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Community and Q&A

Masonry Heaters in Superinsulated House

John Hess | Posted in General Questions on

I am considering a masonry heater for a small superinsulated house that I am planning to build. On the Masonry Heater Association website I see that they offer plans for seven different heaters. http://mha-net.org/html/bookstore.htm I’m wondering if any forum members have had experience either building or using them, in particular their 24”x36” Finnish Contraflow? How large of a building can this model heat, in a cold New England climate? Are the plans complete enough for an owner-builder to make a serviceable stove? What would the materials cost to make one?

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Replies

  1. Riversong | | #1

    My old friend was one of the first to build masonry heaters in the US back in the 70's. If you are not an experienced mason, I would not recommend doing this yourself. You can purchase masonry heaters or have one custom built (don't forget a foundation).

    Tulikivi has many models, and Vermont Stove Company makes wonderful soapstone woodstove heaters and ovens.

    You can also buy an efficient woodstove and surround it with masonry for thermal mass.

  2. Lucas Durand - 7A | | #2

    NOt being able to afford a custom (or pre-fab for that matter) masonry heater, I have settled on a soapstone woodstove. Basically it's what Robert described except the thermal mass is built into the stove. There is a local dealer in woodburners that sells "Hearthstone" soapstone stoves, so I may get one of those.
    One thing to keep in mind about a masonry heater or high-mass aritight is that they take a while to warm up before they throw any heat - unlike a cast iron/steel airtight.

  3. Riversong | | #3

    Soapstone is the most amazing material for a masonry heater.

    Compared to concrete, it has 1.5 times the thermal conductivity, 3.2 times the volumetric heat capacity, 2 times the thermal diffusivity, 2.2 times the thermal effusivity (ability to give off heat), and almost 5 times the thermal mass index (3 times that of marble and 4 times that of granite). And it's a very beautiful stone.

    This is the Vermont Bun Baker, about 350 pounds of cast iron and 300 or 1100 pounds of soapstone.

  4. Tibor Breuer | | #4

    I have lived with masonry heaters in the 2 last homes I've built .It's the only way to go if you want wood heat in a conventional well built green home.But now that I am starting to build double wall I think a stove like mine would be overkillI'm actually having conversations to design a much smaller unit to use in a superinsulated home. The question for you,John, is how super insulated are you? If your answer is highly insulated than an efficient small woodstove would do the job a little fire ,twice a day and wouldn't cost that much. My Temp-Cast heater with an oven came from Toronto,Ontario and my mason put it together and faced it with my chioce of brick.All told with the stack $11,000. The mason was $3,000 of it and my delivery to Washington state was $1,000. It's better in some ways than my first but lacks a little bit too. Temp-cast is a contra flow and I highly recommend it for anyone that is not in a double wall home.It's like legos; 33 pieces of t+g cast refractory that took my mason 3 hours to put together once he was up to floor level with the footings on my pre-set foundation.Plans are easy to follow.

  5. Lucas Durand - 7A | | #5

    Ah, the Vermont Bun-Baker... I love that thing. Too bad it's all the way down in Vermont.
    Hey Robert, if you're ever headed out my way, throw one in the back of your truck for me please.
    C.O.D. ;-)

  6. Riversong | | #6

    Mark Twain:

    To the uninstructed stranger it promises nothing; but he will soon find that it is a masterly performer, for all that. It has a little bit of a door which you couldn't get your head in - a door which seems foolishly out of proportion to the rest of the edifice; yet the door is right, for it is not necessary that bulky fuel shall enter it. Small-sized fuel is used, and marvelously little of that. The door opens into a tiny cavern which would not hold more fuel than a baby could fetch in its arms. The process of firing is quick and simple. At half past seven on a cold morning the servant brings a small basketful of slender pine sticks - say a modified armful - and puts half of these in, lights them with a match, and closes the door. They burn out in ten or twelve minutes. He then puts in the rest and locks the door, and carries off the key. The work is done. He will not come again until next morning.

