Are Masonry Heaters a Good Match for Superinsulated Houses?
Or would this high capacity heater be too much of a good thing?
In New York City, it's been considered a real coup to land an apartment with a fireplace. Now, according to The New York Times, those once lucky urban dwellers are having second thoughts. New concerns about the environmental and health hazards of wood smoke, an article this week said, are outweighing the charm of those cheery winter fires.
High-efficiency wood stoves do a much better job of providing heat, with less pollution, than an old-fashioned fireplace. Yet there's been a sharp drop in interest even for wood stoves. Citing industry sources, the newspaper reported the number of wood-burning appliances sold in the U.S. fell from 800,000 in 1999 to 235,000 in 2009.
These facts seem a little odd at a time when energy costs continue to rise, and in truth concerns about wood burning may not translate to rural areas where local sources of hardwood are abundant. Wood heat remains an attractive alternative to grid electricity and fossil fuels.
John Hess likes the idea. As he notes in a Q&A post at GreenBuildingAdvisor, a masonry heater looks like a good option for a superinsulated house he's planning to build. Masonry heaters stove heat for up to 12 hours in a large mass of masonry; they produce very little ash and low emissions.
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In particular, Hess wonders whether anyone can offer advice on plans he found at the Masonry Heater Association Web site for a heater he could build himself.
"How large of a building can this model heat, in a cold New England climate?" he wonders. "Are the plans complete enough for an owner-builder to make a serviceable stove? What would the materials cost to make one?"
They're great, but pricey
"It's the only way to go if you want wood heat in a conventional well built greenEnvironmental building program of the Master Builders Association of King and Snohomish counties in Washington state. Offers four rating systems that cover single family homes, remodels, multifamily dwellings, and communities; ratings are either self-certified or verified by a third party, depending on the level of certification. home," writes Tibor Breuer.
Breuer has had masonry heaters in the last two houses he's built. But now that he's starting to build a double-wall house, a conventional masonry heater looks like way too much heater. Instead, he's looking for a much smaller design that would be more appropriate to a superinsulated house.
"The question for you, John, is how superinsulated are you?" he asks Hess. "If your answer is ‘highly insulated,’ then an efficient, small wood stove would do the job. A little fire, twice a day, and it wouldn't cost that much."
Breuer bought a Temp-Cast heater with an oven, which his mason assembled and faced with brick. The total cost was $11,000. There were 33 cast refractory pieces that took the mason just three hours to put together with easy-to-follow plans. He'd recommend one, but not necessarily in a double-wall house.
They might be a piece of cake for a mason, but Daniel Ernst doesn't think building a masonry heaters from scratch makes a good do-it-yourself project.
"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," he writes. "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."
Too much heat for a small house
Masony heaters, Ernst says, are designed to be fired once or twice a day with a quick fire that soaks into their high-mass walls. They provide their maximum heat output for several hours after a fire has been lit, then taper off over the next 24 to 36 hours.
Assuming the heater is 80% efficient, and using well-seasoned firewood, Ernst estimates output at 220,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. for each 40-lb. load of wood, or a little more than 9,000 Btu per hour.
"This figure doesn't work with a 1,000 sq. ft. double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. house in New England (unless you are looking to create a 1,000 sq. ft. Finnish sauna)," he adds. "You say 'very well insulated,' so it could be [twice] what you really need. So to answer your question, no, I don't think this is the right technology. "
A masonry heater may also be the wrong choice on the basis of cost vs. value. "Why would you work hard to get your heating requirements down to a low level, then install a very expensive heating system?" Ernst asks.
Jamie Malcolm-Brown, whose brother builds masonry heaters with Maine Wood Heat, is thinking along the same lines.
"They are beautiful and efficient but generally overkill for a superinsulated home," he says. "The biggest problem for a superinsulated 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)."
What about a wood stove?
Malcolm-Brown, along with several others, suggests a small, efficient wood stove with an outside air source as a less expensive alternative with a greater degree of control, particularly for the kind of house Hess is planning on building.
Robert Riversong suggests looking at one of the soapstone heaters and ovens made by the Vermont Stove Company. One of them is called the Vermont Bun Baker, which incorporates 350 lb. of cast iron and as much as 1100 lb. of soapstone.
"Soapstone is the most amazing material for a masonry heater," Riversong writes. "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 massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. index (3 times that of marble and 4 times that of granite). And it's a very beautiful stone."
"I would suggest a small, efficient, sealed combustionCombustion system for space heating or water heating in which outside combustion air is fed directly into the combustion chamber and flue gasses are exhausted directly outside. wood stove, 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)," he says.
Another possibility, suggested by Lucas Durand is a soapstone wood stove made by Hearthstone.
But start with a heat load calculation
Determining how much heat can be extracted from firewood isn't as simple as it is for oil, natural gas or electricity. There are more variables. Still, a good place for Hess to start would be with a heat-loss calculation for his house, as Ernst and Riversong suggested.
"If you haven't completed a heat-loss calculation on your structure, you ought to do that before determining your heater type / size," Ernst writes.
Hess could do that himself, with enough study and preparation, or he could hire someone to do it for him.
That, he says, can get complicated.
"So let's look at it from a different angle: There are 3,413 Btu / kWh, and 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."
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