Latest / Bozeman Builders Science / Ep. 6: BBS - Wood Heat
Transcript
- Stereo Mix: Why does wood heat feel so different? You can walk into two different houses that are both 70 degrees, and one of them just feels warmer. The house that has a wood stove burning in the corner is likely going to feel coziest. And I think a lot of people in Montana know exactly what I'm talking about. I've always loved wood heat. I've been around wood stoves basically my entire life. my grandparents still use one as their sole source of heat. My parents had one in the garage for a while. I even rented a house for a couple years that had an older wood stove and I loved it. And now I spend a lot of time working in other people's homes and I still notice it. there's just something different about a house heated with wood. The heat hits your face And your body in a way that is all-encompassing. And even outside the room the stove is in, the house can still feel different. So I've wondered about this for a long time. Is there actually something physically different about wood heat? Or do I just really like wood stoves? And I think the answer is both. A wood stove really does heat you differently than a typical forced air system. It gives off a lot of radiant heat directly to your body and the surfaces around it. But it heats air too, which starts natural convection throughout the house. And because all that heat starts from one concentrated location, the building itself actually becomes part of the distribution system. But there's another part to it. You have nature's TV crackling away in your living room. You can see it, you can hear it. You add another log and physically participate in keeping the house warm. There's a ritual to it that a thermostat on the wall just doesn't have. So I want to get under all of this. Why does wood heat feel so different? How does one hot box sitting in one room actually heat an entire home? How have we spent centuries making that box better? And with all the easier ways we have to heat a building now. Why are there still people who absolutely swear by burning wood? The first thing we have to get out of the way is this. Your thermostat does not completely describe how warm you feel. When we say a house is 70 degrees, we're usually talking about the air temperature. And that's important, but your body is also exchanging heat with the surfaces around you. Think of the walls, windows, and floor, and if there's one in the room, a very hot wood stove. Now, there's a term in building science called mean radiant temperature. Mean meaning average. Basically, it describes the temperatures of the surfaces around your body and how they affect your thermal comfort along with the temperature of the air. Ashray, which is the American Society of Heating, Refrigerating, and Air Conditioning Engineers, obviously have lots to say on this. Their thermal comfort work specifically accounts for both air temperature and the mean radiant temperature. In normal indoor conditions, the radiant environment can have a very strong effect on how warm or cold a space feels. You've experienced this even if you've never heard of the term. Think about standing next to a big window when it's winter out. The thermostat may say 70 degrees, but that cold glass can still make that side of your body feel colder. And now flip that around. Instead of a freezing window, put a wood stove that's hundreds of degrees across the room from you. you can still feel that stove's effect on your face. And it doesn't need to blow hot air on you in order for you to feel it. You're receiving radiant energy directly from that hot surface. It's a big part of why wood heat feels the way it does. And it's also the first connection I want to make in this winter heating series. Later, when we talk about radiant floor heat, we're going to come back to this. A wood stove and a radiant slab are completely different. A stove is a relatively small surface at a very high temperature, while a radiant slab can be an enormous surface at a much lower temp. But both can change the radiant environment around your body. If you've ever stood across the room from a wood stove and felt the heat on your face, you already have an intuitive starting point for understanding radiant heat. But the stove isn't only heating you. Radiation is only part of what is happening. The stove is also heating the air around it. Air touching and moving past those hot surfaces warms up. That warmer air becomes more buoyant and rises, cooler air moves in to replace it, and now we've got natural air circulation around the room known as convection. So right around the stove, you're getting both at the same time. Direct radiant heat from the stove itself. And warmer air moving around the room. But when you walk around the corner where you can't even see the stove, that strong radiant effect drops dramatically. You're actually relying much more heavily on warm air making its way into that part of the home, as well as the surfaces in that area gradually warming up. If you walk upstairs, it may be warmer as heat rises, but the radiant effect decreases even more. Close a bedroom door and it changes again. And now we've left the stove itself and we're starting to talk about the building. Now this is where asking will this