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More Q&A Spotlight
Patricia Appelbaum is in the market for a new wood-burning stove, one without a catalytic element, to provide mostly supplemental heat for her 1,600-square-foot home. There are a lot of models to choose from, and that’s part of the problem.
“We understand that a non-cat stove needs a minimum temperature of around 500 degrees for an efficient secondary burn,” she says in a post in GreenBuildingAdvisor’s Q&A forum. “But we would like to have enough capacity to heat the whole house occasionally if necessary.”
Appelbaum’s family already has an older wood stove, a large model, that some people in the family like to run at a lower-than-optimum temperature. Before buying a new stove, she’d like to know whether having one with a smaller firebox would make any difference.
That’s the topic for this Q&A Spotlight.
A small, hot fire is best
A fire that burns quickly and intensely is the most efficient kind because it produces the most heat per unit of fuel, and also the least amount of air pollution, GBA senior editor Martin Holladay says. Hot fires also produce less creosote than low, smoldering fires that are starved for air.
“Going to a stove in the 50,000-55,000 Bth/h range can still work for a house with a ~25,000 Btu/h design heat load, but try not to go beyond that,” advises GBA reader Dana Dorsett. “Once the stove is up to temperature, it will still burn cleanly when throttled back to ~25% of its rated fire, which isn’t going to roast people out very quickly.”
The thermal mass of a stove becomes an important factor when the stove is used only intermittently but at high temperature, he adds. But there’s a caveat — when the output of the stove is…
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22 Comments
This conversation begs the
This conversation begs the comment that Masonry Heaters which are often dismissed on this site because of their upfront cost are technically well suited to small efficient homes High temp combustion with low emissions, thermal storage integrated into a central hearth and relatively low output radiant heat panels.
tending a stove overnight
Tending a stove overnight is not much of a problem if you drink 2 glasses of water before going to bed.
Rule of "Thumbish"
Is there no Manual J for sizing stoves? Do high performance structures not require massive under-sizing to what is available on the market? Martin's point is well taken about humans and feul size/type being the "weak link" in determining consistent output.
@ Robert
I agree, I think that is called the Di - Urination swing.
Response to Leo Kloop
Leo,
You don't need a special Manual J for homes heated with a wood stove. Manual J calculates the design heat loss for any home, regardless of how the home is heated.
For more information on the use of wood stoves in small, tight, well insulated homes, see All About Wood Stoves.
Response to Martin
Wow, thanks, great article!
Scott ... I am not seeking
Scott ... I am not seeking glory but I was the one who first mentioned woodheat.org on that conversation.
No nighttime feeding
A modern properly insulated house should be able to go through 8 or 10 hours of the night without a heat source. A modern properly designed wood stove should never be damped down to smolder through the night...that "air tight" phase of stove design is gone. A few stoves with catalytic heater can be damped down because of high burn temperatures in the catalytic chimney, but it is unnecessary and there are still chimney fires being reported with them. By the way I heat a 10,000sq ft house exclusively with wood...no backup. Here are the lessons: 1. Season wood properly to 20% moisture or lower. Feed only 12 hours of the day...a pleasure, not a chore. If your house drops more than 5 degrees f overnight you need to insulate not burn more wood. It is safer not to have a live fire while you sleep. If you have back up furnace just set it to 63 degrees and it will get you through the last of the night in a moderately insulated house. If you have mass in your house a metal stove is best, the mass is the masonry heater, you don't need masonry unless you have a low mass house. Soapstone is ok in small spaces since it has a smoother curve of heat peak than a metal stove ( we use them in our tiny houses of 120 sq ft!) in large spaces their mass is insignifigant. If you are away during the day, wood heat is just a hobby for the weekend. If you work from home it saves good money and if you like exercise without paying for a gym membership then gather your own . One last point: dead trees left to rot release both carbon and methane into the atmosphere and contribute more to atmospheric carbon than if properly burned.
