Winter is here in North Carolina. Frosty grass every morning. Freezing fingers during barn chores. It’s time to get cozy and talk about heat.
First of all, wood stoves are not an automatic environmental impact win. You should know that right up front (more about it later). We always consider environmental impact when we choose but we also take into account other things: comfort, safety, labor commitment and especially resilience. Our wood stove was chosen mainly for resilience.
A merry fire. I found that tea kettle in my aunt’s yard. The black contraption next to it is a fan operated by a peltier motor, which turns heat into electricity, saving me the trouble of shutting off the fan when the fire dies. I was skeptical of this inexpensive little device, but it’s been going strong through one and a half heating seasons, and it really does move the hot air back to the bedrooms. I’m impressed.
I grew up in Alaska, where the weather is legitimately trying to kill a body at least nine months of the year, and sometimes also on random June afternoons. As a child, one of my family’s worst encounters with hypothermia was on a random June afternoon. We had electric heat set in the mid-60. We had a two-story living room which conclusively proves that some architects are idiots. It felt about 45 degrees sitting on the couch. I’m not a well-insulated person. Maybe that’s why I’m so concerned with heating resilience, even now that I live in the south.
It gives me great comfort to know I’d be warm if the power went out, but I was pretty nervous when I designed my family’s little house with a wood cook stove as our sole source of heat. I’d done short stints in wood-heated buildings, but never long-term. What if I hated it, but I was stuck with it because we were out of money at the end of the build? What if I couldn’t manage a wood-cooked dinner on time with two little kids? What if I was miserable because it just couldn’t keep the place warm enough? But into our third winter, I have to admit it’s one of my favorite parts of the house.
Part of the reason is comfort. I have some chronic pain, and a mid-60s house is pretty uncomfortable to me in the evenings when I finally sit down. It’s the worst part of the day for people with spine issues, when muscles are tired and tight. I was shocked to learn that different temperatures in different zones and at different times is much more comfortable than 67 everywhere always. By the time I’m done eating dinner, my back absorbing all that radiant heat, I’m much more relaxed than I used to be in a cold house. When I wake up in the farthest back bedroom on a winter morning, I’m still cozy even though it’s chill. Even arriving home late from a week-long December trip, it’s nice enough in an hour or two. What a pleasant surprise!
My husband made me this graph with data he collected so I could illustrate for you exactly how comfy I am. See that dip at week 5? We went to visit my in-laws for a week. Thank you husband!
There was a learning curve with cooking to be sure, but not a terribly steep one, probably because our model seems very well designed. It takes a smidge longer to heat up than an electric stove, requires more finesse, and gives you less immediate control over the temperature. That might all sound bad, but the ultimate result seems to be better. I used to burn dinner once in a while, which I hated because I despise waste, and I’m sometimes running later than some people would like (it’s my husband- my kids couldn’t care less).
I’ve burned two things total on the wood stove, which is a significant reduction. The reason is probably that the wood stove is a natural check on frustration. It just takes as long as it takes, and there’s no way to turn the knob up when I’m impatient only to regret it later. Dinner takes me an hour no matter what I cook, whether I’m using a nice gas stove, any of the terrible electrics I had in rental houses, or my trusty Coleman two-burner at a campsite with a cutting board balanced on my knees. Lo and behold, with the wood stove it usually takes me about an hour from striking the match to dishing out food. It takes the same time, but subjectively it feels less rushed.
Cooking apple cinnamon oatmeal on a resilient stove is pretty much the same as cooking it on a regular stove. When I need more space I take the fan and kettle off the stove top.
Never mind burning food; how about burning myself? I admit I used to do that too once in a while on our former electric or gas stoves. Again, I’ve done it twice on the wood stove, another significant reduction. I think the reason is it’s impossible to forget the wood stove is hot, because it’s hot. Not just to the top but also to the front, which is a boon to chilly thighs, and also demands constant remembrance and respect. Our littlest kid was two and a half when we started heating with wood, and even she had no problem learning to walk around the other side of the table. No child in my house has even come close to getting scorched. If she’d been littler, I would have had to engineer a serious safety barrier.
