths of a degree so it holds temp all day long. Checked the energy bill and it is fairly good considering it is a large house and has been very hot and humid!

john

Sorry..forgot to add my main problem.. My ducts are sewating which I am told by everyone in my area that is normal this time of year but I am concerned about the duct boot...it is sweating and the subfloor around the boot is sweating. and yes the techs have checked the system and said it is fine. if anything my units are closer to being undersized. It is a new house and it is very air tight.

Allison Bailes

Ah, that's what I was thinking it might be when I said there might be a problem with the tstat. Your particular tstat is working as designed; it's just not a good design if it's trying to hold the house to a really narrow temperature range. Not being an HVAC techie, I don't know all the details of tstats and at what point they kick on the unit, but I think yours may be set too low. 
 
Regarding ducts sweating, where are they? If they're in a vented crawl space, your best bet is crawl space encapsulation. The next best solution is to make sure they're completely insulated and that the insulation is sealed to keep humid air from getting into the insulation.

john

I've heard of sealing up the crawlspace but 99% of the people here in the south do not..radon issues, water leaks, etc...they think the house needs to breath. Ducts are triple the amount of insulation the code calls for. I don't know what to do.

Katherine E.

We built a new 2010 home in Houston TX and the humidity level ran between 57 to 62%. The HVAC company suggested we install two dehumidifiers and we did but they run continuously between 18 - 20 hours a day???? Now what? Is this normal?

Paul McGovern

Katherine, if the interior RH exceeds ambient, moisture must be coming from within. Consider the possible sources and address ... often, dehumidifiers are merely band-aids ... remember, wet moves to dry ... by drying the inside air are you encouraging movement of moist air into the envelope?

Allison Bailes

Katherine, as Paul said, dehumidifiers are often a band-aid solution, and that's certainly what they seem to be in your case.  
 
There are two things that should've been done when they built the house that would reduce your need for a dehumidifier: (i) air sealing, to keep the humid outside air from leaking in, and (ii) installing a properly sized AC. The AC is your primary dehumidifier, but if it doesn't run long enough, it won't do its job well. By making the house as airtight as possible, you'd have much less need for dehumidification, too. 
 
What's the humidity running in your house now with the dehumidifiers? If it's 40%, you could turn them up a bit to reduce what you're spending to run them. Keeping the house at 50% RH is fine. The other ways to reduce the runtime is to air seal the house and get a right-sized AC. Before air sealing, though, get a Blower Door test to find out how leaky it really is.  
 
That being said, in a very humid environment like Houston (where I was born!), a dehumidifier is a good idea to deal with those cloudy, muggy days, when it's not hot enough for the AC to run much but you have trouble making the house comfortable because of the humidity. You shouldn't need two of them, and they shouldn't be running as much as yours do, however. 
 
The other thing to keep in mind is that if your relative humidity before was 57 to 62%, your house performs like a typical house. It's not terrible, and you probably won't start growing things at that level. When it gets above 70% is when you've got a really serious problem.

Katherine E.

Thank your for the input on my high relative humidity with my new home. The a/c company did come and replace a larger a/c unit with a smaller one thinking that would help but still did not bring down the rh. The funny thing is - when the outdoor relative humidity is low then there is a low relative humidity indoors but when the humidity levels are high (above 70%) then the relative humidity levels indoor are high also? Would this indicate there may be a leak in the system???

Paul McGovern

Katherine ... is your system outside the building envelope? if so, the culprit could be duct & equipment leakage. Allison's demonstration of the effects of static pressure, using a ball and vacuum (see the Energy Vanguard website) was very enlightening. It shows that moving air has less static pressure than the air around it. When the moving air is confined by ducts, plenums, and equipment, it creates a vacuum of sorts and wants to suck the surrounding air in, thus encouraging system leakage. Sounds to me like the system might be sucking in more humid air than the coils can dehumidify. I am no HVAC expert & would like to hear from David Butler or others as this situation is not uncommon.

Marie jean

I have a newly installed air conditioning. I am experiencing mold, a/c line is clogged,and the coil is icy. What could possibly be happening.

Bruce

Can I adjust this formula down a degree or 2. In Houston our design temp is 94. If its going to be 93 tomorrow and I set the t-stat to 74 instead of 75 will I get the same results? 
will this give a good run time comparison? 
Thanks

Jessica

I can't seem to figure out what's wrong with my AC situation. We have a 70 year old 1000 sq ft home and recently upgraded our AC unit to a 16 seer 2 stage from a 10 seer single stage. Both are 2 ton. The humidity level in my home will not go below 59%, even if I let the AC run for over an hour at a time. I have set the AC to 1 CPH for both 1st and 2nd stage, and I let the temperature get FREEZING in the house, but to no avail. What gives? Is it because we are a drafty house? Could it be that the attic stairs *next to the return register* are VERY poorly sealed? Please help! I don't know what to do to fix this and I want to do something fast before we get mold or mildew.

Chris Marriner

So, My AC system died last spring, and I was prepping to put new windows (impact resistant, low e) in my house... My house is approx 2200 sq.ft in Florida, south of Tampa... I had a 4 ton system, and knew that would be too big... my HVAC tech replace it with what was here and said he could tune it... Now my house averages 55%rh or higher... Up into mid 66-67...  
 
I talked to him about it and he wants to bring some attic air into the return so it will run longer... I think that adding a whole house dehumidifier, like the Williams air sponge, or April Aire would be the best solution... Any feedback??

