Understanding SEER Ratings in AC units

Summer in New York is hot and humid and unless the building is really well built it requires air conditioning. We install HVAC systems and always recommend the most energy efficient units, which is currently the Fujitsu 26 SEER Halcyon Inverter mini-split.

We are using this unit in the Harlem Passive House job. There will be one unit per floor and will supply both heat and cooling. It is pretty amazing that such a small heating source will be able to heat the apartments, but that is the genius of Passive House construction.

For window mounted AC’s we suggest the 12000BTU 25 SEER TOYO Hybid DC Inverter Thru wall. A SEER 25 for a window unit is really amazing too. They aren’t cheap and I have not done the calcs but their efficiency will definitely save money in the long run.

It is important to understand SEER when choosing an AC system.

This article on SEER ratings gives a great overview of what SEER is. It is reprinted below:

What is SEER?

SEER stands for seasonal energy efficiency rating. It’s a benchmark of how much electricity a given A/C system uses to deliver each Btu/h of cooling power compared to another one under a standard, fixed set of rating conditions. This benchmark system is managed by the Department of Energy, and they decide what the testing standards are. The higher the SEER number is, the higher the score the system has received on the D.O.E. mandated efficiency tests.
Many don’t understand that the emphasis here is on the word ”system”. And the outside unit is only one part of the cooling system. You also have the inside cooling coils (sometimes called the evaporator or A-coil). The blower system in the furnace (or air handler) is also a part of the system as far as the SEER testing goes, because the SEER rating is based on the amount of cooling produced by x outside unit and x cooling coils – using x air volume delivered by x blower system. The blower motor in the furnace or air handler uses x amount of electricity, as does the outside unit. And the SEER rating tests are basically calculating how much total electricity the system uses to deliver its BTU of cooling. So the SEER tests are conducted with x furnace (or air handler), x outside unit, and x inside cooling coils. In other words, there is technically no such thing as a “13 SEER” outside unit when you come right down to it. The same outside unit could end up with a higher or lower than 13 SEER score, depending on the inside coil and furnace (or air handler) being used to test it with.
To make a long story short, the SEER rating is given to a combination of typically 3 components.

The math is quite simple, i.e. a SEER rating of 12 vs. a SEER rating of 10, means that the 12 SEER system received an efficiency test score that was 12/10ths better than the 10 SEER system. So the 12 SEER system would be rated about 20% more efficient than the 10 SEER system. But the SEER test conditions assume a fixed outdoor temperature of 82 degrees, and a fixed indoor temperature of 80 degrees, with a fixed indoor humidity of 50%. There’s a bit more to that, like the cycling efficiency, which we’ll explore later.

In the old days, A/C systems were given only an EER (energy efficiency rating, without the “seasonal” prefix). EER ratings are still used today in addition to the SEER rating.

The EER rating is a lot simpler, because it’s simply a measure of how many watts of power the system used to deliver each Btu/h of cooling power with the system running constantly over a period of time (steady state efficiency) under a fixed set of conditions. That fixed set of conditions was an 80F indoor temperature with a 95F outdoor temperature (instead of 82F like the SEER tests use). So if the system delivered 60,000 Btu’s of cooling power each hour and used 6,000 watts of power, you would merely divide the 60,000 Btu’s of cooling power by the 6,000 watts of electricity it used to deliver that amount of cooling. 60,000 divided by 6,000 = 10. So the system would be given a rating of 10 EER.

But some felt that EER system was not a good system for measuring the efficiency of the system over time, because it failed to take into account the fact that it takes some systems much longer to reach peak efficiency after they come on. And no system runs constantly throughout the entire cooling season. This factor was not considered in the EER tests. The other issue was that in most parts of the country, the system is not going to be running in 95F outdoor temperatures used for EER tests very often (if at all) during the cooling season. So the 82F outdoor temperature was chosen for the SEER tests.

So the SEER system was born a few decades ago to include those factors (cycling efficiency and 82F outdoor temperatures) into the testing process.

So the SEER rating conditions are: 80F indoor temperature with 50% indoor RH, and an 82F outdoor temperature. And the cycling efficiency is then factored in. The math was similar to the EER rating up to that point, i.e. after the system ran until it reached peak efficiency, and the system put out 60,000 Btu’s per hour of cooling, and the wattage used to accomplish that was 6,000 watts, the basic raw efficiency score would be 10.

