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My Big Fat Oversized Air Conditioner

Hvac Oversized Air Conditioning System Massive Enormous Condenser

Back in 2009, I had a new air conditioner installed our condo. The previous one was an ancient 25 years old and barely limping along. It wasn’t cooling much, and the summer electric bills had risen. So, I did a Manual J load calculation on our condo to find out what size we should replace it with. I wanted to install one that wouldn’t be too big so it would have long runtimes and keep our place cool and dry. Well, I put one of my new data loggers to use this summer to find out if I succeeded. The (doctored) photo below, which is not our condo, might give you a hint how well I did.

Air conditioner runtime on a hot day

Once you have an air conditioner installed, it’s not hard to tell if it’s oversized. All you need to do is time how long it runs on a hot day. If it runs a lot and still keeps the place cool, you’re in good shape. If it runs only a few minutes at a time and spends the majority of the time turned off, your AC is oversized.

In the world of air conditioning, “hot day” has a specific definition. There’s this thing called summer design temperature, which is the temperature that your location goes above only 1% of the time. Out of the 8,760 hours in a year, the summer design temperature is the temperature exceeded only 87.6 hours, on average.

Your air conditioner should be sized to run almost continuously at the design temperature. That’s the idea behind a Manual J heating and cooling load calculation, a protocol developed by the Air Conditioning Contractors of America (ACCA). But there’s some slack built into it, so you don’t need to worry about being hot on a day when the temperature is a few degrees over your design temperature. If you’re getting runtimes of 45 minutes or more per hour when it’s near your design temperature, your AC is probably doing a pretty good job.

How much capacity did we need?

As I mentioned, I did a load calculation on our condo before I got the new AC. The result was that we needed about 1.6 tons of air conditioning capacity. As I wrote in an earlier article about how to find out if your AC is oversized, I wanted to go with the 1.5 ton AC, but I chickened out and put in a 2 ton unit instead.

Our place is about 1500 square feet, so that’s 750 square feet per ton. The standard rule of thumb that a lot of contractors still use around here is to install one ton of AC capacity for each 500-600 square feet of conditioned floor area. Since our place is old and leaky (yeah, yeah, cobbler’s kids and all that), 750 square feet per ton is pretty good. But, was it good enough?

Measuring our air conditioner runtime

Well, this summer I got finally put a data logger on our air conditioner to see just how oversized it really is. I previously wrote about using a stopwatch to determine that our AC was oversized because the on-periods weren’t long. I measured about 9-10 minutes max on a hot afternoon shortly after I got the new AC. The data logger I used in my latest experiment was the Hobo UX90-004 from Onset.

data logger motor on off onset hobo

All you do is stick it on a motor (it’s held there by magnets) and it tells you when the motor is on and when it’s off. I opened up the furnace and stuck it on the blower motor, as shown in the photo below. The display shows how many total hours and minutes the motor ran while the logger was deployed, but you get a lot more detail when you download the data.

data logger motor on off air conditioner runtime

Show me the data!

After sorting through a few months of data, I settled on one day that seemed to be perfect for illustrating how well sized our air conditioner is. That day was last Friday, 29 August, and I looked at the 24 hour period from midnight to midnight.

I also made sure to keep the indoor temperature constant. The thermostat was at 75° F for nearly the whole day. The only time it wasn’t was in the early morning morning when I lowered it to 74° F.

You can see the data below. The first graph shows the average temperature for each hour of the day (green), the amount of time the AC was off during each hour (red), and the amount of time the AC was on during each hour (blue). (The temperature data are from Weather Underground.)

air conditioner hourly runtime energy vanguard 600

Our design temperature in Atlanta is 92° F. Last Friday, we had a good 5 hours when the temperature stayed right there around 91-92° F. We had another couple of hours above 89° F, so it was perfect for seeing if our AC is oversized.

The first thing to notice is that there was only one hour in the whole day when the AC was on more than it was off. From 6 pm to 7 pm, the AC was on for 31 minutes and off for 29.

