Image Credit: Tulikivi
More Energy Solutions
Over the past two weeks I’ve written about wood stoves and pellet heating. This week I’ll focus on another way to burn wood cleanly and efficiently: using a masonry heater.
A masonry heater, also called a masonry stove or Russian fireplace, is a wood-fired heating system that is fired intermittently at very high temperature to heat up the large quantity of thermal mass, which then radiates heat into the home. The heater has a circuitous path through which the flue gasses flow. Here, the heat is transferred to the stone, brick or other masonry elements of the heater.
Key benefits of masonry heaters
From an environmental standpoint, masonry heaters burn fuel very rapidly at a high temperature. This results in very complete combustion with little pollution generated. Except when first starting the fire, there should be no visible smoke.
From a performance and comfort standpoint, masonry heaters take a long time to heat up, but they then continue radiating heat for a very long period of time, typically 18 to 24 hours. The outer surface of a masonry stove never gets as hot as a cast-iron or steel wood stove, but it retains its heat much longer. The surface area provides a large radiant surface, contributing to comfort.
Operation of masonry heaters
Unlike a wood stove, where you typically start a fire and then keep it going for a long period of time by adding fuel, with a masonry heater you operate it in batches, and the fuel is typically entirely burned by the time the next fire is started. This means that you have to start a lot of fires — which some people will find less convenient.
Because the firebox may not be very large in a masonry heater and because a fast-burning, intense fire is desired, the firewood is cut and split differently. Often the length of acceptable firewood is less than with a wood stove (sometimes as short as 12 inches), and the optimal diameter of split wood is smaller — typically 3 to 5 inches.
Because the heat from a masonry heater won’t warm up a space quickly (it may take several hours for the outer surface to reach peak temperature and peak heat delivery), it isn’t as effective as a wood stove at quickly taking the chill off. You need to plan ahead. And if it’s going to be a sunny autumn day and you have a lot of south-facing windows, starting the masonry heater in the morning may result in a period of overheating later in the day when the solar gain peaks.
Some masonry heaters include bake ovens or warming areas built into the modules, offering a nice feature for those interested in wood-fired baking. Others include integral benches for seating.
Product options
Masonry heaters are often custom-built, and such units can satisfy a wide range of design needs and special requirements. Because they are large and heavy, provision must be made for such units — such as a concrete slab or concrete bearing walls beneath the heater. The Masonry Heater Association of North America is an excellent resource on masonry heaters and includes a directory of masonry heater builders.
There are also some manufacturers of modular masonry heaters that can be assembled relatively easily. The best-known manufacturer is the Finnish company Tulikivi. Tulikivi heaters are made from soapstone or ceramic and are available in a wide variety of styles, both with and without bake ovens. Some include integral bench seats.
If a house has the space for it, a masonry heater is often the best way to heat with wood. In new construction, particularly in rural areas, it’s definitely worth looking into.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. He also recently created the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
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28 Comments
Wood Heat
I recommended a Temp-Cast to a friend who was building a house SW of Denver a few years ago. I also made some insulation upgrade recomendations and to bring in natural gas from the road. I hope he has since started using his wood fireplace because he has lots of standing dead pine on his property due to pine beetle.
http://www.tempcast.com
More efficient maybe, but "... the best way to heat with wood"??
Since the heat load at 5AM can easily be 5x that of the heat load at 2PM the concept of a large radiating thermal mass isn't very flexible, and will overheat for some parts of the day unless it's under-heating at some other part, even if you're slavishing managing that mass by trying to time the burns.
There's a certain elegant simplicity to the large masonry heaters, but it's something of a throwback. A much smaller mid-mass soapstone or ceramic woodstove with a much more modest thermal mass is probably a better choice for most than the giant grand masonry heaters that radiate for 18-20 hours and take hours to come up to temp. And some of those smaller beasts are north of 80% efficiency at mid-fire. Something that reaches it's peak output in an hour or so that can burn all night at low fire is a much easier mass to manage.
Thermal Performance
I too like the look and feel of the massive masonry stoves, but I am curious about their burn characteristics. On one hand, I can see how a full draft fire intended to throw out as much heat as possible in the shortest time would be a relatively hot fire that would burn up all the volatiles that, if unburned, would be emitted as smoke. And a fire of that nature would be logical if you are using it to charge a thermal flywheel. But wouldn’t that slow process of charging the thermal mass suck heat out of the fire, preventing it from reaching its hottest potential?
