Whether you’re comparing wood or gas appliances, you’ll see an efficiency rating listed in the product information. Between uncertain fuel costs and increasingly strict air quality regulations, more and more consumers are looking for the most efficient heaters they can find. But what do efficiency ratings actually mean? How are they measured?
The bad news is that there isn’t one standard rating system used by manufacturers, nor are they required to tell you which system they’re using. The good news is that by learning a bit about what the ratings mean, you can put manufacturer claims in context and compare apples to apples when buying a stove or fireplace.
What does “efficiency” actually mean?
In the simplest terms, the efficiency rating on an appliance tells you how much of your fuel will become usable heat in your home. A pound of wood, gas, or pellets contains a certain amount of potential energy; some of this energy will be used in starting and sustaining combustion, some of it will go up the chimney (carrying away particulates and other undesirable byproducts of combustion), and the rest will come out into your home as heat.
Wood Heat: EPA vs. Real-World Performance
Wood stoves and inserts are sometimes advertised with two efficiency ratings: EPA and cordwood/real world performance. EPA testing procedures are focused mostly on particulate emissions, and are very highly standardized. These tests use milled lumber (often pine), precisely stacked, with the stove draft set to its lowest (and smokiest) setting; this is also referred to as a “crib wood” test. While this is useful for determining how much air pollution a stove will generate, it has very little to do with how efficiently your stove will convert firewood into usable heat in your home. Many companies choose to do their own testing with cord wood (usually hardwood) and realistic venting setups, in order to give their customers a more accurate idea of stove performance. Unfortunately, there’s no requirement that companies tell you which efficiency rating they’re listing. For environmental impact, focus on the emissions listing (gm/hr of particulates released into the atmosphere) rather than the listed efficiency rating. For real-world efficiency, a good rule of thumb is that 67%-75% is a good range for quality non-catalytic stoves, with catalytic stoves occupying the 75%-83% range.
(Note: just to make this even more confusing, the EPA is currently in the process of revising its compliance and listing standards. While this process is ongoing, look for “cordwood/actual measured efficiency (CSA)” in your stove literature.)
Gas Heat: Steady-State vs. AFUE/EnerGuide
Gas stoves, inserts, and fireplaces are rated in two different ways. The Steady State efficiency rating, which offers a higher and more impressive number, is taken only once, after the appliance is fully heated up and burning steadily. While this number is technically accurate, it has very little to do with how your stove will perform over the course of its lifetime, and under real-world conditions. We prefer to work with vendors who list the EnerGuide, P4, or AFUE (annual fuel usage estimate) ratings. These efficiency readings reflect performance over the entire burn cycle, from ignition to cool-down, and use realistic venting setups. If you’re looking at efficiency ratings, and seeing numbers in the high 80% range, you’re probably looking at the steady-state efficiency of the fireplace. The more honest EnerGuide, P4, and AFUE rating systems give numbers ranging from around 60%-70%. A 65% efficiency rating is actually quite good, and an 85% rating, although it sounds much better, probably doesn’t reflect an actual increase in real-world efficiency or performance.
To sum up: efficiency ratings can be confusing, and don’t always give a complete picture of how an appliance will perform in your home. However, with a little bit of time and research, you can put those numbers in context when comparing different products, and make a more informed decision about which stove, fireplace, or fireplace insert will be the best fit for you!