Note that the graph above isn’t showing the actual on/off cycles. I binned the data so that each red column shows the number of minutes in its hour when the AC did not run, and each blue column shows the number of minutes the AC was on in an hour. When it got hot in the afternoon, there were about three cycles per hour.

air conditioner on off cycles energy vanguard 600

Over the whole 24 hour period, the air conditioner went through 35 cycles. It started the day turned off and ended the day turned off and came on 35 times in between. (The red columns at the beginning and end don’t show the whole off periods. The first one shows only 29 of 53 minutes off, and the last shows only 4 of 63 minutes turned off.)

A few interesting numbers

I’ve been studying these data for a while now, and there’s a lot to see here. Here are some of the numbers that jumped out at me:

  • Maximum on-time: 14.8 minutes
  • Minimum on-time: 7.2 minutes
  • Maximum off-time: 64 minutes
  • Total on-time: 315 out of 1440 minutes (22%)
  • Mean on-time per hour: 13.1 minutes
  • Mean on-time per cycle: 9.0 minutes

The takeaways

Some of the main lessons we can learn from these data are:

  • I wouldn’t have had any trouble with the 1.5 ton air conditioner.
  • There’s a time lag between when its hottest outdoors and when we have the highest cooling load indoors.
  • There’s a huge difference between the longest and shortest off periods (54 minutes) but not so much with the on periods (7.6 minutes).
  • Even with an old, leaky condo, the 500-600 square feet per ton rule of thumb results in greatly oversized air conditioners.
  • I didn’t do a good job with that Manual J.
  • At 9 minutes per cycle, our air conditioner isn’t doing as good a job at dehumidifying as it could. (I’ve got some really interesting data about humidity coming up soon, too. Stay tuned!)

Rick Chitwood’s favorite strategy

I got an email from Rick Chitwood a couple of weeks ago in which he mentioned how he likes to do air conditioner sizing and installation. Here’s what he wrote:

  1. Use Manual J to estimate the size of AC required;
  2. Use HOBO’s to monitor run times (at design temperatures) and indoor temperature – so you know if you did Manual J correctly.

Of course, if you get the Manual J wrong, as I did, using data loggers doesn’t change that. It does help you figure out how to get the load calculations right and make sure you do better on the next job, though.


There you have it. The data don’t lie. If you want to find out how oversized your air conditioners are, just get out your stopwatch. If you want to get a better picture of what’s happening, get yourself a data logger.


Related Articles

Why an Oversized Air Conditioner Is a Bad Idea

We Are the 99% — Design Temperatures & Oversized HVAC Systems

How to Tell If You Have an Oversized Air Conditioner


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This Post Has 50 Comments

  1. Now that you know is there
    Now that you know is there anything that you can do about it? Reduce the refrigerant? Build an addition? Change the compressor?

  2. Judging by that, even a 1.5
    Judging by that, even a 1.5 Ton would have been oversized. I’d be curious to know where you think you went wrong on the Man J. 
    I’m looking forward to your humidity article. My own system is configured to produce a 23*F temp split resulting in humidity of 45% +/-. That lets us set at 77-78*F most of the time, though I’m known to drop it to 76*F in one zone when eating or drinking coffee.

  3. I hear people talking a lot
    I hear people talking a lot about AC over sizing as the #1 cause of high indoor humidity (in a humid climate), but I would like to submit a heresy: that infiltration from indoors is also a big reason. Remember the AC is naturally going to be over sized any day which is not close to the design temperature. 
    Should we be thinking about the coil temperature? I don’t know anybody who is measuring that and publishing any info. 
    Interesting subject, thanks for writing about it.

  4. Exactly what happens most of
    Exactly what happens most of the time. Contractor “chickens out” and doesn’t trust man J. Contractor then fudges inputs so program spits out 500sqft per ton.  
    I’ve got 2tons on my 1600sqft house and my run times look a LOT different than yours. AC basically runs “wide open” when outdoor temps hit between 95-100 (97 design temp for OKC). A house and Condo have VERY different loads though.