wood stove heat 2,000 square feet starts getting a little too simple, because we've actually got two separate questions. Can the stove produce enough heat for the building? And can the building move that heat where we need it to go? Which are not the same problem. Picture a two story house with a big open living room and an open stairwell right next to the stove. Now compare that to an old single story house with a hallway. Six little rooms and doors everywhere. You could put the same stove in those two houses and get two very different results. You can absolutely have enough total heat output and still have the living room roasting while a back bedroom is cold. The stove is producing heat, the house just isn't moving it evenly. And then we have the other side of the equation. How fast is the building itself losing the heat? This leads to questions regarding how well insulated it is. how much air leakage is occurring and how cold it is outside versus inside. You can't really separate the heater from the house it's trying to heat. That's why I don't like looking at heating equipment as isolated systems. The stove is just one part of a larger system. And in this case the system is actually much more complex. You have the stove, the fuel, the chimney, the air, the building itself, and the people operating within it. All of those pieces affect the end result. And I think that unevenness is actually part of what gives many wood heated homes their personality. Near the stove you've got strong radiant heat. across the house you're mostly getting convection. Upstairs, the warmest air naturally wants to collect, but inside a closed bedroom, you may not be getting enough heat at all. The temperature isn't perfectly uniform everywhere. Now whether you consider that cozy or annoying, Depends on many things outside of personal preference. So we've talked about how the stove gets heat into the house, but before it can do any of that, we've got to make the heat. And that means combustion, wood needs oxygen to burn, then the products of that combustion, aka the smoke, need somewhere to go. And that's obviously where the chimney comes in. And this is where wood heat connects directly to something we've already talked about on this show. Pressure. You'll often hear that chimney draft works because hot air rises. And that's not wrong, but there is more to it. The gases inside a working chimney are hot, which generally makes them less dense than the colder outdoor air. In a perfect world, that difference acting over the height of the chimney helps create a pressure differential that moves combustion gases up and out. But then wind, chimney temperature, geometry, as well as the pressure inside the building can all affect what actually happens. And think about what that means for the house. If air is leaving through the chimney, replacement air has to come from somewhere. The house doesn't care where that comes from. Air leaves, air needs to enter. And that's the exact same underlying idea we run into when we talk about radon and stack effect. If air leaves a house through the upper portions of the building, through a dryer or a bath fan or a chimney, pressure relationships inside that building change. Replacement air will find an entry path. That's why I've said before when discussing radon entry that the house doesn't care where its replacement air comes from. If the pressure difference exists and there's a pathway, air will move. A wood stove is operating inside that same environment. It needs combustion air. Its chimney is trying to draft, and at the same time, the dryer may be running. None of those things automatically means you're going to backdraft a wood stove, but they're all part of the same pressure system. The Department of Energy's Building Science Guidance specifically warns that natural draft fireplaces and similar combustion appliances can be vulnerable to backdrafting when other exhaust equipment depressurizes the building. We've already touched on this in previous radon episodes. And this is one of the reasons I want to get as much of this info out there as possible. The physics don't care what industry we're talking about. I can be investigating radon in one house and talking about a wood stove in another, and suddenly we're back to the same fundamental idea. Air moves because of pressure differentials. Now, people have been trying to improve this system for centuries. and once you look at a wood stove as a system instead of just a metal box, the history starts getting more interesting. Because humans have been solving basically the same set of problems for a very long time. Things like how do we get more heat from the fire into the room? Alright, now we've got more smoke, how do we keep the smoke out? The wood stove Is burning wood too fast? How can I slow it down without negative side effects? How do we send less useful heat up the chimney? And that's the engineering story. We start with an open fire in a cave. You get direct radiant heat, but you also get smoke and very little control. If we move that into a fireplace and a chimney, now we've got something important. We've created a dedicated pathway for combustion products to actually leave the shelter. But a traditional open fireplace still has a giant opening between