I have really enjoyed a small centrally located wood fired bake oven in our single story ICF home for about 10 years now. I originally felt it would be a big gamble to install it and have it work properly. It is a very small oven ( about 26 " inside diameter ), well insulated, and uses very little wood. You learn pretty fast that you cannot have any exhaust fans on anywhere in the house while it is being burned. I have a return air duct located about 16 feet away and it has never been a problem, only the bath vents.

The critical factor for any wood burning appliance is going to be matching the firebox to the flue size to the chimney height. The factory built appliances have this nailed down pretty good. It's the cathedral ceilings, bath fans, return air ducts, nearby doors, and sometimes high wind or muggy air conditions that will cause you grief in any structure. They are just compounded in an ICF envelope. But, if you want to burn wood for heating, cooking, economics, or just for atmosphere, and are only in the planning stages, it would be wise to seek out a certified hearth specialist. The training for this profession has come a long way since the wood burning renaissance of the " 1970s ".

Clothes dryer. I forgot to mention the clothes dryer. I can use it after startup of the oven, by opening a window in the utility room. Not a big deal for a two person household, but I can see how little inconveniences like that could add up, if you are up for the compromises !

Posted By Lbear on 25 Jul 2014 11:35 PM

Posted By Leon Possible on 25 Jul 2014 08:22 PM
Jeez! If the guy wants a fireplace, he should have a fireplace. Everyone gets so cranky on here.

Nobody's stopping him from getting a "standard, open wood burning fireplace." He posted a question on a green building forum about incorporating an extremely inefficient and gross polluting, "standard, open wood burning fireplace" into his house design. People responded advising him not to choose such a fireplace but to choose alternative designs that are not as bad as his original design choice.

It's like going on a Vegan Forum and asking how to roast a pig. The response shouldn't be surprising to his question.

actually, you didn't offer any alternative just criticism.

secondly, this specific forum is ICF, and the OP is building an ICF home.

Posted By ICFHybrid on 25 Jul 2014 04:35 PM
Eugenepan, my house is a lot smaller than the one I should have built and I'm hurt that you don't recognize my personal sacrifice in the interest of saving scarce resources, but you don't see me attacking you personally or calling you names, do you?

Let me explain something to you. If you want to defend your choices with data, that's fine, but attacking the size of MY house has nothing to do with whether or not an open fireplace is a good idea.

And don't be so defensive if your ideas don't go over well. Take it in stride.

you have attacked me personally if you search back far enough.

i didn't have any ideas on this entire thread, so nothing to be defensive about.  just don't like how lbear is always criticizing people on their initial posts.

i am sure someone can provide the data, but i am pretty sure that a person building a 3400 sq ft ICF home with a fireplace is using less resources over the life of the home than someone who is building a 6,000 sq ft. ICF home.

if it is not a good idea to have an open fireplace, it is a much worse idea to build a 6,000 sq ft home.

and don't be so defensive if your house size doesn't go over well.  take it in stride.

Posted By sailawayrb on 25 Jul 2014 06:52 PM
I agree Dana. I don't put much faith in any efficiency numbers as stated by wood burning device companies. It is unfortunate that independent test was done on the lowest efficiency breed of masonry heater and not even best of that contra flow breed. Bottom line, you really can't use even the best wood stove in an energy efficient home, they just release too much heat too quickly at their design operating point. Even the worst masonry heater solves that issue. If you live where your have your own personnal supply of endless renewable wood, masonry heaters are hard to beat. I would also say that our masonry heater reaches peak efficiency very quickly. You start it by lighting paper placed at top of the wood pile. It reaches white hot burn phase in a couple of minutes and 20 lbs of wood is gone in less than an hour.

You can run a Hearthstone Tula cleanly at ~11-12KBTU/hr out- that's below the design heat load of many PassiveHouses.  In almost ANY house (except very tiny low mass houses) it takes a long time to heat up the thermal mass of the house at 12,000 BTU/hr.  When the loads are that tiny you don't really need the thermal mass of masonry heaters- the thermal mass of the house is enough.  A lot of not-super-insulated houses have heat loads of less than 30,000 BTU hour (the peak firing rate of that unit)- it's possible to right-size wood stoves and run them continuously at clean burning firing rates during the winter.   The modest thermal mass of a soapstone stove is enough to deal with shoulder season issues.