  5. Did you install an air
    Did you install an air conditioner or a heat pump? We have to consider heating if using a heat pump. I am just a home owner that enjoys your articles. Keep up the good work. I have a one story brick ranch house with 1800 square feet of conditioned space that was built in 1975. In 2008 I had a HVAC contractor remove the old oil furnace and air conditioner and install a Water Furnace Envision geothermal unit. The oil furnace was in the attached garage. The HVAC contractor wanted to install a vertical unit in the location of the oil furnace which was the simplest change because the return is in the attic and the supply metal ducts are in the crawl space. The new unit would extend too far into my garage so I got them to hang it from the ceiling. The return duct was replaced with the new unit and is round flexible duct. The metal supply duct is in the crawl space and left as is. It needs to be sealed and insulated but that job is on hold until funds are available. The unit probably runs about like yours in cooling. I am sure the house would be more comfortable if it ran more. Since I am 66, I am not quite satisfied with the comfort during the heating season. I would like the supply air to be hotter and I know sealing and insulating the supply ducts would help. I live in southeast Virginia.

  6. I’d love to see you add in
    I’d love to see you add in some data to show the effects of short cycling on efficiency. As I understand it the SEER rating of an air-conditioner is not taken until the air-conditioner has had at least 10 minutes of run time so since your average runtime was down around 7 to 9 minutes you never reached the rated efficiency also an air-conditioner uses approximately 15 times as much power in the first minute as it does in the 10th minute so you’re 35 cycles in a day at 90° means your system would have been much more efficient if it had ran all day long and never turned off. This is why I am an advocate of variable speed compressors they much more closely match the load and allows a greater margin of error for the system designer I am also seeing the efficiency of variable speed compressor systems to be much greater than what was reported by the manufacturer because they correct sizing issues.

  7. Did I miss something on the
    Did I miss something on the math? One of your “lessons learned” was, “Even with an old, leaky condo, the 500-600 square feet per ton rule of thumb results in greatly oversized air conditioners.” You went with 750 SF/ton and said you would have been OK at a 1.5 ton unit, or 1,000 SF/ton. What guess would you make for a new code-current house (not a Passive House) but one that is built tightly? And since Manual J would go so far beyond that, how would you get to those correct numbers?

  8. Allison – Was the equipment
    Allison – Was the equipment you installed a variable/dual speed unit? Would that have made any difference? Did you do a blower door test to reference the equipment sizing?

  9. I’m always amused when people
    I’m always amused when people write or talk about the right size air conditioner for “the house”. Air Conditioners should be sized for the occupant and not the house. The occupant who likes the house at 80 degrees on a 100 degree day needs a different system than the person who wants to be 65 degrees. The situation is even more pronounced when it comes to heating … older people … and cold climates. 
    Even the best sized air conditioning system is over sized 80 to 90 percent of the time. The real problem is single size systems. Two speed and variable speed equipment is the answer for everyone who values their comfort more than a few extra days on vacation. 
    And you’ll still be able to use your beloved Manual J. Just don’t forget to ask the client what temperature they want to keep the house.

  10. @JAMIE CLARK,  &lt
    Wherever did you get these numbers? Can anyone document a source, hopefully not hearsay? 
    “…As I understand it the SEER rating of an air-conditioner is not taken until the air-conditioner has had at least 10 minutes of run time… 
    …also an air-conditioner uses approximately 15 times as much power in the first minute as it does in the 10th minute…”
    I have a device named TED5000 on my whole house electric usage, which lets me watch electric usage second by second if I wish. My observation directly tells me the AC runs at a near constant wattage. Emphatically I say the 1st minute is nowhere near 15X the wattage of the 15th minute of run time. If the board permitted, I would show you a graph proving this. 
    So much of understanding the world, depends on finding things you think you know, which aren’t actually true. Hope this helps.