Is it heresy to suggest a
Is it heresy to suggest a gas-fueled stove as a backup heat source for a high performance house? Has the same advantages of a cosy point-source heater with high radiant factor, the convenience of a pellet stove without the supply issues, and the luxury of point and click ignition and near instant gratification. Natural gas would be the economical fuel of choice, but even a propane-fired stove would be a better bet than a backup generator beefy enough to run a heat pump. We've had several clients choose to do this and one tells me that they use the stove instead of the ducted air system in all but the coldest weather. As the house is compact and well-insulated and they seldom close the interior doors they find the minor temperature variations around the home quite acceptable.
Response to Ven Sonata
Ven,
You wrote, "A modern properly designed wood stove should never be damped down to smolder through the night."
I understand your position, and -- as long as you have enough kindling -- there's nothing wrong with starting a fire from scratch every day.
This morning it is -9°F, which isn't unusual. (After all, it's January.) I put wood in our stove before I went to bed last night, and when I woke up there were red coals in the stove. I find it much more convenient to put a log on the remaining hot coals than to kindle a fire from scratch.
Martin, re: kindling or smoldering...that is the question.
Yes, for many years I only thought in terms of the overnight fire and the joy of coals in the morning to start the new one. But for the last 3 years I have changed my opinion to just letting the fire go out around 9 pm without any damping down....cleaner chimney, higher burn efficiency. Kindling is split for a year ahead for two stoves and starter is a propane torch, no paper. Yes, kindling takes a bit of time, but is not as heavy as splitting firewood. Since we use substantial amounts we make a science of splitting. Use an old car tire on top of a big round of wood, place the small section to be split in there and tap down on a small kindling axe with a short heavy hammer. Now this saves body mechanics of swinging, the danger of glancing blows and the necessity of picking up stray kindling and flying pieces. The tire keeps the pieces from falling on the ground. In the end, no more volume of wood is used but more small splits are required.
let's discuss sealed air intake ...
I would love to hear the firebox pros on about that.
I can't imagine a rightly efficient sealed house, operating a stove which robs conditioned air in quantity.
Lastly, what do you think about euro products with claims such as these:
http://www.burley.co.uk/images/brochure_images/Burley_Fireball.pdf
Response to Jin Kazama
Jin,
I have no opinion on the claims made for the Burley Fireball stove manufactured in the U.K. Many North American manufacturers of EPA-approved stoves make similar claims.
Concerning ducted outdoor combustion air for wood stoves, the topic was fairly thoroughly discussed in this GBA article: How to Provide Makeup Air for a Wood Stove.
Wood stove suggestions
I have had 30 years of happy wood heating from a Vermont Castings "Resolute" stove and would recommend that stove to Ms. Applebaum (even if she had to find a used stove). My house is also about 1600 s.f (and well insulated). The stove is centrally located and adjacent to the main return duct for the house (my backup is an air-source heat pump). I can move the stove's heat around the house with the blower of the air handler.
The aspects of that stove that make it excell are that it is made of cast iron, has a thermostatic bi-metal-controlled air inlet which can be set to keep the fire burning at whatever level you desire. It also has an internal damper at the smoke outlet so that once the fire is established, you can close the internal damper and force the exhaust gasses to pass through circuitous passages in the stove's shell to give up more heat to the metal and raise the output efficiency. And, most remarkably, the internal castings are replaceable. After 25 years of use the back plate in the combustion chamber became cracked and deflected and I was able to obtain replacement castings from a Vermont hardware store to restore the stove to like-new condition - a remarkable feat in this age of "throw-it-away consumerism."
next generation wood stoves
If waiting is an option it sounds like there might be some great advances in woodstove technology in the coming years as experimental designs make their way into production and as EPA regs force industry to innovate.
The Alliance for Green Heat organized a couple of contests in 2013 (The Woodstove Decathlon) and 2014 (www.forgreenheat.org/stovedesign/workshop.html) that brought together experts from industry, academia, and government to address the question of how to build a better woodstove.
The testing results that came out of those meetings look promising--innovative designs with much lower emissions, longer burn times, sophisticated sensors and controls etc.
Unfortunately the woodstove industry as a whole has not embraced the idea of making a better product, but instead has mounted fierce resistance against EPA's effort to limit emissions from wood burning stoves and boilers. The arguments for regulation are very strong though: research has confirmed that particulate pollution causes a lot of harm to public health and that soot is a major climate change contributor.