Our stove is the Vermont Bun Baker. You can cook on top of lots of different models that are more efficient and cheaper, but the Bun Baker comes with an oven. This was a silly thing for me to spend a lot of extra money on, since I’m really not a baker. What can I say about this absurdity? When designing a house for the first time it’s easy to get fixated on maximum functionality. I definitely let my anxieties during that difficult process seep into my actions in weird and surprising ways.
The oven works fine. I do use it, but only in late December, January, and February, because the stove doesn’t run enough to get it hot in the other heating months. I use my sun oven in the summer and I personally don’t miss baking in the shoulder seasons.
Another advantage disguised as a drawback is that it takes time and attention to heat the house with wood, which encourages me to slow down and hibernate. Spring, summer and fall border on frenetic because there is a lot to do and I’m naturally inclined to go go GO! When I come inside to make a winter dinner, settle the mind and sit on the tile floor to light the stove, it’s a sort of relaxation. It’s a ritual that focus me on my task. If we were the type of household where no one is home except to sleep and bathe, rather than the type that has reorganized our lives around home and being there, it might be pretty inconvenient to heat with wood.
Our stove is fitted with the optional water jacket that lets us draw hot water off of it. This is a scary idea to lots of people because wood stoves used to heat water in sealed, pressurized loops. When water turns to steam it expands in volume 16,000 times, turning your metal tank into a bomb. People died. That isn’t possible with our setup.
Not pretty, but totally functional. The grey tank is our electric hot water heater, not usually on. The blue tank is a pressure relief system. The red device is the efficient electric pump, and that thing surrounded in bubble wrap is the heat exchanger. The fill valve is the yellow handle. The power supply is the green cord. The wood stove is on the other side of the wall.
In our system, the hot water is in two loops. One loop is pressurized from the city water to the water heater to the sinks and shower. The other loop is open to the air (“vented”), and this is what gets directly heated by the fire. The water heats in the jacket, and as it heats it becomes less dense, rising naturally up a copper pipe to a heat exchanger. From the bottom of the heat exchanger it returns via another pipe to the jacket (you can see this copper return pipe in the picture at the top of the post, and the riser pipe insulated with silver stuff next to it). This is called a thermosiphon and it operates only by gravity. So cool! There’s a fill valve and a little bit of clear plastic extension sticking up from a T at the top, so I can see that I’ve filled it enough.
The other side of the heat exchanger is plumbed to the pressurized household water in a loop operated by an efficient electrical pump with a temperature-sensitive switch. The switch senses the temperature of the water in the unpressurized stove-side loop and turns on the pump at 130 degrees. The circulating pressurized water draws heat off the unpressurized loop through the exchanger.
On cool, one-fire winter days this setup acts as a preheater, reducing the electricity needed to bring the water up to full heat. On colder two-fire days it heats the water entirely, for just the minimal electrical cost of running the pump.
On the one hand, I’m extremely proud of this little system. The idea got hold of me and just wouldn’t let go. I had to try it. I read everything I could find on thermosiphons, and based on that I basically guessed about which makes and models of pump, heat exchanger and switch I needed and how to arrange them, and darned if the thing doesn’t do exactly what I hoped it would. It definitely saves us electricity, helping to invert our usage. See the graph below: we use the least electricity in the coldest months when other households use the most, therefore driving less extra demand for electricity at peak times, which is more emissions-intensive. That’s about the most successful outcome for one of my plans, ever.
My husband the data fiend made this graph of electricity use comparison between my house and my parents’ house, which is in our neighborhood. Thank you husband! You’ll notice we’re lowest when they’re highest. How is our usage so low?? The wood stove is part of it, but don’t worry, I’m writing a whole post about the rest as we speak.
On the other hand, there are some significant drawbacks. The littlest one is that cool water is drawn into the tank as you shower, but it sits at the bottom and doesn’t get to the tap until the hot runs out or unless the pump mixes it up. If the pump clicks on while you’re in the shower, the water cools off as the loop mixes cool water into the hot in proportion to the time you’ve been showering. This is easily remedied by unplugging the pump before I get into the shower. But, if I forget to plug it back in once I’m clean, the stove loop will overheat and spill steam out the vent into the room the water heater is in. No big deal, but not exactly a great feature.