  11. Allison, don’t beat yourself
    Allison, don’t beat yourself up for screwing up the Manual J. I’ve found it to be wildly oversized (30-50%) on the few projects I have now tracked. So you didn’t screw up, Manual J is screwed up, IMHO. 
    Would a 2 stage 2 ton be a good fit here? Why does it have to be a single stage when full load only happens 90 or so hours a year? Low stage would be under 1.5 tons then, depending on manufacturer. 
    I’ve been meaning to shoot a little video of me driving Rachel’s 180 mph Nissan 370Z using the gas pedal as an on-off switch. It won’t be smooth… That’s what most HVAC equipment is – a 300 HP car with an on-off switch, not a throttle. How can we get a throttle on these things, and make them 120 HP instead?

  12. I love your honesty, “I
    I love your honesty, “I chickened out and went with the 2 ton” Wish i had a nickel for every time i did that. Great Article. Love the new Hobo’s

  13. To clarify the efficiency of
    To clarify the efficiency of an air-conditioner is measured by watts to BTUs. And air-conditioner does use the same amount of wattage the first minute as it does the 10th minute but it produces nearly 0 BTUs the first minute of run time and only after the evaporator coil is fully saturated does it give its maximum BTUs according to Trane that takes approximately 7 1/2 minutes . Energy Star has a great graph showing you the EER rating by the minute of run time  
    Regarding your Ted 5000 unit I would not treat that as gospel as I had one and uninstalled it because I found the data unreliable.

  14. How do you use the indoor
    How do you use the indoor temperature along with the HVAC run times to make sure that your manual J is correct? 
    How do I see the “lag time” and correct for it? 
    Do you know if there is a data logger that can be affixed to a supply vent to log both on and off time and temperature and humidity? 
    Where can you look up design temperature for your: (is it area or equipment or what your equipment is set to?) 
    Does anyone know of a professional that can determine if your equipment is properly sized and commissioned in SC? 
    Are there any other determinants like blower speed, and how can you find out what your blower speed is set to? 
    How would a split system act differently from a single unit heat pump? 
    About what humidity do you want the indoor air to be? Is it different for summer and winter?  
    At what humidity do you need to worry about mold? 
    You mentioned that your manual J said that you would need 1.6 tons for 1500 sq feet in an 1980’s built condo. So I am assuming your condo was leaky compared to today’s standards. But if you had gone with the “Rule of Thumb” at 1 ton per 500 sq feet it would still have been over-sized even for a leaky home. When was the Rule of Thumb devised and HOW leaky were those homes compared to the 1980’s homes? How much less leaky are the new homes, not LEEDs but just average built home of today?

  15. Allison, are you gonna take
    Allison, are you gonna take up where Phil left off?… 
    Where is the proof you did a bad job with your manual j?  
    That statement implies manual j leads to proper sizing.  
    Do you have scientific evidence to support this presumption, or is it a lazy conclusion?  
    If I contend you did an excellent manual j, how would you prove me wrong?

  16. Sorry to be picky, and it
    Sorry to be picky, and it doesn’t detract from the post, but I believe the 1% applies to the hours from June through September (2928 hours). With respect to getting Manual J “correct” there is no such thing. It is squishy and you can get a wide variety of answers. When used in a very conservative way (to minimize the estimated load) we found that the old Manual J (version 7) overestimated the sensible cooling load by 50% of the actual sensible load. The new Manual J produces even higher sensible loads. See: Appendix B 
    “For 83% of the units the actual load was substantially less than the Manual J. The 
    average maximum sensible load in a design day was 67% of Manual J for houses with 
    thermostat settings near 75°F. 
    Unit #6 had a sensible load in excess of Manual J. The problem causing the excess 
    sensible load in this house is not known, but it is more than a low thermostat setting. 
    This unit points out an important factor about the practice of oversizing air 
    conditioners, when an air conditioner is oversized, the oversizing hides problems that 
    otherwise would get attention and be solved.” 
    On another note our studies show Manual J underestimates the latent load in many cases.

  17. My suggestion on the solution
    My suggestion on the solution is to replace only the compressor with a 1.5 ton compressor (and an adjustable TXV) then you will have a very efficient system (just like a two speed system in low speed). That is what we did on two of the Stockton homes.