Allison Bailes

Chris M.: If your HVAC contractor really wants to bring attic air into the return, then you need to find another contractor. Not only will that not solve your problem, your energy bills will go up. 55% RH isn't bad, but if you're regularly getting up north of 65%, (especially if you spend significant time over 70%) then yes, you may want to look into using a dehumidifier. It's too bad you just replaced the AC.

edda harbert

my air conditioning runs all day once a while kicks off at night. is it normal to stay on allday? WE HAVE IT SET AT 74 

Ashok Aiyar

My 3-ton AC unit is way oversized for our two-story 1600 sq ft town home in New Orleans. This unit was installed by the previous owner right after Hurricane Katrina. The 3-ton unit replaced a 2-ton unit that was probably closer to the required size. 
 
During the average summer day, the average run time was 5 minutes (and even the 5 minutes was the default setting using a programmable Honeywell thermostat). The AC ran for about 4 hours a day, and even though the temperature was set at 75oF, it felt hot and muggy all the time. As is typical with short-cycling, the indoor relative humidity ranged from 60-68%. Typically, there were about 40-50 AC compressor starts every day. 
 
All that changed after I read several of your (Allison Bailes) posts and a few articles by John Proctor. First, I replaced the Honeywell with an ecobee thermostat because it was simpler to program various installation settings such as the minimum compressor run time. Based on information from both of you, my compressor is now set to run for a minimum 20 minutes at a time. In addition, I have added a small 40-pint portable dehumidifier into the mix that is downstairs (the HVAC return register and thermostat are upstairs). The dehumidifier is set to maintain 50% RH. I also have Vornado fans circulating air throughout the house. After several weeks this summer, I now have the following averages 
 
Sleep cycle (21:30-06:25), Set Temp = 80, Av. Temp = 77.6, %RH = 45.6 
Wake cycle (06:30-09:25), Set Temp = 77, Av. Temp = 75.4, %RH = 47.3 
Away cycle (09:30-17:25), Set Temp = 80, Av. Temp = 78.6, %RH = 48.3 
Return cycle (17:30-21:25) Set Temp = 77, Av. Temp = 75.5, %RH = 47.5 
 
The AC runs for an average of 5 hours a day, with 14-16 AC compressor starts. 
 
The house feels very comfortable throughout the day - even during the Away cycle. With the fans, the perceived temperature is a few degrees lower, low enough that a comforter is needed at night. 
 
Without the dehumidifier in the mix, the %RH ranges between 50-55%, so the tiny dehumidifier certainly helps - I have to empty the 9.5 pint bucket once a day. The dehumidifier consumes about 6 kWh per day (measured using a Kill-a-Watt). 
 
So far, this "Rube-Goldberg" solution has worked well to compensate for an oversized compressor. I'm guessing the longer run times and fewer starts are probably better for life of the AC compressor as well. 
 
Do you think this is a reasonable approach? What are the negatives to doing so (apart from the increased power consumption - however, oddly enough, my electric bills this year are about 15-20% LOWER than last summer - I cannot understand why)?

Charlie Conger

Those humidities seem well controlled. We did something very similar in my sons house in Houston. He has a 3 ton AC in 2300 SQ ft new home. It had a sealed attic and the AC ran very little, even when set to 75 degrees. Reducing the ventilation rate on the external ventilation controller and adding a 70pt per day portable dehumidifier made a huge difference in humidity levels ( now about 45% ), comfort, and smell. The monthly bill went up about $20 but the comfort is much better and they don't freeze all the time. This was just an experiment to see how it worked. He will be installing a central dehumidifier which is at least 3 times as efficient as soon as he can afford it. 
I am convinced that in humid climates dehumidifiers are almost always helpful. They allow independent control of temp and humidity. This is not the case for AC's except for the Top of the line models that allow much better control using better logic and variable speed blowers and compressors.  
 
Of course all of this is a process and the house should be well insulated and tight BEFORE you do anything else. 
 
BTW I have also had success by INSULATING the temp sensor inside my Honeywell thermostats with a short strip of weatherstrip ( foam tape). This works because most Honeywell thermostats try to control the temperature too tightly often to less than +/-1 degree. While this may be OK for moderate /dry climates it does not work well with oversized AC units that can change the inside temp 1 degree in less than 10 minutes. By insulating the temp sensor it takes longer for the thermostat to react. You can temporarily test this by building a foam box and putting it over the whole thermostat. Alternatively block all the air vents on the thermostat by sealing them with blue removable masking tape.  
 
I would not recommend disassembling your thermostat unless you are very comfortable with such activities. 
 
In the less hot months it made an even bigger difference in his house.

Scott

As an HVAC technician with 21 years of experience, I have to disagree with the concept of oversized systems causing high humidity. My home A/C system is 4 tons in the midwest. Manual J would probably call for a 2.5 or 3 ton system. Lazy, cheap contractor didn't do a load calc. It runs 5-7 minutes per "on" cycle on a warm day, never more than 10 minutes. Out design temp is 95 and it will cycle off on a 106 degree day. But It dehumidifies just fine. With a modern 13+ SEER system the evap temp will fall to 45 degrees within the first 30 seconds of running. A 45 degree evap will be collecting full moisture if the dewpoint of the entering air is 50 degrees or more. One caveat: the blower must cycle. With a continuous blower you will need 80% runtime or more to dehumidify most homes due to re-evaporation of moisture during the off cycle. I never recommend a continuous blower for that reason.