    All day long and until past midnight all parts of the room will be delightfully warm and comfortable, and there will be no headaches and no sense of closeness or oppression. In an American room, whether heated by steam, hot water, or open fires, the neighborhood of the register or the fireplace is warmest - the heat is not equally diffused throughout the room; but in a German room one is comfortable in one part of it as in another. Nothing is gained or lost by being near the stove. Its surface is not hot; you can put your hand on it anywhere and not get burnt.

    Consider these things. One firing is enough for the day; the cost is next to nothing; the heat produced is the same all day, instead of too hot and too cold by turns; one may absorb himself in his business in peace; he does not need to feel any anxieties of solicitudes about the fire; his whole day is a realized dream of bodily comfort.

    America could adopt this stove, but does America do it? The American wood stove, of whatsoever breed, it is a terror. There can be no tranquility of mind where it is. It requires more attention than a baby. It has to be fed every little while, it has to be watched all the time; and for all reward you are roasted half your time and frozen the other half. It warms no part of the room but its own part; it breeds headaches and suffocation, and makes one's skin feel dry and feverish; and when your wood bill comes in you think you have been supporting a volcano.

  7. John Hess | | #7

    These prefabricated stoves are lovely, but outside my budget. My house will be less than 1,000 sqft, and will be double-wall, so very well insulated.

    I thinking that these plans sold by MHA might be for heaters built of refractory brick and solid common brick, with perhaps a manufactured door. I was hoping someone had built from these plans and knew the level of difficulty for such a project, the cost of materials, and what size house they might be appropriate for (or the BTU output per burn).

  8. Riversong | | #8

    John,

    What's the design heat load for your house?

    In a 1000SF superinsulated house, a massive masonry heater is overkill. The BTU output is determined by the quality and moisture content of your wood, the efficiency of the combustion and the size of the firebox.

    In a very efficient stove, you might get 4-5,000 BTU per pound of dry wood. The delivery of that heat (btu/hr) will depend on burn time and heat transfer time in the mass.

    I would suggest a small, efficient, sealed combustion woodstove, perhaps with a masonry surround - or, if you want to cook in it as well, something like the Vermont Bun Baker (which is available with or without the soapstone surround).

  9. John Hess | | #9

    Robert, I haven't yet learned how to calculate heat loads. Perhaps you could give me a rough estimation for a 24'x38' house with FPSF, double-walled, and double-pane windows, on a somewhat shaded site, in a cold, northern climate.

    You've recommended a small sealed-combustion woodstove because you think a masonry heater would be overkill. Are you suggesting that small masonry heaters won't work properly? Is there a certain minimum size they must be before they work well?

  10. Daniel Ernst | | #10

    John,

    I built a Temp-Cast masonry woodstove, used it for five years. I've wrestled with the same questions. Based on my experience, I'll provide some input:

    * Don't try to build a custom unit out of firebrick, unless you have A LOT of masonry experience, have worked with refractory mortars, and fully understand masonry heater design. You can build a pre-cast or kit unit if you have some experience with masonry, but you'll first need to do a lot of studying and learning. The refractory kit cores are fairly easy to construct (paint by numbers). Building the chimney and transition to the chimney, as well as the brick or stone facing, this requires a higher level of skill.

    * It's been a few years since I've looked at this, but you'll probably spend ~ $5,000 for a refractory core kit, then you'll need the facing, which will add ~$1,000 (a lot less or a lot more, depending on the material). The firebrick kits are cheaper, less than half of the refractory kits, as they use only a limited number of refractory pieces. They send you the throat / lintel / gaskets / door, and you buy the firebrick from a local supply yard. This lower material cost is offset with increased labor / construction time. But firebrick isn't cheap either. Call around to find a supplier. I'd guess that you'll spend $1,000 just on firebrick. Anyway you dice it, you're still looking at $2,500 - $3,000 for even the cheapest variation, and you would spend a lot of time getting to that cost level. Remember that doesn't include the chimney, which can be substantial $$.

    * If you asked a masonry heater builder to do the work, you're budget would probably start at $10,000 and go up from there, depending on facade details.

    * Masonry heaters are designed to be used 1X - 2X per day. They burn very quickly. They need to be constructed with enough mass to absorb the heat of the fire, then release that heat throughout the next 24 hours. There's no such thing as a tiny masonry heater (i.e. contraflow model).