A bigger problem would occur in a winter emergency. The whole point of the wood stove is that we still have heat if utilities fail due to bad winter weather. If the power went off, we could be fine. We have a little solar generator that would run the pump. But if the water also went off for a really extended period we’d be in bigger trouble, and here’s why:
If I fire the wood stove with the jacket empty it’ll warp, break the watertight seal and make the whole setup useless. Also, the energy has to be drawn off that water, or it’ll steam the room pretty good which could lead to mold over time. Finally, the water must be drawn out of the water heater and new cool water added, because while it would be impossible for that little exchanger to heat 40 gallons to steaming and explode the water heater, it’s still scary to have that much water so hot. It would take days to get too hot, even in cold weather. But I’m not willing to bet we’ll never have a utility interruption longer than a few days.
Without running water, the water heater would have to be hand-filled and hand-emptied. It could definitely be done with some modification and the right set of funnels and elbows, but it’s not the sort of thing I want to be fooling with in an extended wintertime emergency.
Assuming our minds and bodies were occupied with more essential tasks such as parenting in a blackout, we’d be faced with the choice of draining the jacket, running the stove and letting the whole setup fail, or letting the house cool off. Even with low solar gain, cold days and no heating, our data says the house doesn’t drop below 54 degrees, which is totally survivable. Just not comfortable for small children and people with spine problems, who would already be under some stress.
We would be reluctant to let the setup fail because of the third drawback: it was freakin’ expensive. By the time I paid the premium for the optional jacket, bought the components and paid a plumber to solder it together, this is an $800 piece of technology.
It would have been cheaper and more edifying if I’d hooked it up myself, but I was absolutely frazzled after 17 months building our first house. I wasn’t confident I could learn soldering quick enough and well enough to not screw the whole thing up. It was Thanksgiving, my relatives were coming and we needed to start heating and cooking inside immediately. I had an undiagnosed selenium deficiency causing spectacular fatigue and hair loss. My husband was working his standard 50+ hours a week, and I was home with an unusually challenging four-year-old and also a two-year-old. Basically, life got in the way.
It’s disappointing, though, that one of the coolest things in my house isn’t really scalable or even a very good demonstration because it’s prohibitively expensive. “The bougie-est water heater ever,” I think my husband termed it. He’s completely right. It doesn’t return enough either in saved money or in reduced environmental impact to justify the cost, complexity or embodied energy in the parts. But I just had to try, you know?
While we’re being honest about drawbacks, let’s talk about the drawbacks of a wood stove itself. First of all, burning wood releases carbon dioxide. How much carbon dioxide, for any particular stove? That’s really hard to tell without equipment to measure. One best guess is 2.5 tons per cord of hardwood such as oak or walnut (a cord is a neat stack 4 feet wide by 4 feet tall by 8 feet long)
We currently burn some walnut because one fell next to the house. I sold the usable portion of the trunk and root ball; we’ve burned limbs and also sticks as kindling. We also burn some hundred-year-old pine out of the houses we’ve torn down. This softwood is less dense than hardwood so it probably emits less. It’s too insect-damaged to reclaim even for my funky aesthetics, but it’s better to burn it than bury it in the landfill where it would emit methane, a shorter-lived but much more effective greenhouse gas.
However, there are some contortions in our collective reasoning about burning wood that we should examine. It’s generally considered a renewable resource, which sounds great and it is, sort of. Trees do regrow, but only if they are allowed to regrow. If the soil isn’t too damaged by their felling, if the land doesn’t get used for something other than forest causing some pretty wild swings in soil carbon, if the trees planted to replace them are a good fit for the ecosystem and are cared for so they survive (I’ve learned the hard way that young trees are fragile). If, and this is a big if, we don’t cut trees faster than they can regrow.
