Understanding SEER

So, what is SEER?

The Seasonal Energy Efficiency Ratio (SEER) is an efficiency rating required by the federal trade commission to be labeled on air conditioning and heat pump equipment. It was developed in 1979 with the help of the DOE (Department of Energy) and the ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers) and is used to express an air conditioner's (or heat pump's) energy consumption during an average period of cooling.

Technically, a SEER is a ratio composed of the cooling output of an air conditioner (or heat pump) measured in BTUs, divided by the energy input of the air conditioner consumed in Watt-Hours, as the air conditioner is operated over a period of time in a controlled environment which simulates the average outdoor cooling conditions experienced across the United States.

It is important to note that the SEER label on AC equipment is essentially the result of tests performed in a laboratory setting that serves to simulate an averaged seasonal "cooling" environment which includes testing AC equipment at a constant indoor temperature of 80°F at 50% relative humidity, and at an outdoor temperature of 82°F at 39% relative humidity, over a period of time. These exact testing procedures are formed by the AHRI (Air Conditioning, Heating, and Refrigeration Institute) and are documented in articles called: The ANSI/AHRI Standard 210/240-2023 and AHRI 210/240-2024.

And, although the testing procedures performed in the lab are very accurate, the controlled laboratory testing environment is very different from any real environment found at a home. Which means a SEER rating may or may not reflect the actual energy consumption of your home.

It is also important to note that SEER evaluations are performed across an entire package of components, and tested as a single set supplied by the manufacturer. Meaning, manufacturers that are having their equipment's SEER evaluated supply 3 pieces of equipment that are all used during a SEER evaluation, including: An Outdoor Condensing Unit, a Furnace, and an Indoor Evaporator Coil, all 3 of which are all tested at once and used to calculate SEER - Simply purchasing a new 20 SEER condenser and plugging it into your existing furnace and evaporator coil does not upgrade the entire system to 20 SEER.

Sure thing! So, if you don't wish to read the AHRI's and DOE's documents on exactly how SEER testing takes place, allow me to sum it up real quick:

AC equipment for which a SEER rating is going to be determined is set up inside a laboratory setting consisting of 2 side by side rooms. Since air conditioning equipment transfers heat from an indoor environment, to an outdoor environment, one of the rooms in the laboratory simulates an indoor environment, and the other room simulates an outdoor environment. For the most common types of AC systems, including AC systems that use a single speed compressor and a furnace that delivers a constant volume of air, the indoor and outdoor testing environment is as follows:

It's important to note this because the difference in temperature between the outdoor and indoor environments for which an air conditioning system transfers heat between has a big impact on the air conditioning systems energy consumption and performance - The greater the difference of temperature between the environments where the heat is being transferred to and from, the more work the system has to do in order transfer the heat from one area to the next, and therefore the greater the energy consumption.

This means, at 82°F outside, and 80°F inside, the SEER evaluation doesn't test equipment at peak demand times, and therefore does not accurately show peak demand time savings - SEER is an evaluation that tests air conditioning equipment during a period of moderate to low demand - At a 2 degree difference it could be argued that you might as well just open the windows of your home.

Anyway, the equipment is then operated within these simulated indoor and outdoor conditions as described above, and over the course of time the energy consumption and cooling capacity of the equipment is gathered through testing equipment and logged as a series of recorded EER (Energy Efficiency Ratio) values. In the end, the results that were collected throughout the testing procedure are averaged as a single EER (Energy Efficiency Ratio) which is then multiplied by a PLF (Partial-Load Factor), and a SEER is born.

So this equation may seem kind of confusing, but that's because the AHRI defines each of the variables within the above equation separately throughout the ANSI/AHRI Standards. Basically what this equation is stating is: SEER is equal to the average of the EERs collected during test B, multiplied by the Part Load Factor. Let's talk about these variables very quickly:

PLF stands for Partial-Load Factor. The Partial-Load Factor is a factor that takes into account the fact that AC equipment isn't always operating at 100% capacity when it's consuming energy, such as when it is cycling on and off during a cooling period, as well as the fact that the heat load in the occupied space doesn't always match the total capacity of the HVAC equipment. Therefore, the PLF is a variable that defines the AC equipment's cycling efficiency.