The concept reminds me of those outdoor wood boilers, and their infamous reputation for making a lot of smoke when building up the fire. Their problem is that the heat exchanger, circulating loop, and water they contain constitutes a sizeable thermal mass, so it takes a long time to raise its temperature after firing. Until the heat exchanger reaches full temperature, its relatively cool surfaces quench the flames and let the volatiles exit unburned as smoke.
Ron, they burn like a jet
Ron, they burn like a jet engine fired up. Alex's, blog is right on. Dana mentions a good point too.
Another masonry heater manufacturer
EcoFirebox is another source for masonry heaters. http://ecofirebox.org/ American made, based in Maine, they use a precast modular approach that allows them to easily configure different sizes and fit different spaces. It also allows smaller heaters than the massive traditional russian heaters, and brings this into reach of smaller, lower budget projects. I learned about this from a client in Maine who is installing one in their own house. http://www.flickr.com/photos/lamidesign/8156477664/in/set-72157629895832660/
How Heaters Work
Dana there is an elegant simplicity to masonry heaters and it carries through in the concerns you bring up. They do not require a lot of management because they tend to self regulate by reducing their heat out put if the home is warm and increasing output as the house cools. basic physics of heat transfer come into play here. That is not to say its impossible to overcharge a heater and be forced to crack a window but this is less likely with a Masonry Heater then with a iron or steel stove or even with a small soapstone parlor stove.
Ron AJ's comment is typical, the refractory design of the firebox brings combustion temps up around 2000 F which is how the low emissions are achieved, then the heat is captured and cooled in the long flues usually entering the chimney at about 300 F The HeatKit Contraflow heater we typically build has tested emissions as follows PM .68 g/kg CO 18g/kg For comparison EPA's averages are PM 7.3 g/kg CO 70 g/kg for Phase II Wood Stoves and PM 2.1 g/kg CO 20 g/kg for Phase II Pellet Stoves
Rocket Stove
That's an interesting looking company and product Gregory, thanks for sharing.
Here's another take on the traditional masonry heater--a quick-heating Rocket Stove: http://www.richsoil.com/rocket-stove-mass-heater.jsp
Masonry Heaters akin to Ground Source Heat Pumps
Having built (or maybe I should say assembled) and lived with a masonry heater, as well as most other types of wood stoves, I thought I would share a few comments:
Although they may not be the best way to heat with wood---that statement is probably too broad---they certainly represent one of the most efficient and enjoyable methods. If you like to set in front of a wood stove come evening, there is nothing like a roaring fire from a masonry heater.
Regarding efficiency, they are in the top tier.
Although you have to start a fire everytime you use one, they do not require the constant tending of a typical wood stove. Start the fire and it's out in ~ 3 hours. Seal the chimney. You are done. More enjoyment, less fuss. (Note: A small propane torch can really minimize the difficulty of starting a fire, and eliminate using paper or other fire starting materials besides kindling.)
Output from a masonry heater peaks near the end of the firing process, then it gradually cools for the next 12 - 36 hours---depending on the size and mass of the heater, as well as the amount of fuel consumed during the firing AND the heat loss rate of the home's thermal enclosure. If fired in the evening, a masonry heater's peak output generally matches a house's peak heat load. So although they are not quick or responsive, they can be operated in a way that provides a good foil for passive solar designs.
I didn't see any mention of fire safety. Because masonry heaters burn at full throttle, there is very little chance for creosote to form. So the risk of a chimney fire is very low with masonry heaters. That is not the case with your average wood stove, where many people throttle/dampen the fire to minimize the output, thus producing a smoldering fire that increases the risk of creosote build-up. Whatever the case, if you burn wood, make sure to inspect / clean your chimney at the start of the heating season!!!
There is no comparison between a masonry heater and an outdoor wood furnace/boiler. The former is a clean burning unit that operates >80% efficiency, the latter is a constantly smoking beast that might reach 30% efficiency (and that's before you figure in the electricity required to operate the pump and blower(s)). Average size masonry heaters---like a Temp-Cast or Tulikivi---do not completely cool before the next firing. If fired once a day, the refractory/brick core will still be several hundred degrees above ambient when you next load and fire the unit. There is not a condensing surface.