the room and the chimney. That means it can move a lot of indoor air up and out of the building. And as a dedicated heater, a traditional open fireplace generally isn't very effective. The EPA's current residential wood heater rules don't even regulate ordinary open fireplaces as wood heaters. That's in part because the EPA's review found that typical fireplaces aren't effective heating appliances. But think about that. We've come so far that our original only source of heat is now not even designated as one by the EPA. So think about the next engineering evolution. We put the fire in a box, and now we can start controlling the amount of air getting to the fire. We can expose hot iron or steel surfaces to the fire and the space around it, we can control the route that the smoke takes before it even reaches the chimney, which then allows us to start extracting more useful heat before those gases leave the building. And none of this is a new idea. Even early stove designs experimented with things like baffles specifically to get more heat into the room and reduce smoke. Benjamin Franklin's stove design used internal baffles. For exactly that reason. But the important part is the problem people were trying to solve. How do we get more useful heat out of the fire before we throw the exhaust outside? And that question leads us directly to modern wood stoves. A modern wood stove is doing more than just holding a fire. From the outside, it really doesn't look much different. It's a metal box with a door, potentially some fancy glass window, and a chimney. You put wood in it, light a fire, and heat comes out. But what is happening inside can be considerably more sophisticated than an older one. When wood burns, it releases combustible gases. along with particles and other combustion byproducts. If those materials don't burn completely, some of that usable energy leaves the stove as smoke. One of the biggest improvements in modern wood stove design is burning more of that before it goes up the chimney. Now, there are two broad ways you'll commonly see that done. One is secondary combustion. Depending on the stove design, you may have an insulated firebox, baffles that keep the combustion gases in a hot environment longer, and preheated secondary air introduced above the main fire. A solo stove is Believe it or not, actually a great example of this, even though it's not a box. Instead of allowing those gases to immediately head for the chimney, we're just giving them better conditions and another opportunity to burn. The other approach is known as catalytic combustion. Those stoves route the smoky exhaust through a catalyst, which then promotes additional combustion to occur at lower temperatures than it likely otherwise would. And some modern wood stoves combine both approaches. Catalytic and non-catalytic designs are the two major approaches manufacturers have used to achieve cleaner combustion, and current federal standards actually require newly manufactured and regulated wood heaters to meet specific particulate emission limits. So when we compare an older wood stove with a modern certified stove, we're not just talking about a pretty door or a nicer piece of glass. The combustion system itself has actually evolved. We're trying to control the fire better, keep the combustion environment hotter where we need it, burn more of what would otherwise be lost to smoke. And then we're able to extract more usable heat and reduce what leaves through the chimney. It's the same fundamental goal humans have been working on for centuries. We're just much better at controlling it now. And let's not forget about the fuel. All that combustion technology only works with the fuel you give it. And this is another place where there's a simple rule with a system underneath it. You've probably heard burn seasoned wood. Okay, well what does that mean? Burn dry wood. Okay, why? Because if there's a lot of water in that wood, some of the energy from the fire first has to heat that water and then evaporate it. This is actually where a fun camping experiment with my dad from my childhood comes into play. If you take a paper cup and fill it about three-quarters of the way full with water and set it in a campfire, what happens? If you're anything like me, you would expect the paper to instantly catch on fire, the cup fails, and all that water spills out and puts out the fire. But annoyingly, that's not what happens. The paper above the water line burns away instantly. But the part of the cup that's touching the water does not. And the reason is actually pretty simple. At normal atmospheric pressure, water boils at about two hundred and twelve degrees Fahrenheit. Once it gets there, adding more heat doesn't just keep making the liquid water hotter and hotter. A huge amount of that energy starts going into changing the water from a liquid into a vapor. So the water is effectively keeping the paper that's touching it from getting hot enough to burn. The fire may be hundreds of degrees hotter than that, but as long as there's water against that part of the cup, The water keeps pulling heat away and holding that paper down near the temperature of boiling water versus paper's ignition point. Once the water boils away, obviously that changes. Now there's nothing