  18. Sorry for the lack of
    Sorry for the lack of response today, but I’ve been too busy to get in here today. I’ll try to get in later and respond to more comments but for now, I’ll respond to  
    Bob Ellenberg: 1500 to 2000 square feet per ton is more and more common for efficient homes. I wrote a while back about the rule of thumb I use when doing quality assurance on our home energy raters and said that if anyone sent me a rating for and ENERGY STAR home that had less than 1000 sf/ton, I was immediately suspicious and would look further into the details. One way to get to correct numbers is to learn how to tweak the software based on experience and real-world data like I’ve shown in this article. 
    Nate A.: I’m not beating myself up about it. The performance isn’t horrible, but we generally run 55-58% relative humidity in the summer (unless I do experiments on my family, like I did recently). Manual J isn’t as screwed as you may think. I was off some but not a huge amount. I came up with 1.6 tons, and that’s probably about a half ton too high. If you do a really good job with the inputs, you’ll still be 15-20% higher than the actual load, so I probably was off by maybe 0.3 ton. Manual J and data loggers go together. I’ll write more about this soon. 
    John Proctor: According to what I read on page 144 in my copy of Manual J (8th Edition, Version 2): “The 99 percent values apply to the total hours for the entire year and the 1 percent values apply to the total hours for the entire year.” The old 97.5% and 2.5% design temperatures were based on just the warm months or just the cold months, but that has changed. 

  19. @JAMIE CLARK,  &lt
    You deserve three cheers for coming right back with a link to EPA to support your statement. Focusing on something objective and verifiable really helps the discussion progress. 
    That said, I have to wonder how up to date is the EPA info. Its EER goes up to 7 while my two ACs are ARI rated at 12.5 and 13 EER (17.25 and 16.0 SEER). 
    There was an interesting study done by FSEC which attempted to measure energy savings and humidity performance in older Florida houses: 
    The expected benefits failed to materialize in this case, the discussion on Page 29 says that engineers have made significant progress in making ACs perform better in the first few minutes of a short cycle. 
    The bigger lesson of the Florida study is you cannot look at only one component of a system. One confounding factor is the duct system leakage is increased by longer AC cycles, leading to worse humidity control, not better. This study tried to go cheap and replace just the condensor and coil, leaving the original air handler in place — a bad plan as it turned out. Energy consumption was nowhere near as low as was originally expected. 
    One thing I think we can agree on, is the system performance is influenced by lots of factors other than equipment sizing. It is my opinion that duct systems cry out for just as much attentiond. I believe a whole house system with the other things done right, could possibly perform well in spite of AC sizing issues. 
    Thanks very much for your posts. 

  20. Very interesting article. But
    Very interesting article. But only telling after the system is installed. Great way for installers to tune their algorithm, but only delivers good or bad news to the homeowner when the hurly burly is done. IF I ever have to buy another system I think I will put in a run time percent in the contract so that it is up to the vendor to make it right. (Listen as emptors cheer, vendors jeer).

  21. Allison, that is what I get
    Allison, that is what I get for getting old. Thank you for bringing me up to date. 
    John PS I would probably put in a 1 ton compressor if it was my house. I assume your ducts are either not in the attic or they are very well insulated and don’t leak.

  22. I kinda like John P’s idea of
    I kinda like John P’s idea of retrofitting a smaller, 1.5 ton compressor. If they made ’em in 1.0 or 1.25 ton models I’d consider that as well. 
    My guess is that the TXV would adapt to the change but might be good to check its model # to be sure.  
    With any luck AHU can be configured for 1.5 tons…likely just a jumper change. 
    I have a client with a 5 ton system in a tiny office, a two story interior townhouse-style office condo well shaded on one side. System is 2-3x oversized and no one is happy. I proposed dropping the compressor size to 2-3 tons along with a TXV and smaller blower motor. 
    There are about 15 identical itty bitty office condos in the complex, all with the same ridiculous 5 ton systems, so I figured if i worked out the details I could do a bunch. Nothing came of it, sadly enough.

  23. Curt We install a BPM motor
    Curt We install a BPM motor that we can set the speed and lower the watt draw a bunch. I presume that the existing unit does not have an ECM. What we actually use is no longer available, it had the ability to switch back and forth from sensible cooling to more dehumidification.