Masonry heaters have a minimum and maximum fuel load. They operate most efficiently with a larger fuel load, fired at least once per day. For a heater the size of a Temp-Cast, that's 40 - 50 lb. of wood . . . something in the range of 250,000 - 300,000 Btu / day. Double that if you fire the unit twice a day.
But looking past all that, do they make sense? Codes in many localities disallow wood stoves for the primary heating system---so the cost of a wood stove is above and beyond what's required. Masonry heaters typically cost > $10,000, possibly a lot more, depending on their size, complexity, and what material is used for the masonry veneer. At that cost, would it be better to invest that money in a better thermal enclosure and a higher efficiency primary heat system?
Bottome line: Like ground source heat pumps, masonry heaters are specialized pieces of equipment, with a high capital cost and low(er) operating cost.
strange ideas about convenience
I found this paragraph curious:
"Unlike a wood stove, where you typically start a fire and then keep it going for a long period of time by adding fuel, with a masonry heater you operate it in batches, and the fuel is typically entirely burned by the time the next fire is started. This means that you have to start a lot of fires — which some people will find less convenient."
It would be a rare person who would find loading a stove once in twenty-four hours to be less convenient than adding wood at irregular intervals throughout the day and night. And I know plenty of people who are used to getting up one or more times each night to add wood to their stove, just to keep the house comfortable. Of course, the house is the big problem in that case. But those who don't add fuel to their stoves during the night probably have to start a new fire every morning, so they are starting at least as many fires as the people who own masonry stoves.
A person having no experience with a masonry stove, and little or none with a more typical wood stove, might _imagine_ that the former would be less convenient. But the reality runs entirely the other direction. The other concerns mentioned in this article and in the comments, are worthy of consideration, but the supposed inconvenience of a masonry stove makes no sense to me. In the end, people get used to anything and everything.
Response to Derek Roff
Derek,
I've been heating my house with a wood stove for 37 years, and I strongly disagree with you.
First of all, I don't wake up in the middle of the night to add fuel to my stove. Nor do any of my friends who heat their homes with wood. Here's the routine: you put some wood in the stove before you go to bed, and you adjust the air intake and the damper. In the morning, you wake up and open the stove door. There are red coals in the stove. You put a couple of big chunks of wood on the fire, and that's it.
There is certainly no need to start a fire from scratch with paper, kindling, and matches. That's what people with a masonry heater have to do.
Here in Vermont, most people I know begin the winter season with a stack of firewood to get them through the winter -- either in a wood shed or in an outdoor pile covered with metal roofing. They don't begin their winter with big bins of kindling. That's an extra task required of people who own a masonry heater.
In January, when the ground is covered with 2 feet of snow, who wants to go outdoors and look for some dry kindling? What a nightmare. No one ever has enough kindling. They end up using egg cartons or cedar shingles that would be better put to other uses.
I much prefer lighting my wood stove just once, in October. I certainly would find it depressing to have to rustle up kindling and start a fire from scratch once or twice a day.
Thanks for correcting me
Thank you, Martin, for showing me the errors in my posting. I guess I, and my friends, need to get better wood stoves (or maybe I just need to get better friends). The wood stove that I use in the workshop will definitely NOT maintain red coals overnight. It's barely warm in the morning. I'm glad to hear that most people you know have stoves that will. That would make a big difference. [And since the Internet is filled with gripes and sarcasm, let me emphasize that none of these words are intended to have inverse or satiric meanings, except the comment about my friends.]
Response to Derek Roff
Derek Roff,
If the wood stove in your workshop has no coals in the morning, and you wish it did, here's what you can do:
1. Make sure that the air intake is shut to its lowest position when you tuck in the stove at night.
2. If so much air is still rushing through your stove that all the fuel is burned up in the middle of the night, that means the stove is getting too much air. The next step is to install a damper in the stovepipe, and damp it shut before you leave the stove in the evening.
3. If one stovepipe damper doesn't work, install a second stovepipe damper.
4. Needless to say, these methods increase creosote consumption and don't result in a very clean burn. If that bothers you, go to the stove store and buy an EPA-approved wood stove.
Indeed, pollution is my main concern
Thanks again, Martin. Your point number 4 is my main concern. Shutting down the intake air increases dramatically pollution from my stove in easily visible and smellable ways. I suspect that the current generation of stoves is a lot better. I will see if I can find one that fits my requirements and tolerance for pollution when the air intake is restricted.