But then the cycling efficiency of the system was measured (unlike the EER tests). This test measured how long it took the system to reach peak efficiency after it came on. If the system reached peak efficiency immediately upon starting, the raw score from the first test became the SEER rating. So in the previous example, the system with a raw efficiency score of 10 on the first test (the steady state test) would be given a 10 SEER rating. But the longer it took the system to reach peak efficiency, the worse the SEER rating would be after factoring in how bad the cycling efficiency was. The reduction of the raw score to obtain the SEER score is done by using a multiplier for the cycling efficiency score. If the system has no cycling losses, like our previous example, then a multiplier of 100% (1.0) is used to multiply the raw score by. So if the raw score was 10 and the cycling efficiency test was 100%, i.e. 1.0, then we would merely multiply 10 times 1 to get the SEER score. But if the cycling efficiency score was 80%, i.e. 0.8, and the raw efficiency was 10, then the SEER score would be only 10 x 0.8 = 8 SEER.

What does the SEER score really mean and how is it used by consumers and the government?

First let’s clarify one point. The SEER system only gives a very vague and often inaccurate idea of exactly what the efficiency of a system will be when it is installed in your home, in your climate, attached to your ductwork at your thermostat setting preference because it’s only testing each system at one set of fixed testing conditions. They couldn’t possibly tell you with any degree of accuracy how efficient a given system would be in your home unless a multitude of different tests covering all of the climate variations, indoor temperature preference variations and indoor airflow variations were performed on each individual system.

Residential A/C systems come in the following tonnages (a ton is 12,000 Btu/h)

2, 2.5, 3, 3.5, 4 & 5 tons

The available SEER ratings for each of those sizes currently varies from 13 to 25 SEER. So if you need a 3 ton system (36,000 Btu/h) and you want a specific brand, then you have a bunch of different SEER choices for that brand of 3 ton system. That’s a lot of tests for the manufacturers to have performed, even when you’re only testing at one fixed set of conditions, i.e. at 80F/50% RH indoor temp with an 82F outdoor temp with a good indoor airflow rate. But that’s what they currently have to do. It would be a very large task to test each of those systems for each combination of condition variations found across the country. That’s why there’s only one set of rating conditions for the SEER and EER tests.

The Federal Government is concerned that we, as a nation, will run out of electricity at some point. So they are constantly looking at the “big picture” of our national rates of energy consumption vs. the amount of that energy we have available to use. They are closely monitoring the balance of consumption vs. supply over time in order to predict what happens in the future, so they can possibly ward off any problems. So from time to time, they increase the minimum SEER ratings that the A/C manufacturers can make. The most recent increase in the minimum SEER ratings that could be manufactured occurred in January of 2006, and it was a biggie to say the least, because they raised the minimum SEER from 10 to 13, which turned out to be a catastrophic blow to consumers. The 10 SEER systems were very affordable, and the 12 SEER systems weren’t much more expensive either. But the jump from 12 SEER to 13 SEER crossed a line into a more expensive required technology. The 13 SEER systems required much larger cooling coils and outside condensing unit coils, with much larger outside cabinets to house them. This caused a huge price increase to be placed on the public. This is why we’re now seeing outside units that look more like a piece of real estate than an A/C unit. There was a great deal of resistance to the decision to increase the minimum SEER to 13 rather than 12. But the 13 SEER mandate was chosen even though it was only 13/12ths more efficient than the 12 SEER systems and was much more expensive. Part of the large initial cost increase was due the fact that the manufacturing facilities were up to that point not equipped or geared to making 13 SEER systems in large quantities as they had been only a small percentage of the market prior to that time. So the factories were forced to retool. The government acknowledged that the 3 point minimum SEER increase was unprecedented, but they claimed that as soon as the factories recouped their initial outlays and began mass producing the higher SEER systems, the price to the homeowners would drop. But they failed to foresee the increases in the materials used in the manufacturing process of these systems. They also failed to realize that you couldn’t fit nearly as many of these systems in a truck to transport them, or that one person could no longer install one of them.

So the government uses SEER ratings as a way to establish a national minimum efficiency standard to help keep us from running out of electricity. That is their MAIN agenda. They are not a consumer advocacy group. They know that the existing, older and lower efficiency systems will die through attrition and will be replaced by systems that meet the minimum efficiency mandates in effect when they are replaced. So their agenda is typically much different than ours as consumers.

OUR agenda as consumers, on the other hand, is to select a system that will be the most efficient for our cooling habits, our climate and our budgets. And our budgets include both the initial cost of purchasing the system and the cost of electricity it will use over the time that we plan to live there.

So the government and us have totally different agendas.