On our land this isn’t a problem; more trees fall than we could ever use for firewood. Many are left to support the creatures that thrive on rotting wood. We won’t even finish burning ancient framing out of defunct houses until the end of next winter at the soonest. But in other contexts, failure of forest regeneration is a problem.
Another issue is that, even though fresh-fallen tree carbon isn’t ancient carbon, it is nevertheless sequestered carbon. It’s held safely in living bodies rather than dangerously in the atmosphere, and we’re letting it out in one hot bright flash. We imagine that this doesn’t matter because a fallen tree would give its carbon back to the air anyway, but that’s not what happens. A fallen tree is fed upon, and some not-insignificant portion of its carbon therefore remains sequestered in other living bodies or in the fluffy forest soil humus (that’s the organic matter in soil). Some is breathed back into the sky, but not all of it, and not instantly.
Processing and transporting wood also has energy requirements. It’s got to be cut and split somehow, and while we split by hand rather than by machine, we’re not badass enough to cut much by hand yet. It’s a very small part of the carbon footprint of our wood, because like I said, the tree fell right next to the house. For a wood stove that needs its feed logged with larger machines and then moved dozens or hundreds of miles, it’s not so small.
Burning wood also releases things other than carbon, some of which are quite nasty, health-wise. Lifespans were shorter when American cities were wood-heated, and they still are in developing nations for exactly the same reason. Keeping the smoke out of the house is critical. Keeping the smoke in the neighborhood at low-enough levels is critical. In short, if you don’t have a patch of land large enough to produce more fallen trees than you could ever burn, you and your neighbors might be making the area too smoky for good health. It’s not all bad; some of those wood-originating aerosols are cooling the planet, even as carbon warms it (here’s a thorough explanation). But that doesn’t mean we should breathe it.
So what’s the bottom line on wood stove carbon?
Some data from my parents’ house allows me to guess how much electricity might heat our little home, sans-stove. They live right up the street, so the climate is comparable. Overall their insulation and air leakage probably isn’t much different; my attic insulation is definitely better, but then again I have a French door that is currently rather poorly sealed (I’m getting to it, I swear).
Our living area is half the size, which doesn’t halve our heating needs because our surface area to volume is larger, and therefore so is our heat loss. Our house is banked into the earth, which reduces some of that loss. They only have small children there sometimes, so their doors get left open less often. (“Why is the door open?” must be my most-uttered winter phrase, second only to, “Yes, you DO need socks. It is 31 degrees. Put on socks.”)
It looks like my folks emit maybe a ton of carbon a year strictly for electric winter heating. I’m not sure exactly how much more they use for winter cooking and hot water, because I haven’t yet used our handy Kill-A-Watt on any of their appliances. If their percentage of household energy used to heat water follows the typical pattern (about 14% of the total) they might be emitting another ton and a half there, just in the winter. They run their electric stove and/or oven at least two hours a day, which means over half a ton from cooking, for a total of at least three tons.
We burn about a cord and a half of wood per winter, which could be about four tons, given that wood cook stoves aren’t the most efficient. This also does all of our winter cooking and much of our winter hot water, in addition to heating. We’re less efficient in terms of carbon emissions, but maybe not by too much. It’s worth it to me, for the peace of mind.
How do you heat? If you have a moment, I recommend you do your own little experiment and check out your carbon emissions by using your electric bill. Compare the difference between, say, October and February if you live in the south, or maybe June and January if you live in the north. See the average difference in kWh/day, then multiply that by heating days, and multiply again by 1.45 lbs/kWh. That’s the average for a unit of electricity from the U.S. grid Mike Berners-Lee gives in How Bad Are Bananas? The Carbon Footprint of Everything. When you do your math remember that’s 2000 lbs to the ton, not 1000.)
Or take the number of cords of hardwood you burn, multiplied by 2.5 tons/cord (if somebody has a really awesome source that supports or contradicts that number, please send it to me). How do your heating carbon emissions look? Is your heat source more or less resilient in case of a power outage, water outage or other likely emergency? What are the other drawbacks and advantages? Tell us below.