there keeping the paper cool and it heats up enough to ignite and then burn. And that's the connection I care about here. If we think about a wet or unseasoned piece of firewood, we're trying to get the wood hot enough to support good combustion, but it's also loaded with water. Before we can get all of that potential useful heat out of the wood, we're spending energy heating that water and then driving it off as steam and a vapor. And while we're spending energy getting rid of that water, we're also making it harder to maintain the hot. Combustion conditions that help the stove burn the wood cleanly. The practical result can be more smoke, poorer combustion, and less useful heat from the fuel. The EPA's older burn-wise guidance, which is now archived but remains useful technical info, recommended less than 20% wood moisture content as a good target. They even recommended actually checking firewood with a moisture meter. Rather than assuming that time alone made it dry. And I like that because it gets underneath another common statement. My wood has been sitting outside for a year, it's seasoned, I promise. Well, maybe, but it's outside for a year isn't actually the property we're interested in. We're interested in how much moisture is still in the wood. Now, obviously the wood species matters, how big the pieces are, how it's stacked, whether air can move through the pile, weather. And storage matter, season it is the rule, but moisture content is why the rule exists. And now the part that we shouldn't romanticize. I've spent most of this episode talking about why I love wood heat, and I genuinely do. But I'm also somebody who works in indoor air quality. So I don't want to turn a fire inside your house into some perfect nostalgic thing and ignore the combustion side of it. Wood smoke is technically pollution. When wood burns, it doesn't just produce the smoke that we can see and smell, it also produces extremely small particles that can stay suspended in the air for a very long time. One term you might likely hear around this is PM two point five. And PM just means particulate matter. And the 2.5 describes the size of the particles that we're talking about. So about 2.5 micrometers across the or smaller. Just take my word for it, it's small. For some perspective, these are actually small enough that when you breathe them in, they can travel deep down into your lungs instead of simply getting caught higher up in your respiratory tract. And that's where the health concern comes from. Those particles tend to irritate and inflame the respiratory system. They can make asthma worse, aggravate existing heart and lung disease, and Particulate pollution has also been linked to cardiovascular problems like heart attacks and irregular heart rhythms. So we're not worried about PM 2.5 because somebody gave a tiny particle a scary acronym. We're worried about it because its size allows it to get into parts of the respiratory system we don't particularly want combustion particles ending up in. And there's actually a really interesting Montana example of this. Libby, Montana had a serious wintertime air quality problem, tied in large part to residential wood burning. Before a major stove replacement program, University of Montana researchers studied the sources of fine particle pollution in the valley and found that residential wood stoves were responsible for a huge portion of the wintertime PM two point five they were measuring. So starting in the mid 2000s, the community replaced roughly twelve hundred older wood burning stoves with newer cleaner models. And researchers from U of were able to study what happened before and after that change. They also didn't just look at the outdoor air. They measured particulate levels inside wood burning homes as well. And overall, they found that replacing those older stoves with newer certified models reduced indoor particulate levels. Now, as with anything, there is a caveat. Obviously not every house improved by the same amount, which is important. Houses are different, people operate stoves differently, and simply changing the appliance doesn't magically control every source of particulate matter inside a building. But the larger point is actually pretty useful. How cleanly we burn wood actually matters. The stove design, fuel, and operating practices all matter. I love the experience of houses heated with wood. And there is absolutely a smell that I associate with those homes. But that doesn't mean I'm saying that smoke leaking into an occupied house is good, harmless, or just something you should accept, because it's not. If you're regularly seeing or smelling actual wood smoke in your living room while the stove is operating, it's something worth investigating rather than shrugging off. Cause that's Just what wood heat smells like is not the case. And then there are other realities. We're dealing with an actual fire, potentially carbon monoxide, and we're producing hot combustion gases and hopefully sending those gases up through a chimney. Incomplete combustion and cooler chimney conditions can contribute to deposits like creosote. The stove has clearance requirements, and the chimney has installation requirements. Smoke alarms and carbon monoxide alarms matter. Inspection and regular