Almost all masonry heaters installed are intentionally oversized for the loads, and intermittently cold-fired at a high BTU rate, letting the mass of the stove itself moderate the delivery of heat. But that means multiple high-emissions start up ramps per day.  With any wood-burner, sizing the burner appropriately is EVERYTHING in terms of minimizing as-used emissions, whether high-mass or low.

Burning 20lbs of wood in an hour is a pretty high BTU rate, and while the peak efficiency might be reached in 10 minutes, it's unlikely to be reached in 2.  Bell-chamber rocket stoves don't even hit peak efficiency in anything like 2 minutes.  Absolutely dry wood is good for about 8000 BTU source fuel content, a 20% moisture content brings that down to ~6500 BTU/lb , so you're looking at a burn rate of something like ~130,000 BTU/hr or higher for an input. Assuming an average efficiency of even 75% over a burn cycle, an output rate of ~100,000 BTU/hr.

Posted By krom on 25 Jul 2014 06:27 PM
if the guy wants a fireplace let him have one, and give him advice to make it work well in an ICF home, or STFU and stay out of his thread.

Burning wood is carbon neutral, so even at 15% efficiency it is more green than electricity, natural gas, propane, or probably even solar (ever wonder what those chinese plants churning out panels dump into the environment?) even windmills destroy the view and kill bald eagles...

Contrary to the simplest models, burning wood is not exactly carbon neutral, and not necessarily lower carb than condensing natural gas heaters unless burned at sufficiently high efficiency.  When wood is burned on an ongoing basis in a better class wood burner like an EPA wood stove or masonry heater at 60-80% efficiency it takes about 25 years before the net wood carbon emissions & forest re-growth to break even with the emissions of the condensing gas it displaced.  Beyond that wood burning becomes a net greenhouse gas reduction. 

At 25% thermal efficiency a wood-fired biomass power plant takes 90 years for the net forest-carbon to catch up with the emissions of a 35% efficiency gas peaker, (and never catches up with a combined cycle gas peaker.)

When burned in 10-15% efficiency fireplace (which would be VERY good for an open hearth fireplace), the "interest" in the forest carbon-bank literally never catches up- it's a unsustainable practice, and higher net carbon emissions than condensing natural gas.

Regulators in Massachusetts commissioned a careful accounting study to determine the sustainability of biomass power genertators to get a handle on when & how much biomass powerplants should be considered a net green gain in comparison to other fuels.  Counter to the expectations of those promoting biomass boilers strictly for power generation, only combined heat & power (CHP) co-generators can ever reach sufficient total thermal efficiency to be a net-positive relative to power generation by simple cycle natural gas turbines operating at 33% thermal efficiency, and only the highest efficiency CHP unit beat a combined cycle gas powerplants + condensing gas heaters.  (While much of that study is MA-specific, it's still worth skimming- there's quite a bit in there, covering everything from how different forestry practices affect both the initial carbon hit and the recover, wood heating efficiency, etc.)

The regulatory/policy result was that newly built biomass power generators need to operate at a minimum of 50% thermal efficiency, and to get ANY verditude credit needs to deliver at least 60% net thermal efficiency.  This is currently only possible in powerplants with a CHP approach.  Existing biomass power generators at lower thermal efficiencies are still allowed to operate, at least for now.  It's not clear if even those efficiency levels are really high enough to be sustainable, but it's a start.

Not addressed in the MA study, when a stable forest/woodlot is harvested it results in carbon being released from the soils as well as the amount of carbon burned in the heating appliance/fireplace. (Some big-picture view of that is discussed in the slides here.)  This means that even the MA study might be somewhat on the rosy side, but not excessively so.  When including soil-carbon aspects it's probably closer to a 30-35 year time frame before the net-carbon of an EPA woodstove beats condensing gas.