  24. Kurt, if you reduce HP of the
    Kurt, if you reduce HP of the motor you will need a lower RPM or X-13. You will burn up the a PSC motor if you change HP w/o using a different RPM motor.

  25. Manual J is a SWAG and you
    Manual J is a SWAG and you knew it. 
    Why didn’t you use historic energy usage, estimated SEER of the old equipment and historic weather data? (I’m assuming in 2009 you still had your electricity bills from the time your AC was still performing well). 
    It’s easy to back out the cooling portion of the KWH usage by looking at the whole year’s bills. 
    Even if you hadn’t chickened out, it could have been tough to find a contractor willing to put in a correctly sized unit.

  26. Kevin D.:
    Kevin D.: Manual J is a lot better than a SWAG. Like all calculations, though, it definitely follows the GIGO (garbage in, garbage out) principle. I didn’t use historic energy usage because we hadn’t been in the condo long enough.

  27. If you had the data logger
    If you had the data logger you could have determined run time of the old system. problem is most systems only run at 2/3 capacity, so that’s not entirely accurate either.. 
    As other post mentioned, oversizing hides underperformance of the AC system.

  28. @ALLISON, 

    It is not obvious how one gets from historic energy usage, to AC sizing. I wish you would write about this sometime in your blog.

  29. Fantastic Article! Allison,
    Fantastic Article! Allison, if you had to guess, how many HVAC systems have had a Manual J run on them? 50% of the new homes in America? 10%?  
    Your blog is so well done. Appreciate all the time you put into this resource. Best, Matt Risinger

  30. For M. Johnson: &lt
    For M. Johnson:  
    I hear people talking a lot about AC over sizing as the #1 cause of high indoor humidity (in a humid climate), but I would like to submit a heresy: that infiltration from indoors is also a big reason. Remember the AC is naturally going to be over sized any day which is not close to the design temperature.  
    While I agree with you that most residential a/c systems are oversized any time the weather is at less than design conditions,I would be hesitant to call “infiltration from indoors” a heresy. In fact I would be reluctant to acknowledge “infiltration from indoors” as a factor at all, as infiltration typically is an outdoor to indoor movement of air. If you are referring to internal sensible and latent heat gains, then yes that is a matter most load calculation programs consider.  
    Should we be thinking about the coil temperature? I don’t know anybody who is measuring that and publishing any info. 
    If an a/c coil is to effectively reduce indoor air dew point temperatures below 55 degrees, then the evaporator coil surface temperature must be below 55 degrees, which often translates to a refrigerant boiling temperature of 40 degrees.  
    What you may be after is an expansion of the discussion of HVAC system design beyond only Manual J and into psychrometrics and coil bypass factor. Oh, and air distribution. If so, I’m game. 🙂

  31. I must make a correction:
    I must make a correction: infiltration from OUTDOORS is what I meant to say. I think you know how imperfect the data is for infiltration, that is typically entered into a Manual J model. In other words, just a guess most of the time. 
    Perhaps I am alone in this, but it would seem most interesting to measure what temperature my coil really does run at. Maybe that would explain why the 2-speed system at one end of the house, seems so-so at removing humidity despite hours-long run times on low stage. And why the installation of a 1-stage system at the other part of the house, seems to result in really good humidity control despite 3 cycles/hour, run times of about 12 minutes each. Both systems are proper ARI matches, 12.5 and 13.0 EER respectively. 
    Thank you.

  32. @ Jamie The SEER rating is
    @ Jamie The SEER rating is based on two steady state tests and one cycling test with six minutes on and 24 minutes off (see What makes a difference is not the watt draw, which is nearly constant at a given outdoor temperature, but the fact that the capacity (delivered cooling) starts at 0 and builds over time. That is what makes a longer run more efficient.  
    @ M. Johnson, I presume you know that the low side pressure you measure on the unit is directly translatable to the temperature of the saturated portion of the evaporator coil.