Tulikivi
Twenty-one years ago I designed and built our superinsulated house around a Tulikivi soapstone masonry heater. We chose the next-to-smallest model for our single-story 1,900 square foot house. We have an open floorplan and an ERV that supplies air to some rooms and exhausts air from the other rooms, distributing the Tulikivi's heat without an AHU.
Before breakfast I start a fire so we can see and hear the fire going while we eat. After breakfast I close up the Tulikivi and the flue. The soapstone gives off heat all day while we are at work...with no fire going in our absence.
To build a fire I wad up a few newspaper pages and line the grate of the firebox with them. I lay the kindling on top of the newspapers. I do not lay the firewood on top of the kindling because the kindling IS the firewood, eleven inches long by 2-in. thick. I light a match, touch it to a wad of newsprint, and wait ten seconds. Then I close the glass doors and hear the "whoomp" as the fire takes off like a jet. The Tulikivi draws like no other stove or fireplace I've used.
When I come home I put my hand on the soapstone. This tells me whether or not I need to build a fire in the evening or wait until morning. If I'm cold coming in from outdoors I'll hug the stove for a few minutes. Try that with your metal stove...or your heat pump.
stoves
Until the advent of “The internet” the word stove meant for me only what you know as The Russian Stove, or Terracotta stove.
I grew with such “stoves”, my grandfather’s house has two. My great-grandfather’s has three.
Each village used to have at least one stove-maker with at least one apprentice. Each stove is a piece of art and each stove is unique as I don’t recall seeing two identical ones. I have seen stoves made from all sort of materials from expensive terracotta to plain mud and sun-dried bricks. They all burn fine, little ash, really heat up the place and all this with no smoke and near to no-maintenance. Most can burn all kinds of wood or pellets. They can be converted to gas in no time. There are some issues with softwoods, saw dust and other wood shavings on regular stoves as they clog pretty fast. Also resinous woods burn too hot and will damage the stove. I have a distant uncle that had himself made a custom stove so he was able to burn saw dust. Sawdust seems to be plentiful and free when you own a sawmill.
I spent a lot of time at my grandparents and I got used with making the fire daily. I realized how convenient and hassle free the terracotta stoves are only when I have had the chance to fire up a 15th century open fireplace. It was moody; if a certain door was opened the draft brought smoke indoor. It kept you warm only as long as the fire was on and was a real fuel guzzler.
Take one of our neighbors example, small house no more than 50sqm with bedroom, living, kitchen and bathroom, two layers of brick, a decent stove, no more than 300$ worth of wood and one can spend the heaviest winter staying only inside and doing nothing but eating, drinking, and shitting thy pants. He used to go out to the woodshed every other day – only if he feet like. Too bad a good stove can’t help with the liver cirrhosis :)
Pyrolysis furnaces are also great, I don’t know exactly the differences in exhaust fumes and efficiency. I was told that they are cleaner and more efficient. I saw that they produce way less ash, however they do need electricity to work. If electricity is readily available you could go for the modern solution only because of the efficiency. Nothing beats the warmth made by a terracotta stove. It is the kind of warmth that you feel penetrating through the clothes; you can feel it as it warms you from the outside to the core, not to mention that the stove looks great.
Thanks for all the info, folks!
I'm building a small home (900 sq ft) in Colorado. I'm interested in a masonry stove but not keen on buying a Tulikivi or similar, as the idea is ancient and beautiful and I'd like to preserve the traditions. I want to avoid the mass production, if possible... Are there other small companies, or detailed instructions on building one myself? Other, alternative ideas? Any info I can get would be greatly appreciated.
Thanks!
At least consider a wood boiler with water storage. Clean, fast burn and excellent control of interior temperature. About 1/2 the space for the same thermal storage.
Along the same lines, I'll need to be able to keep my house at a minimum temp if I go out of town in the winter. Trying to put all the pieces together!
Colorado is a big place with a lot of climate variation, covering US climate zones 4B through 7B.
What is your 99% outside design temperature? What is your design heat load at that temperature at an indoor temp of 68-70F?
