Air Conditioning Energy Ratings

BTUh, kWh, EER, CEER, SEER, SACC – when it comes to understanding air conditioner capacity and efficiency there are certainly plenty of arcane acronyms to sort through. The systems set up to allow objective comparison of air conditioners are hindering that very goal because of the many different measurement systems and terminology. Before discussing the different terms I would like to quickly explain what an air conditioner does: it pumps out heat. Much like a sump pump pumps out water from a basement or crawl pace, an air conditioner pumps heat out of your house. You need a sump pump that can pump water out as fast as it leaks in, or your basement will flood. With air conditioning, you need one that can pump heat out of your house faster than it leaks in, or your house will still get hot. The key point is that the air conditioner moves heat. In the United States we measure heat in British Thermal Units, or BTUs. The rate at which in air conditioner is able to pump out heat is given in BTUs per hour, or BTUh. This tells us how many BTUs of heat the unit can remove operating for an hour. The air conditioner’s electrical use is measured in kilowatts hours, or kWh.

EER Energy Efficiency Ratio

The EER is the easiest measure to understand. It is the cooling capacity in BTUh divided by the energy use in kWh. It tells you how many BTU of heat removal (cooling) you get for each kWh of energy. However, there are some things not taken into consideration. First is that it is a steady state test, meaning the unit is up and operating at full efficiency before any measurements are taken. No consideration is given for the energy used at startup, shut down, and while plugged in but not running. Also, all measurements are done at design condition, which is 80°F, 50% rh inside and 95°F outside.

   

SEER Seasonal Energy Efficiency Ratio

The Department of Energy devised this measurement for rating central air conditioning units in 1978 to address some of the issues not addressed by EER. Namely, cycling losses and operation at more than one temperature. The idea is that SEER is supposed to show the BTUh/kWh over a season, not just at one steady state condition. To simulate seasonal operation, SEER testing includes cycling and operation at  three different test conditions: 80°db/67°wb inside and 95° outside, 80° db/67°wb inside and 82° outside, and 80°db 57°wb inside and 67° outside. A unit’s SEER is generally higher than its EER because the SEER includes operation at milder conditions. Currently, the minimum SEER in the northern half of the US is 13 while the minimum SEER in the southern half of the US is 14.

CEER Combined Energy Efficiency Ratio

The DOE devised CEER in 2014 specifically for window air conditioning units. CEER is similar to SEER in that it measures efficiency at two operating conditions: 95° and 83°. It also includes the energy used while the unit is plugged in but not operating. A unit’s EER and CEER normally end up being very close to each other with the CEER being slightly lower.

SACC Seasonally Adjusted Cooling Capacity

The SACC was devised in 2017 to measure the efficiency of portable air conditioners. By portable, they mean the ones on wheels where the whole unit sits in the room and an exhaust duct is placed in the window to carry hot condenser air out. It is similar to the CEER in that it measures capacity at both  95° and 83°. It also includes adjustments for heat gains (cooling losses) from the exhaust duct plus loses due to infiltration caused by having to stick the exhaust duct out the window.

How do you convert between these different methods? You don’t because they each have different testing specifications. There are some formulas offered, but they can’t determine the differences in how different units will respond at the varying testing conditions.  The best you can do is understand each rating and use them to compare units with similar ratings. Just as the EPA mileage estimates don’t really tell you what your mileage will be with that new car you just bought, these ratings will not really tell you the energy use for your new air conditioner for a year. So which rating system do I believe is the most reliable? Honestly, the simplest and oldest one: EER.

Desired Air Conditioning Temperature Drop

I was recently asked for a formula to determine the temperature drop between the return air and supply air of an air conditioning system. While it is logical to check the temperature drop, determining exactly what it should be is not as simple as plugging in readily available numbers into a formula. Two operating conditions can have a pronounced effect on the results: the relative humidity of the return air and the amount of airflow. Most air conditioning systems condition the air two ways. They cool the air, referred to as sensible cooling; and they take water out of the air, referred to latent cooling. Only sensible cooling creates a temperature drop. Removing water from the air takes system capacity. The more water the system removes from the air, the less capacity is left for reducing the air temperature. Standard airflow is 400 CFM per ton for most systems, but that does not mean your system is actually operating at 400 CFM per ton. If you move less air across the coil, the air will be cooled a little more. To determine the temperature drop you must know the outdoor ambient temperature, the return air dry bulb, the return air wet bulb, the CFM of airflow, and the system’s sensible cooling capacity at that condition.

Take for example, a system that is removing no water out of the air operating at 100% sensible cooling with a standard 400 CFM per ton of airflow and producing 12,000 Btuh per hour. The temperature difference is calculated as TD = 12000/(400 x 1.08) = 28°F TD. If the airflow is reduced, the TD becomes 12000/(350 x 1.08) = 32°F. Increasing the airflow would make the TD 12000/(450 x 1.08) = 25°F. In humid climates, the latent capacity can easily be as much as one third of the total capacity, reducing the sensible cooling capacity to 8,000 Btuh. These same airflows would then give TDs of: 8000/(400 x 1.08)=19°F, 8000/(350 x 1.08)=21°F,8000/(450 x 1.08)=16°. In reality, these TDs would be a little off because the overall system capacity would be a bit less with the decreased airflow and a bit more with increased airflow. The system capacity will also change depending upon the outdoor ambient. The 12.000 Btuh per ton is a nominal figure based on the AHRI rating condition of 95°F outdoor ambient, 80°F indoor dry bulb and 67°F wet bulb.

You can try to account for duct gain by reducing the expected TD by some amount: say 3°F - 5°F. However, it is really difficult to use TD at the registers because the duct gain from one system to another can vary a lot. Ducts in the attic will pick up more heat than ducts in a crawl space. Duct leakage also has a big effect. If 10% of the air entering the coil comes from a 150°F attic, that obviously will affect the delivered air temperature. The rule of thumb people have used for many years is a TD of 15°F to 20°F across the coil, not at the registers. Looking at the above calculations, you can see where this comes from. However, it is also easy to see how little you actually know if you don’t really know all the operating conditions, the system airflow, and the system capacity at those conditions. If all you do is measure the return and supply air temperatures at the registers, you don’t really know much.