  33. Holy cycle city, Ductman!&amp
    Holy cycle city, Ductman! 
    Can you increase the swing on your thermostat so that you don’t have quite so many cycles? 
    Also consider testing your unit with a lower blower fan setting. Expected result, better humidity control with longer run times.

  34. @John Proctor  &lt
    @John Proctor 
    Actually I do not do this for a living, rather am a nerdy homeowner who tries real hard to get things right. I am flattered, but no I actually did not know that at all. With thermometers so ubiquitous in the modern world, I assumed the #1 way would be to measure the coil temperature directly. High tech controls such as Carrier can routinely tell us fan speeds and external static pressure, maybe someday they will add coil temperature as well? 
    Thank you very much.

  35. M. Johnson said: &lt
    M. Johnson said:  
    “I must make a correction: infiltration from OUTDOORS is what I meant to say. I think you know how imperfect the data is for infiltration, that is typically entered into a Manual J model. In other words, just a guess most of the time.” 
    With Manual J based computer programs like Wrightsoft, blower door infiltration numbers can be entered into the program. Ideally, building envelope performance testing (blower door, duct blaster, etc.) would coincide with a Manual J calculation, and that prior to a crisis moment of complete HVAC system failure driving hasty decisions.  
    “Perhaps I am alone in this, but it would seem most interesting to measure what temperature my coil really does run at.” 
    You can get close via a psychrometric chart and actual (real time) system performance data. Entering wet and dry bulb temps, leaving wet and dry bulb temps, actual airflow across the coil in CFM, and coil bypass factor can yield much beneficial info. Generally, if the refrigerant inside your evaporator is boiling at 40 degrees, the leaving air temperature will be around 55 degrees. This will vary with the entering wet bulb temperature, of course. 
    “Maybe that would explain why the 2-speed system at one end of the house, seems so-so at removing humidity despite hours-long run times on low stage.” 
    Two stage systems use the full coil surface while reducing the pumping capacity of the compressor. If the indoor blower is not slowed along with the reduction in compressor pumping capacity, the coil can become warmer than it should, and experience more bypass factor than is beneficial for dehumidifying. Proper refrigerant charge is very important for good dehumidification. Systems that are overcharged or undercharged will not dehumidify well. Duct leakage, especially on the return air side, is another factor, in terms of system performance. Supply side duct leakage (with ducts outside the conditioned space) turns your house into a giant vacuum pump, drawing hot, muggy air inside. 
    “And why the installation of a 1-stage system at the other part of the house, seems to result in really good humidity control despite 3 cycles/hour, run times of about 12 minutes each. Both systems are proper ARI matches, 12.5 and 13.0 EER respectively.” 
    Many variables there. Could have better airflow, better refrigerant charge, less duct leakage, less restrictive ducts and registers.

  36. Thanks for the super
    Thanks for the super informative article. 
    I am just now running Manual J for my 2241 sq ft 1993 A/C ready tract house in San Diego, CA., so I read with great interest! 
    You have certainly given me encouragement to stick to my Man J calculations indicating a 2.6 Ton A/C.I plan to go for 2.5 ton Goodman SEER 13 with R410a and hire the best rated Goodman recommended contractor in Sandy to install. The internet prices look good so far.  
    I only need A/C in September, our hottest month, when hurricanes in Mexico bring us high humidity. This year’s hurricane Odile was a whopper.  
    I went to Man J because my house is 1727 sq ft 1st floor, 514 sq ft second floor, with 18 foot and 10 ft 1st floor ceilings. So I going by sq ft didn’t seem like a good idea. 
    Man J extimates 90,388 BTU heating, and the existing 100,000 BTU Comfortmaker has been more than adequate for the last 20 years, so that’s consistent. 
    Typical So Cal construction, Med style stucco/tile roof, a big box. Like Taco Bell meets mini McMansion. 
    I am sympathetic with A/C pros. They need a blast of Arctic air to get the homeowner to sign off on the job, so they can move on. 
    Maybe variable or two speed compressors will be the comfort and efficiency solution. More expensive than calculations, but what the hey.  