Generally, residential AC equipment handles the heat load of an occupied space by cycling on and off as the heat load within the occupied space rises and falls. Therefore, the part-load factor is used to predict this "on and off" behavior and adjusts the energy efficiency ratio accordingly.

For example: Most AC equipment is sized to handle a heat load that is based off of a worst case scenario that rarely occurs within the occupied space, that is to say, your home may have a 5 ton air conditioning system which can transfer a heat load of 60,000 BTUs per hour, however, there are few times when a heat load of 60,000 BTUs is present in your home all at once, which then has to be removed by your 5-ton air conditioning system (which would require the system to operate for a full hour).

Instead, your air conditioning system periodically cycles on and off and handles the heat load through periods of cycling. This behavior, of cycling on and off, plays a big role in determining the total energy consumption over an entire season, which is what SEER attempts to predict and the PLF attempts to account for.

Manufacturers actually have the option to run a separate test to determine the equipment's PLF when they are having their equipment's SEER evaluated, or they can opt out, and simply use a default PLF of 0.875. Equipment PLF is not something that is published by the AHRI after it is determined, but a study has been done that essentially works backwards in order to solve what this default PLF variable represents, which concluded the default PLF factor represents a start-up time of 76 seconds, and a maximum equipment cycling time of 3.125 cycles per hour (which is considered very poor).

So a default PLF resembling poor performance is obviously a good thing for consumers. If manufacturers could simply opt out and receive a default value that resembled an AC system that performed better, which resulted in a better efficiency rating, then they most likely would, as we are constantly reminded by various sources to spend more money initially on purchasing equipment at a higher SEER.

So, in most cases the manufacturer most likely does choose to have their PLF evaluated. Modern equipment includes various features that allow cyclic performance to score much better than calculations made using the default PLF, including features such as: Time Delays, Variable Speed Compressors, Variable Speed Fans, and Electronic Expansion Valves, all of which are components that would contribute to a lower PLF than what this default value represents.

This value is again however, just another variable which will change between every single residence - A partial-load factor obviously depends on, among other things: The Available Heat Load, The Length of the Refrigerant Line-set, and the Duct System Design within a home, all of which are factors that are going to vary between every single home and equipment installation.

PLF, as well as the entire SEER evaluation, also makes the assumption that installed AC equipment has been properly sized to the home and calculated by a professional contractor who understands how to perform a proper residential heat load calculation according to the ASHRAE's (American Society of Heating, Refrigeration and Air Conditioning Engineers) heat load calculation guidelines listed in a document called the Manual J.

Home construction companies typically have absolutely no idea how to properly size HVAC equipment to a residence, and, throughout the cookie-cutter home construction process, many homes are built with HVAC issues stemming back to design and installation failure. There is no law which requires a heat load calculation when a home is constructed, yet there is a law that requires a minimum SEER efficiency for AC equipment, which assumes a home's heat load has been properly addressed by properly sized equipment.

The home inspection process, which all homes must endure and successfully pass in order to be bought or sold, simply addresses code violations and safety concerns - A home may look very nice on the inside, and pass a safety and code inspection, but behind the illusive curtain of beautiful interior design that most home builders expect shopping consumers to never see beyond exists a very different story of undersized or oversized AC equipment and very poor duct systems, which isn't detected until the home is inhabited by the buyer for a full season of AC run time.

If you are one of those people who constantly complains of a room or two within your home that constantly remains too hot or too cold, then unless your HVAC system is having a mechanical issue, or duct work has simply fallen apart, then you're most likely experiencing an issue that stems back to poor practice.