    * I'll use the Temp-Cast model as an example. It is fairly average in size as far as masonry heaters go; it has a basic footprint close to 24" x 36", same as what you're investigating. It's faced with 4" of stone, brick, or block. And it needs to be fired with 40 - 50 lb. of wood in order to operate in an efficient manner. Once you fire the unit in late fall, you generally keep firing it on a daily basis.

    * All wood species are capable of producing 8,660 Btu / lb. But that's only the theoretical energy---at 0% moisture content. Well seasoned firewood still has ~20% moisture content, so 20 - 25% of the wood's stored energy is used to drive off this moisture. This happens during the early stages of the fire, which is the also the "dirtiest" phase. So now you're down to 2X what you really need. So to answer your question, no, I don't think this is the right technology.

    If you haven't completed a heat-loss calculation on your structure, you ought to do that before determining your heater type / size.

    Another point is the high cost of a masonry heater. Why would you work hard to get your heating requirements down to a low level, then install a very expensive heating system? If a prefab soapstone heater (like the Hearthstone model) is outside of your budget, then so is a masonry heater.

    I hope this was helpful to you. Good luck!

  11. Daniel Ernst | | #11

    John - We must have posted at the same time. I didn't see your last post.

    If you don't know how to complete a rough calculation on heat loss, then you would need to read and study this subject quite a bit to get comfortable with the process. There are some simplified spreadsheets out there, but you still need to understand the inputs.

    Or you could hire someone. They'll need to know more detail than you provided, like the wall and roof insulation values / thicknesses, window specs (U-Value, SHGC), window size and orientation, foundation details, estimated air leakage rate (ACH), etc, etc.

    So let's look at it from a different angle:

    There are 3,413 Btu / Kwh

    220,000 Btu = 64.5 Kwh

    Do you think that you would use 64.5 Kwh per day, even if you used baseboard (electrical resistance) heaters?

    Maybe this will give you a better perspective on the "overkill" comments above.

  12. Jamie Malcolm-Brown | | #12

    John,

    You can use the Home Heat Loss Calculator over at http://www.builditsolar.com to get a general idea what your heating requirements will be.

    My brother builds masonry heaters with Maine Wood Heat. They are beautiful and efficient but generally overkill for a super-insulated home. The biggest problem for a super-insulated house is that you can't turn off the masonry heater. Once it is has been fired you have to wait for all the stored heat to dissipate into the house (you don't want to have to open windows in the middle of winter to be comfortable). A small and efficient wood stove with an outside air source would be less expensive and give you more control for a small super-insulated house. Good luck with your decision.

  13. Riversong | | #13

    Jamie,

    Your comment reminded me of a guy I worked with down in rural Tennessee, who lived in a typical Appalachian shack, fired up his potbelly coal stove until it was glowing red and then regulated the house temperature by how much the front door was open.

  14. ROY HARMON | | #14

    Is it possible to build a very small masonry heater?

  15. Tibor Breuer | | #15

    That is what I am trying to work on,like I said in my previous post.I would like to try to mimick the old time Monarch wood fired cook stoves where you have the small firebox on the side and then pull a lever to make the fire go around the oven when you want to bake, before going up the stack.In my opinion,a small refractary unit used to heat an oven would be enough to heat a double wall home.The hot oven on my Temp-cast is a byproduct of heating my home but this idea I'm working on ,will be heating the home as a byproduct of firing an oven.

  16. Daniel Ernst | | #16

    Roy,

    Several European manufacturers build smaller units. They accomplish this by forming refractory pieces with very tight tolerances, and use these as both the "core" and the "facing" of the heater. Supposedly, you still get the long "smoke path" that increases the efficiency of thermal transfer. But I wonder about their durability . . . since they heat up and cool down before the next firing, which is not optimum for masonry, even if it is high-quality gasketed refractory masonry. Personally, I would be more comfortable with a gasketed cast iron stove, or one of the soapstone woodstoves mentioned above.

    Here's an example of a unit out of the Netherlands:

    http://en.tigchelkachels.nl/tigchelkachels

    Their smallest unit, the 4D model, is fired with a 12 - 13 lb. load. Note that you would have to use small diameter pieces, not your average American firewood. The 6D uses 15 - 16 lb. per firing.

