BTUs can be very confusing.
Simply put, the definition of a BTU (British thermal unit) is the measure of the heat content of a fuel source. In mathematical terms, a BTU is equivalent to the quantity of heat required to raise the temperature of a pound of water by 1 degree Fahrenheit.
That may sound pretty straightforward, but there’s a lot going on under the surface, especially when you’re trying to understand BTUs in the context of a wood burning furnace.
Wood furnace manufacturers provide all kinds of BTU specifications in their manuals and marketing materials. You may see terms like “maximum input capacity,” “minimum overall heat output rate,” “maximum delivered heat output,” and plenty more.
But what do any of these terms mean? Which ones matter? How do they explain what the furnace does and how it actually performs?
At HY-C, we manufacture forced air wood burning furnaces under our Fire Chief and Shelter brands. And in this guide, we want to clear up some of the perplexity around this complicated unit of measurement.
We’re going to cover 4 important BTU metrics for wood burning furnaces:
By the time you’re finished here, you’ll understand not only what these metrics mean, but you’ll also be able to use them to compare and contrast any make and model of wood furnace using a clear, set standard.
Note: throughout this guide, we’ll use actual laboratory numbers derived from tests on our own Fire Chief FC1000E and Shelter SF1000E wood burning furnace models.
Maximum Peak Input
Let’s start with maximum peak input, as it’s a measurement that drives some of the other metrics we’ll be talking about.
Simply put, maximum peak input is the highest possible amount of wood fuel energy that can be added into a wood burning furnace. For our furnaces, the maximum peak input is 289,000 BTUs.
Now, let’s take a look at how we determine that number. There are three variables to consider:
- The volume of the firebox (in cubic feet)
- The weight of the firewood (in pounds per cubic feet)
- The energy content of the firewood (in BTUs per pound)
In our furnaces, the firebox (i.e., the “chamber” that houses the wood fuel) measures 3.4 cubic feet.
The weight of the firewood is based on an industry average derived from several rounds of EPA testing data. They’ve found that, on average, wood furnaces hold 10 pounds of firewood per cubic foot of firebox.
Finally, there’s the energy content of the firewood itself — again, based on EPA data. The agency has found that, on average, 1 pound of dry cordwood contains 8,500 BTUs of heat energy.
From here, we can do some simple math.
We can multiply the size of our firebox (3.4 cubic feet) by the weight of the wood per cubic foot (10 pounds) to get 34 pounds — the total weight of the wood fuel load in our firebox.
Finally, we can multiply that weight (34 pounds) by the amount of energy in a pound of wood (8,500 BTUs) to get 289,000 BTUs.
In other words, the maximum peak input — the highest possible amount of wood energy we can put into our furnace — is 289,000 BTUs’ worth of wood.
Maximum Peak Output
Once you understand maximum peak input, understanding maximum peak output is easy.
A furnace’s maximum peak output is the highest amount of energy (in BTUs) it can possibly deliver to a living space once it has burned through its largest possible fuel load.
In our furnaces, that number is 143,500 BTUs.
This means that when you add the largest possible fuel load to the furnace (34 pounds of wood containing 289,000 BTUs) and the furnace burns up all of that wood, it will have delivered 143,500 BTUs’ worth of energy to the living space.
In other words, maximum peak output is the highest possible amount of heat energy a wood furnace can deliver when it burns its largest possible fuel load.
Average Heat Output
A furnace doesn’t run on full blast every second. Rather, it runs in cycles, adjusting to deliver just the right amount of heat based on the setting of the thermostat and the actual temperature in the home.
Let’s think about this in practice.
Say it’s 30 °F outside, and the furnace’s thermostat is set to 74 °F. To overcome that 44-degree temperature difference, the furnace will have to start working hard to put out heat. Once the home is warmed up to 74 °F, though, the furnace can lower its output a bit.
As the furnace winds down, though, the house begins to cool, and its temperature may dip down to 73 °F. At this point, the furnace will need to kick back into gear, cranking out more heat energy to keep the home at your desired temperature.
All of this is to say that wood burning furnaces run in cycles. And the average amount of heat the furnace puts out per hour as it burns a fuel load is called its average heat output.
The chart above represents a typical burn cycle. As you can see, during some hours, the furnace will put out quite a bit of heat, and during others, it produces much less. On average, though, its BTUs per hour come out to 46,435 — the actual average heat output for our wood furnace models.
Simply put, average heat output is the amount of heat energy you can expect your furnace to deliver to your living space during each hour of its operation.
Emissions
Like cars, wood burning furnaces give off emissions. Wood is an organic fuel source, and it gives off byproducts like ash, soot, creosote, and more when it’s burned.
Recently, the Environmental Protection Agency started regulating the emissions levels of wood heaters. As of 2020, EPA standards maintain that wood heating appliances that burn cord wood must produce less than 2.5 grams of material per hour.
The tested emissions of our furnaces are 1.07 grams per hour — well within the EPA’s regulations.
But what does this have to do with BTUs?
Well, there’s another way to look at that 1.07 grams per hour measurement: pounds per million BTUs. This is the same rate simply expressed in a different number — the pounds of material emitted by the furnace every time you burn one million BTUs’ worth of wood.
In our furnaces, one million BTUs are produced for about every 118 pounds of wood burned. And, with this amount of wood, the furnaces emit about 0.14 pounds of material.
So, while some agencies or manufacturers may present their emissions in terms of grams per hour, others may express emissions in terms of pounds of material per million BTUs.
How Should BTUs Affect Which Furnace You Consider?
Now that we’ve looked at BTUs in a few different ways, there’s a larger overarching issue to consider: what does this all mean when you’re actually buying a furnace?
The point of this guide is to help clear up any confusion over BTU specifications. As we’ve seen, BTUs can be expressed in a number of different ways in an attempt to influence buyers’ perceptions about a furnace. But as a buyer, you deserve to understand what it all really means.
One manufacturer may say, “Our furnace is capable of 350,000 BTUs” and be referring to maximum peak input.
Another manufacturer may say, “Our furnace is capable of 180,000 BTUs” and be referring to maximum peak output.
Still another may say, “Our furnace is capable of 60,000 BTUs” and be referring to average heat output.
None of these numbers are incorrect. The manufacturers aren’t lying, but their figures don’t share the same context. It’s comparing apples and oranges.
When you’re making an important and expensive purchase, you need to be able to compare apples to apples. With the information in this guide, you can decode all the jargon around BTUs and make a truly informed buying decision.
With that in mind, be sure to take a look at our guide to the best wood burning furnaces on the market. It compares and contrasts four industry-leading models to help you find the one that’s right for you.
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