So although I like my woodstove (a Harmon TL300) I look forward to being able to replace it with something better.
I would vote for a masonry
I would vote for a masonry heater on all counts but portability. The idea of mass immediately invokes images of a huge and imposing object, and that is certainly what you see pictured the most. In truth, the design and sizing of a masonry heater, if done correctly, is much more likely to result in a good fit for the use it will see than picking from a choice of stock sizes of metal stoves.Ven's soapstone stove is what I would consider a miniature masonry heater. Of course, all require some training in the process of combustion to be used efficiently.
I don't use kindling anymore. I keep a pot of dead coals handy, and lay a small pile of it down with a few twists of birch bark on top. The regular wood is laid on this.The whole arrangement is positioned so incoming air is funneled right to the coal. A single match sets it off. The birch bark, which burns very hotly, starts the coals which quickly get hot enough to get the wood going.
A new stovewould be more efficienct (response to Carl)
The first round of EPA emissions requirements were drafted in 1988, 27, not 30 years ago, so the upgrades happened AFTER your version of the Resolute. Pre-1988 stoves were rarely ran better than ~60% combustion efficiency even at full fire, and less than 50% at half the max firing rate. Most current EPA complaint stoves sold today test north of 75% efficiency, (many are north of 80%, some hit 90%) and lose very little when throttled back as long as they are brought up to a sufficient temp to light off the secondary burners (or catalytic converter.)
The current Resolute Acclaim tests at 75% efficiency (but at a respectably low 3.4 grams/hr of particulate emissions): http://literature.mhsc.com/vermont_castings/spec_sheets/VC_ResouteAcclaim_Spec0511.pdf
If you've been running it in an auto- damper mode most of the time, it's likely that you could cut wood consumption by a third or so (and the particulate emissions by 3/4) going with a newer stove. It's not clear if it's possible to retrofit yours to make it perform like the new ones- most non-catalytic EPA complaint stoves have internal manifolds pre-heating the secondary burner combustion air injected at the top of the firebox to light off the unburnt smoke & gases. In most cases accommodating those changes would require modification of the interior shape of at least some of the fundamental castings, even if the outside shape never changed. It's may be worth asking the manufacturer though, if you're up for a summertime stove-rebuilding project. The combustion air valves on new stoves never fully close (or have guaranteed minimum bypass air), which makes it impossible to fully starve the flame down to a complete smolder, which results in extreme particulate emissions as it dies out.
Having in the past lived with an early 1980s pretty-good Jøtul, then more recently a similarly sized post-2010 Hearthstone (that tests north of 80% efficiency), the difference in operation & efficiency is dead-obvious. That Jøtul is probably still in use (don't know the current owners of that place), but it WOULD be worth replacing rather than repairing at this point, just as a 1980s vintage cast iron boiler would be worth replacing with something more efficient, even if still repairable. The technology has improved that dramatically. (My in-laws have the exact model & similar vintage Jøtul in their circa 1980 house, but aren't using it as a major heat source, and have commented to us on how much burn time & heat we get out of our stove relative to theirs- it's not just a figment of my memory. If I had to guess I'd hazard it runs about 50-55% efficiency at full fire, not more.)
With the newer even more stringent proposed emissions standards it would be hard to meet spec with combustion efficiencies under 80% (or even 85%), but it's not clear how soon fully compliant models will be on the market, given the voiced opposition.
Old men, new tricks
Jin … a wood stove needs about 10 – 20 CFM of fresh air to burn right. That question has been discussed at GBA many times and the myths of the need for special make up air supplies (or risk compromising air quality) persist. In fact, it was that question that got me coming to GBA. I urge you to read this site:
http://www.woodheat.org/the-outdoor-air-myth-exposed.html
Even in an airtight house, 15 CFM is not a lot of air to use or pull in and in most houses' fugitive infiltration can easily do it without affecting conditioned space air quality or adding much cost (i.e. slight increase in negative pressure through inactive dryer vent, bathroom vent, kitchen exhaust, ERV/HRV vent, door and window frames gaps and weak weatherization). Stack effect, wind forces and pressure differentials have significantly greater affects than 15 CFM.