maintenance does too. I'm intentionally not turning this into a wood stove install or operation class. Obviously, specific requirements depend on the stove, the chimney, the system itself, the manufacturer's whatever, as well as applicable code. The point is simpler. A wood stove is a heating appliance. But it's also a solid fuel combustion system with a fire inside your house. And enjoying it doesn't mean ignoring that. So why do people still do it? Because when you list the drawbacks, it sounds ridiculous. You've got to get the wood. Maybe you buy it or cut it, you gotta split it, stack it, cover it, let it dry, you carry it in the house, sweep up the bark in the dirt, remove the ash, kill the spiders. Maintain the stove, maintain the chimney, tend the fire, and if you're trying to keep the whole house comfortable, you've also got to learn how your particular building and stove behaves. They're constantly interacting with one another. Meanwhile, there are heating systems where you just turn a thermostat to 70 and go on with your life. It does it all automatically for you. So why choose the wood stove? And honestly, around here, I hear a lot of the same reasons. It's cozy. It's dependable. If you've got access to wood, it can be inexpensive. And sometimes people can get some of or all of their fuel essentially free in exchange for their own labor. Depending on the stove and the installation, it can give you heat during an electrical outage without depending on the same systems as an electrical furnace. There's a sense of independence to having part of your winter heat sitting in a wood pile in the shed outside. And some people genuinely enjoy the work, myself included. Which makes no sense if the only thing we're measuring is convenience. But convenience isn't the only thing people value. There's something extremely direct about wood heat. You get cold, you build a fire, you put another log on, the flames change, the coals heat up. You feel the stove get hotter and the house gets warm. You can see it working right in front of you. There's one house I still think about when I try to explain this to people. It was on the north side of Bozeman around the old railroad station area, and it was a very old home. They had obviously added onto it and remodeled it over the years, but the original section of the home was very much a cottage. They had these big tiles throughout the kitchen and around the wood stove area. And it was a cold, crappy day out that day. Like wet, drizzly, miserable. And we both walked into that house and got next to the wood stove and just had that involuntary sigh when you've been cold and suddenly you aren't anymore. It just feels different. And that's really the question that started this whole series for me. Why does that feel so good? But now I have a much better answer than just because I like wood stoves. I'm standing near a very hot surface that's directly radiating energy toward my body. That stove is heating the air around it and it creates its own natural convection. The heat is moving through a building that has its own layout, heat loss, and pressure relationships. The fire is consuming air, the chimney is trying to maintain a draft. And the stove itself represents generations of engineering aimed at getting more useful heat from the wood. While sending less smoke and wasted energy out the chimney. And then layered on top of all that physics is the human part. There's still just a fire sitting there. Nature's TV in your living room. So why does woody feel different? Because it actually is different. A wood stove doesn't just raise the temperature of the air until a thermostat is happy. It directly changes the radiant environment that your body is always interacting with. It heats air and creates its own air movement. Its ability to heat the rest of the house does depend on the layout and performance of the building, but modern wood stoves are the latest version of something people have been trying to improve for a very long time. How do we turn a fire into useful heat for a building without wasting so much of that heat? And how do we do it without filling the house with smoke? And yet that's still only part of why people love them. There's something satisfying about seeing where your heat comes from. Your fuel's stacked outside. You get to put another log in and understand exactly what that action is going to do. And you can't beat walking in from a miserable, wet winter day and standing next to a stove and immediately feeling your body warm back up. There's days I've been soaking wet, chilled to the bone, and I know a hot shower is going to be the only thing that fixes it. However, a wood stove does the exact same thing. And I don't think the building science takes anything away from that experience. For me, understanding what's actually happening makes it more interesting. And we're going to carry part of this directly into the next heating system that we talk about. Because when we get into radiant floor heat, we're going to come back to that feeling of standing near a wood stove. It's the same fundamental mode of heat transfer. but a completely different way of using it. So maybe instead of putting one extremely hot box in the corner of the room, what if we just heated up the entire floor?