As it happens I live in an urban forest, just down the street from an urban logger (tree maintenance & removal guy), inside a quarantine area for Asian Longhorn Beetle.  To keep my person wood burning on the more-sustainable side I try to use as much of the storm damage and scrap trees that were marked for removal (for any number of reasons) in lieu of the more standard cordwood sources.  The price is right (mostly sweat-equity). Since it's currently illegal to move wood outside of the quarantine area without first running it through a chipper, putting it in sealed containers,  and trucking it to a local biomass generator (who has the protocols in place for handling potentially infested wood) burning it in the wood stove is lower-carb than the ~25% thermal efficiency biomass powerplant solution to the disposal problem.

BTW: Coal fired power plants kill more birds per megawatt-hour of output than wind turbines or PV (or concentrating thermal solar).  Nukes do too.  It's not as clear with natural gas.

BTW: Coal fired power plants kill more birds per megawatt-hour of output than wind turbines or PV (or concentrating thermal solar). Nukes do too. It's not as clear with natural gas.

The bird mortality is something brought up by numskulls against alternative energy technologies. It's really quite a non-issue. Bird mortality due to wind turbines is measured in the low hundreds of thousands, while feral cats alone kill about 500 million birds a year. Windows account for about a billion dead birds.

Dana, our single daily 20 lb firing results in an average of about 6500 BTUs/hour during the first 12 hours and an average of about 4500 BTUs/hour during the last 12 hours. These rates are ideal for our place. I love having the free oven baking time provided by our masonry heater too. We burn wood from our annual property wildfire defense/prevention maintenance. Yes, much of this wood could be chipped, but that would involve using another fuel source too and this wouldn't address disease or insect infested wood issues either.

So is a Masonary fire place more energy efficient than Wood-burning stoves; yet more expensive? And use less wood?

Yes, the worst masonry heater is likely significantly more efficient than the best wood stove. Whether you trust the absolute efficiency numbers published by manufacturers of either is another matter. Yes, masonry heaters are very expensive and take up a lot of space too. As such, they often become the defining architectural element inside the building. I believe the following is an accurate description of a masonry heater and is an excerpt from the masonry heater performance software on our website:

Energy efficient homes may only require 3,000 to 8,000 British Thermal Units (BTU) of heat gain per hour. For most conventional woodstoves, this is well below their critical burn rate for operating cleanly and they will start to smolder. If you operate most conventional woodstoves at or above their critical burn rate, you may easily overheat an energy efficient home. The only solution for this dilemma is to have many small firings, which is not very convenient. Therefore, wood burning and energy efficient homes are not normally compatible unless you have some way to burn the wood at or above the critical burn rate to allow operating cleanly and you have a way to store the excess heat that is created and release it as needed without having to accomplish frequent, inconvenient firings.

We prefer masonry heaters which provide the solution to this problem and have actually been the most efficient way to heat a home with wood for over a hundred years. Unlike fireplaces or woodstoves, there is very little heat loss because the exhaust gases are circulated through the masonry heater several times before going up the chimney. There is very little pollution because masonry heaters burn the wood very quickly and operate at about 1700 degrees so as to fully burn what even certified woodstoves cannot burn. Masonry heaters store and slowly release radiant heat over a 24 hour period accomplished by only one or two firings per day. Masonry heater surfaces never get extremely hot like stoves and do not overheat and excessively dehumidify your home which your sinuses will greatly appreciate. A masonry heater can also be located to absorb solar radiation and store this form of heat energy too. Therefore, masonry heaters are similar and compatible with hydronic radiant floor heating and passive solar heating. As a side benefit, you can have a nice masonry oven that is available for energy free baking duties perhaps 10 hours per day and you can also have heated benches for you, your guests, and your pets to enjoy all day. Constructing a masonry heater is a relatively simple DIY project and there are many good kits available in the marketplace to do this. Please be sure to fully research and comply with all your local building code requirements.

sailawaybr - thank you for the response! I didn't know any of that, I thought wood-burning stove or outdoors wood-burning stove was the way to go. I didn't even consider the house "over-heating" due to a wood-burning stove. I guess now the question for me to research is how expensive is a Masonry oven related to alternative heating sources?