  37. I’m surprised your 2241sqft
    I’m surprised your 2241sqft house leaks 90,000BTU per hour at 44f degrees (San Diego design temp). I think you input a value incorrectly, I have a hard time believing a 1993 house would be that leaky.

  38. You could well be right, that
    You could well be right, that’s my main concern. I’ll definitely go over my calculations again.Thanks for the input!  
    I was got much lower 48,273 BTU value, when I entered concrete slab with R-5 edge concrete slab, but got same 2.5 ton cooling requirement. 
    California does not require edge insulation on slabs, so I used un-insulated slab on my latest calculation and got 90,000.  
    My existing builder-installed gas furnace is 100,000 BTU and has worked well for the last 22 years. I only use heat on a month or so, only the coldest nights. 
    I have no real experience with gas furnace sizing,other than this, so I had no idea.  
    One thing, this really would be a 3500 sq ft house, except the living room has an 18 ft ceiling, and the 2nd floor is only 514 sq ft—similar to a loft. 
    There are 207 similar homes in the tract, so I’m assuming the builder used California minimum insulation values.  

  39. Bob— 

    Just one more thing. 
    The BTU breakdown says that 67,381 BTUs out of 90,388 are being lost through floors with no slab insulation. 
    With R-5 slab edge insulation, only 25,295 BTUs are lost through floors. 
    I’m assuming no slab insulation, but I don’t really know for sure. 

  40. Something isn’t right, not
    Something isn’t right, not sure what. If temps are so mild you only need heat 1 month a year, a 50k should heat the house. Oversizing won’t hurt too much, since it’s only 1 month per year you use it. Did the 100k ever run ALL NIGHT or does it just give a quick burst of heat to warm things up in he morning?

  41. The carrier infinity system
    The carrier infinity system has a thermostat that works off both temperature and humidity. Honeywell also has several options.

  42. I am not familiar so much
    I am not familiar so much with the Manual J, but in commercial building ASHRAE calcs the 1% is applied to 4 months summer hours, June-September. So the design temp is exceeded 29 hours instead of 87.

  43. Try lower blower fan setting.
    Try lower blower fan setting. It will surely work.

  44. My one story attached villa
    My one story attached villa is an interior unit and a very tight house. It’s only 1123sq feet. What is the right sized unit I should have for my home? My son and I have experienced high humidity in the home and breathing problems. We are ab out to spend 5k to have a dehumidifier/ventelator because we have been told we are suffering from tight house syndrome. Would downsizing my HVAC unit solve that problem?

  45. Just recently started reading
    Just recently started reading your blog, really good information. I wanted to question the ASHRAE 1%, I believe it is stated as 1% of the hours from June through September, so less than 30 hours total instead of 87 hours.

  46. @ Bonnie We cannot tell if
    @ Bonnie We cannot tell if the unit is too large (except that they almost always are) without knowing the climate your home is in, the insulation, windows etc. If the air conditioner runs very short cycles when it is a maximum temperature outside, then it is oversized, but we cannot tell how much. If you are in a wet climate, then a dehumidifier should do the trick. I am not sure you need a ventilation system.

  47. Lee H.:
    Lee H.: ASHRAE changed from the 97.5% and 2.5% numbers based on the hours only in the heating season or the cooling season, and now the hours are design temperatures are based on all the hours of the year.

  48. I have a 2400 sq ft house (up
    I have a 2400 sq ft house (up and down) with a 3 ton AC unit (3 years old) installed by the previous owner. I can’t get the temperature below 78 in the house. Tech came out and confirmed the unit was poversized but operating properly with correct amount of refrig. He concluded the A cool was freezing up. He reduced the fan speed but that only helped a little. What can I do besides replacing the unit?

    1. Tom S: If the A coil is
      Tom S: If the A coil is freezing up, the correct solution would be to increase the fan speed to prevent it from becoming so cold which makes the condensation form ice. A high fan speed would provide greater air flow through the coil and increase the coil temperature. Reducing the fan speed would only make the problem worse. (Less air flow even colder coil.)

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