    Of course, you'll pay for this kind of technology. Their smallest starts at 5,510 Euro (2009 pricelist), and they go up from there.

    Tom Trout (in North Carolina) sells Tigchelkachels on the US market. He also builds custom masonry heaters around our neck of the woods (VA), and is very involved in the Masonry Heater Association. If you're serious about investigating your options, he's an excellent resource. You can find his contact information here:

    http://mha-net.org/html/members.htm

  17. Daniel Ernst | | #17

    Here is a picture of the Tigchelkachel 4D unit:

  18. Riversong | | #18

    Is it possible to build a very small masonry heater?

  19. Riversong | | #19

    this idea I'm working on ,will be heating the home as a byproduct of firing an oven.

    Baking and space heating don't always coincide in time. And that's still a very expensive baking oven that requires a foundation and chimney.

    I would recommend getting either something like the Vermont Bun Baker, which does everything you want it to do, or consider getting a wood-fired cookstove which may be sufficient to heat your house as well as cook and bake (and possibly produce hot water).

  20. Lucas Durand - 7A | | #20

    Here's something else that might be of interest - for those with a sense of adventure. They're called "rocket mass heaters" and appear to be a very do-able home-made cob heater.
    Potentially they're quite "green" since the materials shouldn't be very difficult to source locally. Also they apparently function like a traditional masonry heater and are possibly as efficient as well.

    I say "possibly" because I have never seen or used one myself - just something I came across on a permaculture forum. But you can be sure the permaculture crowd wouldn't be interested if it wasn't a more sustainable way of burning wood.

    Here's a video link too:
    http://www.youtube.com/watch?v=9uh2VExcdbY&feature=player_detailpage

  21. John Hess | | #21

    Daniel, thank you for your clear description of costs and heat output.

    Jamie, thanks for the most helpful link to the Home Heat Loss Calculator.

    Using the calculator I come up with "Design Loss" of ~18,000 BTU/hr for my planned house. I assume this is only at the coldest temperatures that I did the calculations for (-10*F). Does this figure pass the "smell test" for a 900sqft double-walled house? That is twice the 9000 BTU/hr that Daniel mentioned that he estimated his Temp-Cast produced, with a 40-50lb charge of wood. So I'm not sure if my calculations are correct, or if I'm not correctly understanding how to use the heat output figure that Daniel supplied?

    The smallest Tigchelkachels takes a 12-13 lb charge. I haven't yet found on their website an estimate for BTU output for their heaters.

    Tibor, if you get more information on small masonry stoves I'd appreciate if you would post it on this forum.

  22. Riversong | | #22

    John,

    My last 12" thick walled house, with nearly 2000 SF of conditioned space (outside measurement), had a design heat load (at -10°F) of 18,700 BTU/hr unoccupied and 16,300 BTU/hr with internal gains for 3-person occupancy.

    So you should be about half that.

  23. John Hess | | #23

    Robert, I suspect you used a more accurate technique to measure your design heat load, but I would be curious if you were to run your house numbers through this heat loss calculator (http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm) to see if it comes up with a figure similar to those you derived?

    In running the calculator for my house design I wasn't sure what values to put in for a Shallow Frost-Protected Slab? They don't seem to care how much insulation you put under the slab (because most of the heat loss is through the edges), and they wanted to know the effective R value per foot of perimeter (http://www.builditsolar.com/References/Calculators/HeatLoss/SlabR.htm). I'm not sure what that means? Maybe they are not expecting the foundation/slab perimeter to be insulated? Would it make sense to plug in a value of "20", assuming two inches of rigid foam on either side of the foundation?

  24. Daniel Ernst | | #24

    John - That calculator is very simplified. Your airtightness assumptions can literally through your results to the wind (pun intended;)

    For reference, compare it to the PHPP spreadsheet. Click on the PHPP 2007_English-Demo.xls link):

    http://www.passivehouse.us/passiveHouse/DesignTools.html

    Also, the winter design temperature is very different from your average temperature. See this link:

    http://www.engineeringtoolbox.com/us-outdoor-design-temperature-humidity-d_296.html

    For January, it shows Burlington, Vermont having a -10 F design temperature, but an average temperature of 19 F. What happens to your 900 sq. ft. house when it's 35 F outside, the sun's a shining, and that masonry heater is still kicking out plenty of Btus? Do you open the windows?

