One of the great developments in world history was the unearthing of the Rosetta Stone, which finally allowed scholars to decipher the once-inscrutable hieroglyphs of ancient Egypt. If you’ve ever had to do some minor maintenance work on your HVAC system or go shopping for new parts, you’ve probably yearned for a Rosetta Stone of your own to make some sense out of all the letters and numbers that heretofore have had no meaning to you whatsoever. “HSPF 7.9”? With no context, that may as well be some squiggly lines and a picture of an owl.
Fortunately, explaining different HVAC ratings won’t require you to learn an entirely new writing system. You’ll simply have to become familiar with a few initialisms, some numerical scales, and the general underlying concepts of how your furnace and air conditioner should perform. If this guide does its job, pretty soon you’ll practically be speaking a new language—the arcane argot of HVACese.
MERV
We’ll start with MERV, a topic we’ve explored in this blog before. MERV—which you can feel free to pronounce as “Merv,” rather than “M-E-R-V”—is how the HVAC industry measures the strength of an air filter. MERV stands for “Minimum Efficiency Reporting Value,” and concerns itself with the minimum efficiency, or baseline performance, of an air filter against particles of varying sizes. The MERV scale traditionally runs from 1 to 16, with ratings 13 through 16 typically reserved for industrial settings that lie beyond the needs of an average residence.
What can a MERV rating tell you? It indicates the average arrestance, or the ability to capture particles that try to pass through, as well as how small those particles can be. At MERV 1, the lowest rating, a filter will capture roughly 65 percent of all possible particles and at least 20 percent of all particles ranging from 3 to 10 micrometers—those are one-thousandth of a millimeter—in diameter. MERV 12, on the other hand, will have an average arrestance of over 90 percent. Not only will it capture at least 90 percent of particles in the 3-10μm range, but it also specifies efficiency levels that lower MERV ratings don’t even concern themselves with. MERV 12 boasts 80 percent efficiency against particles from 1-3μm, well beyond what the naked eye can even see.
A standard MERV rubric is full of information regarding micrometers and percentages that don’t tell you what you really need to know. As a layperson, you’re more interested in the practical applications of MERV ratings. Here’s a quick and dirty—though we want as little dirt as possible here—breakdown. MERV 1-4 will reliably capture the big stuff like lint, mites, and pollen—all common allergens that no one wants to breathe. MERV 5-8 will do all that plus smaller particles like mold spores, another sworn enemy of people with allergies. At MERV 9-12, filters start catching fumes and emissions, making them valuable for industrial and automotive use. All the way at MERV 16, filters catch bacteria, making this the rating of choice for cleanrooms and laboratories.
Pretty simple: the higher, the better, right? Wrong—there’s always a tradeoff. Filtration necessarily means resistance, and the more resistance a filter puts on airflow, the harder your HVAC system has to work—especially as a filter fills up. If your HVAC system can’t handle a high MERV rating, imposing one upon it will do more harm than good.
AFUE
AFUE stands for “average fuel utilization efficiency.” Simply put, does your furnace get good “mileage”? Whether your furnace runs on natural gas or fuel oil, it cannot be 100-percent efficient. As a chemical process, combustion itself is too inefficient. Some heat energy—and in turn, your energy dollars—will necessarily be wasted in your furnace, as some of that heat flies out the flue with the exhaust fumes. You can only hope to get the highest efficiency possible. Since 1992, the United States Department of Energy has mandated a minimum AFUE of 78 percent, contributing to more sensible energy usage throughout the country. Traditional furnace models will range from that 78-percent baseline to 90 percent.
Of course, not all models are traditional. The advent of the high-efficiency furnace, which uses a secondary heat exchanger to recapture lost heat from the exhaust, boosts the AFUE past the threshold of a traditional unit and into the 90s, with maximum AFUEs of 97 percent. Electric furnaces give off no exhaust at all—their AFUE is an immaculate 100 percent.
As with MERV, certain costs and drawbacks attend moving higher up the AFUE scale. Higher efficiency means a higher retail price. The pursuit of perfection in AFUE can be dangerous, as electricity costs more than gas or oil in almost all regions of North America. The secondary heat exchangers of high-efficiency furnaces are prone to fracture and failure after hard use, leading to water leaks or even a dangerous carbon monoxide leak. Your responsibility as a consumer is to weigh the short-term costs of the unit against the long-term savings in fuel consumption, as well as taking maintenance needs into account.
HSPF
Not everyone relies on a furnace. For people who outfit their homes with heat pumps, the HSPF, or heating seasonal performance factor, provides the key to understanding unit performance and energy costs. HSPF ratings aren’t arbitrary or qualitative. It takes hard numbers to arrive at this figure. The HSPF is the pump’s heat output in British thermal units divided by kilowatt-hours of energy input—the lower the divisor, the higher the quotient, or your HSPF, and that means month-over-month savings on electricity bills. Ideally, a heat pump should perform at 8 or higher. A heat pump must achieve a minimum of 7.7 HSPF for sale in the United States, and a unit whose rating exceeds 8.5 can attain the coveted Energy Star certification.
Applying the Ratings
With the different HVAC ratings explained here, you can be a more informed customer and client as you deal with the regular and emergent needs of your HVAC system. Understanding the pros and cons of higher MERV ratings can help you find the filter that’s right for you. As an AAF Flanders filter distributor, Remember the Filter offers filters with superior filtration capabilities for superior HVAC systems. Making sense of AFUE can help you weigh the benefits and liabilities of furnaces that promise percentages nearing 100. And if heat pumps are better for your climate, you can do the math and discover just how much energy you are slated to save. For all the unfamiliar letters and numbers, the language of HVAC is not that hard to decipher after all—there may even be just enough time left over to start brushing up on your Latin again.