A Fujitsu 12RLS2H cold-climate 1 ton ductless mini-split heat pump wall-mounted above the historical snow drift line can deliver 11,500 BTU/hr @ 70F indoors, -15F outdoors, and even more if it's 50F indoors. The 3/4 ton 9RLS3H is good for 11K @ -15F out/70F in. This series has a "minimum heat" operation mode that keeps it at 50F indoors, which is below the minimum normal heating mode setpoint. Unlike comparable Mitsubishi units, they will not shut down no matter how cold it gets. (Mitsubishi units can turn off at temps as warm as -18F, and don't automatically restart until it's a balmy -13F outdoors.)
A tight, IRC 2018 code-minimum 900' home would have a heat load of about 10-11,000 BTU/hr at an indoor to outdoor temperature difference of 70F with the ventilation turned off, so with either of those mini-splits it probably wouldn't lose ground on 50F until it's cooler than -20F outside, and it wouldn't hit the freeze point indoors until/unless hell freezes over.
There are lots of online resources for DIY high mass "rocket stoves" and multi bell chamber aka "Russian Stoves". Start googling!
https://richsoil.com/rocket-stove-mass-heater.jsp
http://blog.dragonheaters.com/wood-heat-storage-flues-vs-bells/
http://www.stovemaster.com/html_en/articles_3.html
https://www.pinterest.com/pin/465418942716692461/
Thanks so much for your response, Dana. I will look into it all!
I am unsure of the outdoor design temperatures. I found a couple of possible answers but nothing in the EXACT area where I'm building. I'll attach what I found here. My county is in a hardiness zone 5 . I bet if I call the county they would help me.
If I have an insulated concrete foundation, and exposed concrete floors inside (this is my preference), would the heat from the masonry fireplace keep my floors fairly warm or would I want radiant heated floors or to put wood floors in? My feet are always cold...
Maybe also consider rocket mass heaters for clean burns, super efficiency, minimal input (materials and labor) and fantastic heating: https://freeheat.info has some good info/movie/plans.
AngusRocket,
Impossible to know whether they are a good option or not when the proponents make such absurd claims about their efficiency.
Impossible? It seems you could look into it an make an assessment of your own.
They may be considered, in many cases (and codes), modified masonry stoves.
platinumtweed,
I'm not saying they aren't a good idea - in fact I find them really intriguing. My point is that like a lot of other innovative building equipment or materials they would probably gain wider acceptance if their advocates didn't make unreasonable claims for them.
The front page of the site AngusRocket linked to claimed:
- They produce 2% of the C02 of electric heat. A completely meaningless statistic, which ignores what the source of the electricity is.
- They use 1/10 the wood of other wood heating methods. Even if you agree that a rocket heater was 100% efficient, wood stoves are in the 70% range. Where do you get 10 times the heat from a 30% difference in efficiency?
- They reduce your heating bill by 98%. Another hyperbolic claim with no comparator.
Looking further into these concerns, the "2% of CO2" claim is based on average energy product CO2 emissions for the entire energy sector.
Regarding the efficacy of a potential tenfold increase in the efficiency from a 70% efficiency stove,
from https://richsoil.com/wood-heat.jsp#faq:
"First, "75% efficient" isn't really 75% efficient. 16% is allowed for heat going up the chimney, so it is actually 59% efficient. And that was from the best burn in a lab under optimal conditions. An excellent operator might be able to get about 35% efficiency. Most people run their "75% efficient wood stove" at 3% to 15% efficiency.
"We get the "one tenth" number from people that have replaced conventional wood stoves with a rocket mass heater reporting that they now heat their homes with one tenth the wood."
Seems fair - a bit unclear, but fair.
Regarding heating bill, I'll give you that one. Yeah, a claim to reduce your heating bill 98%is hard to validate.
Hi Malcolm, there are lots of places using them - some of which offer open days/tours so perhaps a good idea to visit one independently and see if the claims are absurd or not? I can appreciate it seems incredible but they really are worth investigating and can be a fantastic option. Thanks for the comment :)
AngusRocket,
They seem very useful, and their effect on fuel use and emissions in third world countries for cooking has been transformative. One of my fishing buddies and I are thinking of building a rocket cooker at his cabin to boil broths and cook crabs, which will free us from dependence on propane. I just wish when new ideas come up they get looked at with clear eyes. When proponents make wild claims it makes it too easy to dismiss them altogether.
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