    To figure out your woodstove's output, take the pounds of wood and multiply it by an estimated efficiency / output. A smaller stove (like the Tigchelkachel) will likely be less efficient than one of the larger masonry heaters, as the flue gases will reach a higher temperature, and you'll send more heat out of the chimney). So take Robert's 4K - 5K Btu / lb. figure and multiply it by how many pounds of wood you burn a day.

    4,000 Btu / lb. * 12 lb. = 48,000 Btu

    If you burn twice a day (once in the morning, once in the evening), then double that figure.

    IMHO, if you are going through the trouble of creating a 900 sq. ft. house, with double-stud walls, and you're going to insulate the ceiling very well, and focus on airtightness---then you want a heating system that can respond quickly, with very little lag (ramping up or down). You want to have the ability to respond to the weather. And you want something that has a small Btu output. A masonry heater doesn't suit this kind of building.

    I'm sure there are plenty of people in your area that could help you nail down a heat loss calculation, but it's a time consuming process to do it right. They'll need your floorplans and a lot of detail on your building assembly. Why not hire somebody to help you? It will probably keep you from making a more costly mistake. And you'll have some assurance in your decision making process.

    "An ounce of prevention is worth a pound of cure." Benjamin Franklin

  25. Riversong | | #25

    John,

    I entered my house into the BuildItSolar heat loss calculator and, though it used effective slab edge loss rather than separate slab bottom and edge loss the way I do, it came out almost identical: 18,473 BTU/hr design heat load (not counting internal gains).

    I found their effective slab edge table to be useless. A formula for calculating effective slab edge R-value per foot perimeter is:

    1.21 + (0.214 x R-value) + (0.0103 x R-value²)

    This is from the book Modern Hydronic Heating by John Siegenthaler.

  26. Amy Clark | | #26

    Hi John,

    We would be happy to discuss small masonry heaters with you. Our masons here at Maine Wood Heat Co. do have experience with the style and size heater you refer to. We actually have a design concept you may be interested in known as "Scott's Heater." It's a small pre-fabricated soapstone unit you can see here on our Web site:

    http://mainewoodheat.com/2008/08/masonry-heater-gallery/?album=6&gallery=73

    This may be a good size for a small super insulated home.

    I hope this information is helpful, John. Please contact us at (207) 696-5442 if you'd like to talk with us about your project.

    Warm Regards,

    Amy Clark

    Le Panyol
    Maine Wood Heat Company
    254 Father Rasle Rd.
    Norridgewock, Maine   04957
    Phone:  207-696-5442  
    Email: [email protected]  
    Web:  http://mainewoodheat.com

  27. John Hess | | #27

    Daniel, thanks for the links to the Passive House Planning spreadsheet and the Outdoor Temperature table. I lived for over twenty years in a house with large south-facing windows, and heated by Electric Thermal Storage heaters. They had some of the same problems that a masonry heater might have. It was a challenge to charge the heaters at night (during the less expensive electric rates) and not have them too hot during the sunny days, but still have enough heat in them until late the following cold night when they started charging again.

    Not having lived in a superinsulated house, or heated with a masonry heater, I’m having trouble understanding your or Robert Riversong’s concern with matching the two together – though I am listening carefully. I understand why a large masonry heater wouldn’t be a good match for a small house, but I don’t yet understand why a small masonry heater isn’t practical.

    The advantage of a masonry heater over ETS heaters, I hope, is that I could fire the masonry heater in the evening with a larger load of wood to heat throughout the cold night, and then fire with a smaller load in the morning, depending on daytime solar and temperature conditions. You mention that in a small well-insulated house that the heater should be able to respond quickly to temperature changes. But my assumption was that with a superinsulated house there would be less internal temperature fluctuations than in a conventional house, and so the even heat generation of a masonry stove would be ideal. Where is my thinking muddled?

