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A 3-Ton Air Conditioner Will Rarely Give You 3 Tons of Cooling

Even when a cooling system is perfectly designed and installed, three factors can reduce an air conditioner's capacity

Posted on Jun 5 2013 by Allison A. Bailes III, PhD, GBA Advisor

Today I'm going to give you three reasons why your 3 ton air conditioner isn't really a 3 ton air conditioner. Of course, there are more than three reasons, starting with the fact that it's not 3 tons in weight. That unit refers to cooling capacity and harkens back to the days of ice. I'm also not talking about any of the multitude of reasons having to do with improper design, faulty installations, or lack of maintenance — topics that I discuss frequently enough already.

No, today I'm going to tell you that your 3-ton (or 2-ton or whatever size you have) air conditioner may not be what you think it is, even when everything's designed, installed, commissioned, and maintained perfectly. David Butler wrote about two of these reasons in a guest post on ACCA's Manual S protocol for selecting HVAC equipment two years ago, and that's a great article for understanding some of the subtleties.

So, what are these three reasons?

1. Nominal vs. actual capacity

When we talk about air conditioner capacity, we're usually giving the nominal size. A 3-ton air conditioner has a nominal capacity of 36,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. per hour, but the actual rating using the operating conditions specified by AHRI is rarely the same as the nominal capacity. For example, the air conditioner shown in the AHRI certificate below is referred to as a 3-ton unit (36,000 BTU/hr), but it has an actual capacity of 2.8 tons (34,000 BTU/hr).

2. AHRI's indoor operating conditions vs. actual operating conditions

As David Butler discussed in his article on Manual S, AHRI ratings are done for an indoor dry bulb temperatureAir temperature as measured by an ordinary thermometer. of 80°F and indoor wet bulb temperature of 67°F. ACCA recommends using an indoor design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year). (dry bulb) of 75°F and relative humidity of 50%. That's closer to the actual conditions that most homes actually operate at than AHRI's conditions.

Let's think about the temperature difference and see what effect that might have on the cooling capacity. Which way do you think it would go if we bring cooler air into the air conditioner than it was rated for?

Well, let's frame that a little differently. Is it harder to cool cooler air or warmer air? The answer is the former. The lower the temperature goes, the harder it is to remove more heat from it. Just ask the folks at the Microkelvin Laboratory at the University of Florida, where they get about as close to absolute zero as is possible.

If it's harder to cool air at 75°F than air at 80°F, then that means the 2.8-ton air conditioner above isn't going to be even 2.8 tons. To find the answer here, you have to factor in the humidity levels, too. As David wrote, it's a moving target, but the net result of AHRI's operating conditions is that your air conditioner's capacity is lower than it's rated (unless you keep the thermostat at 80°F or higher).

3. Outdoor operating conditions for AHRI vs. actual operating conditions

AHRI uses 95°F as its outdoor test temperature, so if your outdoor cooling design temperature differs from that, your AC capacity will again vary from the AHRI rated capacity. In this case, we get a little of that lost capacity back here in Atlanta. Our design temperature is 92°F, which means that the air conditioner has an easier job of dumping heat into the outside air than it would if it had to dump the heat into 95°F air.

If you live in Tucson, Arizona, with a design temperature of 103°F, however, your 3-ton air conditioner has now dropped in capacity again. It's just harder for that refrigerant to give up those BTUs to air that's hotter.

More reasons for variation

It's important to remember that the three reasons above don't have anything to do with poor design, installation, commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. , or maintenance. Plenty of other factors related to those issues also affect capacity:

  • Bad ductwork
  • Dirty filters or coils
  • Poor air flow through the condensing coil
  • Improper refrigerant charge

These are not good reasons to oversize an air conditioner!

The takeaways

The main thing to be aware of is that you need to know more than just the result of the Manual J cooling load calculation. A Manual J report may say you need a 3-ton air conditioner, but for the three reasons above, you might really need to install a 3.5-ton unit. That's why Manual S, the equipment selection protocol, is so important. The thing is, though, that even with these issues that mostly reduce the capacity of your your air conditioner, most air conditioners still end up oversized.

I recently heard someone say that thumbs are great things, but it's good to recognize their limitations. Having opposable thumbs allows us to write a letter, examine a mulberry, and hold a glass of beer, but no matter how great they are, thumbs can't design HVAC systems. Rules of thumb don't work. You need to know how things really work and do the math for that.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


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Image Credits:

  1. Energy Vanguard
  2. AHRI
  3. jetsandzeppelins, from flickr.com, Creative Commons license
1.
Fri, 06/07/2013 - 12:29

Tons at the register - a useful concept from LBNL
by Brennan Less

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Potentially a good concept and idea to incorporate here would be 'tons at the register', a concept developed by researchers at LBNL to reflect the performance of the entire system. As you noted in your 'more reasons for variation' section, duct leakage, airflow and other factors have large influence, certainly larger than the difference between nameplate/nominal capacity. Bad distribution systems can waste half of the unit's capacity (labeled or actual at the coil) during peak demand, and improved systems can reduce the required equipment size by one ton in a typical CA central valley home.

http://epb.lbl.gov/publications/pdf/lbnl-41957.pdf
http://epb.lbl.gov/publications/pdf/lbnl-45315.pdf

Cheers!


2.
Fri, 06/07/2013 - 17:25

Response to Brennan Less
by Allison A. Bailes III, PhD, GBA Advisor

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Yes, you're absolutely right. I love the 'Tons at the Register' concept! Thanks for sharing those links.


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