SEER system rating credibility

I’ve researched this at length, and have come to the conclusion that while such a rating system is necessary, things don’t always end up as they were intended when you create a scenario where large manufacturing corporations are included in the rulemaking processes that are designed to regulate them. This just doesn’t help the credibility of the system at all. And this case is no exception. While the corporations are not given the power to veto or modify any provisions of the rules, their recommendations are often heeded because the government wants to keep them happy. Some of these corporations are major campaign contributors. And that certainly can’t be factored out. I’ve read the published minutes of the meetings held for the purpose of modifying the SEER provisions, and it becomes obvious that the D.O.E. often bows to the wishes of these corporations and modifies the SEER rules in their favor.

We must also keep in mind that like every law and rule enacted, an expert can often find and exploit loopholes. Unfortunately this has occurred in the SEER testing process, because some of the manufacturing company engineers are extremely clever. Here are a couple of them:

As previously mentioned, in the steady state testing process, the raw SEER score is the system Btu/h divided by the power consumption. The lower the power consumption is, the higher the raw SEER score will be. The power consumption includes the power consumed by the blower motor in the furnace (or air handler) inside the house. So the engineers figured out that they could use a smaller blower motor during the test and get a higher SEER score. The problem with this is that this motor would never begin to come close to being able to move enough air when the furnace or air handler was attached to the ductwork in a typical house.

Then there is also a loophole in the cycling efficiency test. As previously mentioned, the cycling efficiency test reduces the raw score of the SEER test to yield the final SEER rating score. The loophole is that the system doesn’t actually have to go through the cycling efficiency test at all, because if they know that the cycling efficiency is very poor, they have the option of taking a fixed penalty for that test rather than actually having the test run. So if they know that the fixed penalty is going lower the SEER score more than the actual cycling test would, they will merely accept the penalty and receive a higher SEER rating than the system deserves.

So there are credibility issues with the SEER testing process. And as previously mentioned, the SEER testing process only shows us the efficiency of the system at a given window of test conditions that aren’t typically applicable to us.

How and where are the SEER tests performed?

The SEER tests are done in a lab, under conditions that cannot be duplicated on your installed system. So you can’t determine the SEER of your system. This is because the raw SEER score testing (the steady state tests) are done with mechanically maintained conditions of 80F & 50% RH indoor and 82F outdoor conditions. They are able to maintain the required indoor conditions by using a “bin” of 80F, 50% RH air, which is piped into the blower system. They are heating or cooling and adding or removing humidity from that air to maintain that 80F & 50% RH condition. And the “outdoor” unit isn’t really outdoors. It’s typically inside, in an area that’s being maintained at 82F. Under the actual conditions found in a building, when you continue to run the A/C, the temperature and humidity in the building will drop. And the 82F outdoor air temperature obviously can’t be maintained without intervention either.

Aside from the extra cost, what are the other downsides to the higher SEER systems?

The short version is that, as a rule, the higher the SEER ratings are, the less humidity the systems are able to remove, because the cooling coils aren’t as cold – so less water condenses on them.
And most of the higher SEER systems lose cooling power more rapidly at higher outdoor temperatures because they have smaller compressors.
The extra efficiency is gained by using a smaller compressor and giving the inside cooling coils and the outside condenser coils more heat transfer area. But they also use thinner walled tubing because it has more heat transfer. They also use “rifled” tubing to increase the heat transfer. This can sometimes cause problems with refrigerant leaks in the tubing.

So is the rated SEER of a system totally bogus?

The SEER rating will give you a rough idea of how efficient the system will be compared to other systems. But the more the actual conditions you are using it at differ from the test conditions, the less accurate it will be.

Let’s say for example that you keep your thermostat set at 70 degrees. That’s 10 degrees colder inside than the 80F temperature the system was tested for. Here’s the problem with that:

All systems of the same Btu/h rating and SEER rating will NOT deliver the same Btu/h of cooling when the indoor temperature is 70 degrees. And the same thing applies to the outdoor temperature too. And it also applies to the amount of airflow the blower system in that furnace is capable of delivering. If that amount of airflow is less than the amount the system needs to deliver the proper Btu/h and efficiency, you won’t get EITHER the rated Btu/h or the rated efficiency at any indoor or outdoor temperature.

In short:

The Btu/h and SEER rating of a system is only good for the conditions it was tested at.

Let’s keep in mind that the manufacturers know full well that it would be foolish not to tweak their systems to look the best at the SEER testing conditions (80F with 50% RH indoors and 82F outdoors).

Trust your heating and air conditioning professional to install the proper size system that will deliver the best efficiency and the most Btu/h at the conditions found in your climate, in your home, at your chosen indoor temperature preference AND within the confines of the level of restriction to airflow of your home’s ductwork system.

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