Code requires you to supply that volume of air per occupant so it should be getting indoors already in a planned and intentioned way (i.e. air intake has to happen somewhere). As Martin reminds us, few houses are without infiltration.
Dana … you're brave. Tellin' a man to change his stove and stove operating behavior is like tellin' a man to get a new dog or take an old dog to school.
:-)
Flitch:
I've read through the thread and some other readings ..
Seems to be that nobody has any real data on CFM of stoves, other than a few stoichiometric reaction maths .. i agree that 10-20cfm is not as bad as it seems.
( sure looks like a lot more than that when looking at chimneys around here!! )
But reading those introduced a little bit of fear into my thoughts ... my wife prepares meals at home for living, and we have quite powerful kitchen exhaust fans that she may turn on at maximum settings occasionally when she is cooking with oil or other high pollution stuff she uses ( asian food...don't ask :p )
Would that be a serious enough reason to justify a sealed air intake ? I am not really afraid of the blow-back heat to the intake side, as this can be easily dealt with using only metal components on the intake side and some internal metallic filters etc...
Had time to visit a large stove sales point today; expensive stuff...
All euro imports are greatly overpriced here in Canada, affordable modern looking wood stoves are inexistent ( they seem to add a 1000$ to any stove that looks modern or blocky with a large glass front ) and chimneys to code are more expensive than i thought.
Will have to give it some time and numbers before investing in a wood stove, as i do not believe i could get anything installed properly under 5000$CAD even with some of my elbow juice and this would push a ROI period quite a bit since i do not wish to heat solely with wood fire.
Old thread, but I've been researching this.... so leaving this here for anyone else in the future: CFM data on wood stoves.....
I've found several manufacturers post their EPA test results online. Often times, (I think they may need to for some calculations), the flue CFM is measured, or simply calculated from velocity. Of course, this may be different depending on the scenario of temp deltas, chimney heights, etc, but it does at least provide some data.
You may have to hunt for it, and read through the test results carefully, but often it's in there.
For instance, my Jotul F100 (a very small wood stove), shows a range of flue flows from about 5 to 10 CFM in the test burns. And they have a diagram showing the height of the chimney setup used in testing as well. (14-15 feet in this case, IIRC.)
-Nate
The rationale is not the heat loss, but the backdraft.
The sub-20cfm combustion air requirements doesn't impose a significant heat load, but that's not why outdoor combustion air needs to be ducted to the firebox of a wood stove in a tight house. If drawing combustion air from conditioned space, the tighter the house, the more likely it is that kitchen/bath/clothes dryer exhaust depressurization in combination with unfavorable wind can backdraft the wood stove into the indoor air. Sealed combustion appliances, including wood stoves with air-tight ducted supply air) simply can't backdraft combustion products into the conditioned space air, no matter how many ventilation exhaust fans or running, or the direction of the wind.
Flitch Plate: I'm not telling anybody how to run their wood stove, but I am saying that " "throw-it-away consumerism" notwithstanding, when there are significant technology shifts that improve the pollution & efficiency issues, it's worth considering retiring & replacing the equipment rather than replacing it when it fails. A 1952 Hudson can be repaired forever (as seen on the streets of Havana) and keeping it on the road is cost effective in the short term when it only needs regular maintenance type repairs. But when something major fails on it, fixing it to keep a 15mpg hydrocarbon & NOx puking nearly-brakeless unsafe beast like that on the road for another 30 years isn't the "right" solution. I liked the old Jøtul, it was great stove for it's time! But I like the new stove better (a LOT better). I can SMELL the difference just walking out the front door, and the large reduction in fuel consumption is a nice perk too.
pacific energy vista
We use the Pacific Energy vista which puts out up to 56,000 BTU/hour for our 1600 square foot house with R40 walls and R50 attic in coastal BC zone 4c. Even when its -10c this provides enough heat. In warmer weather we just let it burn for 2-4 hours or just put on a small piece now and then and the thermal mass in the slab and the insulation / sealing holds the heat to the next morning.
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