  28. John Hess | | #28

    Robert, thanks for running your house’s numbers through the BuildItSolar heat loss calculator. I am somewhat reassured that I can use their less-sophisticated calculator with my house design and get a ball-park figure for its heating needs.

    Using Siegenthaler’s formula for Slab Edge R-value I get:

    *Slab Edge R-value = 1.21 + (0.214 x R-value) + (0.0103 x R-value²)
    *Insulation R-Value = 20 (a 2” layer of foam on each side of the foundation @ R=5/inch, neglecting R-value of concrete)
    *Edge Perimeter R-value = 1.21 + (0.214 x 20) + (0.0103 x 20²)
    *Edge Perimeter R-value = 9.61

    Interestingly, substituting this new value of 9.61 for the original value of 20 that I originally used results in virtually no difference in the final BTU/hr Design Loss?? In fact, any value greater than 4 changes the final BTU value by less than 10% or so. Does it make sense to you that slab edge insulation actually doesn’t make a whole lot of difference to the overall heat load? And why is the thickness of insulation under the slab ignored?

    What does make a huge difference in this calculator, as Daniel suggested, is the air infiltration rate. When I calculated a heat load of 18K BTU/hr for a 900sqft house, it was with an assumed infiltration rate of 0.5 air changes per hour. If I changed that assumption down to 0.33 ACH, then I got a heat load of 12K BTU/hr, which seems much more in line with your 18K BTU/hr value for a 2000sqft house.

  29. John Hess | | #29

    Amy,
    Thanks for bringing my attention to your line of masonry heaters. Would you mind answering questions on this thread? I’m sure others would be interested in your responses, as well as myself.

    Regarding your Scott’s Heater:
    * What are its height, breadth, depth, and weight?
    * What is the size of the firebox, and how many pounds of wood can it take?
    * What is the BTU/hr output from a full load of wood, how long does it take to reach that output from the initial firing, and for how long does that output last?
    * Does the smoke pipe exit only from the rear of the heater, or can it also exit from the top or side of the heater?
    * Is the Scott Heater a kit that a homeowner can assemble, or does a company representative need to assemble it?
    * What does the heater, or heater and builder, cost?
    * What are your delivery times?

  30. Riversong | | #30

    John,

    A small house doesn't need more than 0.25 ACH.

    The slab edge is where most of the heat loss occurs from a slab on grade, but there is very little surface area compared to walls, windows, and ceiling - so the overall contribution to the design heat load is minimal.

    I think Siegenthaller's formula ignores sub-slab insulation because it assumes that subslab ground temperature stabilizes over time at the approximate average between indoor and outdoor temperatures, and the total heat loss is proportional to the amount of edge (where the heat is moving toward).

    I use a simple UA (or A/R) at the slab edge and a UA for subslab heat loss which uses double the installed R-value (since the delta T downward will be half of that to the outside air).

    But I still think you're asking for trouble with a massive heater in such a small, well-insulated house.

  31. User avatar GBA Editor
    Martin Holladay | | #31

    John,
    Here's another vote for the proposition that a masonry heater is inappropriate for a small superinsulated house.

    Masonry heaters made a lot of sense in central Europe in the 19th century. They created a warm place for the family to congregate, a place to allow your yogurt to curdle, a place to let your bread dough rise, and even a shelf where Grandma could take a nap. But they were a response to leaky, poorly insulated houses.

    Get yourself a good little wood stove, and you'll be happier -- and you will have saved a lot of money.

  32. Tibor Breuer | | #32

    Martin, A masonry heater is an excellent way to heat any home OTHER than a double wall structure,not just a leaky house.Full combustion,one fire a day, virtually no smoke .I do agree that for John, a simple efficient small stove would easily do the trick.

  33. Riversong | | #33

    If the choice is between a high-efficiency central heating system and a masonry heater at about the same cost, a masonry heater might be a nice aesthetic core to the house. But in an extremely tight and well insulated house, it's like central heat without a thermostat. Once the heat is in the mass, there's no way to turn it off or down and the house can easily overheat on a sunny day.

    If the choice is between a high-quality woodstove for $1000 that can be regulated by the draft control and a masonry heater for $10,000-$15,000, then there's no question which makes more sense - and it ain't the mass heater.

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