I'm, not sure that a generic masonry heaters' footprint will take up as much space as a manufactured wood stove when you consider the clearance to combustible distances necessary to meet installation codes. Unless it has soapstone for its' finished facing the clearance to combustibles is only 4" ! For many wood stoves I believe this clearance distance is more like 12-18". Many models, even the contra flow design ( five run channel configuration ) can be top vented to save the space that would normally be needed for a rear or side installed chimney. But, a top vent model would make installing a heated bench option a little more challenging.

The best environmentally friendly feature, in my opinion, is that if the owner of a medium to heavy weight masonry heater is truly committed to burning cord wood as their main heat energy source, it will very seldom be fired from a stone cold firebox, during the heating season. Even after not being fired for 24 hours the firebox ( assuming it has a properly designed combustion air supply ) can still be well above 200 degrees ( F ). It does take a bit of diligence to anticipate and adjust the wood load and firing schedule to match the possible changes in the weather during the heating season. If you, your lifestyle, or your climate will not allow for that, a masonry heater is not going to be yours' or the environments' best heating solution.

Masonry heaters often have heated wrap-around benches and ovens. They often encompass a significant footprint and as I indicated previously, they often become the centerpiece of the building and a long-term family heirloom. Check out the photos in the gallery section of this website:

http://www.mha-net.org/

Yes, I would agree that you need to take diligence to align the wood load with the forecast weather. Once you fire a masonry heater, you are stuck with the resulting BTUs/hour for the next 24 hours. You also don't ever want to do more than two firing per day or you can damage the masonry heater core.  I also attached a photo of my personal favorite

sailawayrb: Most wood stoves are oversized by more than 2x for their peak loads, leading to lower as-used operating efficiency. But that's not to say they have to be. The comfort aspects of masonry heaters are a far bigger plus than any perceived or alleged efficiency improvement over right-sized wood stove.

It's simply not factual that masonry heaters are inherently more efficient than the best wood stoves. While it IS true that it's harder to screw up the operating efficiency of a masonry heater by how it's operated than it is with wood stoves (thus oversizing of masonry heaters is less consequential), right sizing a wood stove keeps it operating at or near it's rated efficiency. It's simply not possible to operate most masonry heaters in a smolder mode, the way folks with ridiculously oversized woodstoves often operate them.

The notion that wood stoves "... overheat and excessively dehumidify your home..." has no resemblance to the facts. Heating air do not add or remove moisture, and the relative humidity of the air in the house at a given house temperature is the same, whether the surface temp of the wood-burner is 150F (masonry heater) or 500F (cast iron wood stove at high fire.)

Ventilation/infiltration in winter has a drying effect, which is why leaky ducts in hot-air heating systems in air-leaky homes can have a drying effect. Heating with a wood burner does NOT drive infiltration, unless you leave the doors open like an open hearth fireplace" allowing the flue to depressurize the house, sucking in drier outdoor air, which is not how they arre used.

The output rate of the BURNER of a 20lb burn that is done in one hour is 100K+ (independently of how much it's buffered by the thermal mass of the appliance), which means it can't be continuously fired in a low-load (or even moderate load) house. That also means that every burn will be a cold-start, and possibly multiple cold starts per day. The majority of the particulate emissions of wood burners will be in the first 10-15 minutes of a cold start, which is why the EPA only tests steady state burn emissions, when the unit is already at temperature. Right-sizing a wood stove means it burns nearly continuously in mid winter at a firing rate above what it takes to keep the secondary burners working, and only cold-started daily during the shoulder seasons.

There were several high-mass burners entered in last year's woodstove decathlon ( http://www.forgreenheat.org/stovedesign/finalists.html ) but the winner was a sheet steel wood stove (with internal soapstone for a modest amount of thermal mass) with from Woodstock Soapstone. Efficiency counted for 20% fof the score particulate emissions another 20%. Since affordability was also 20%, the mass burners had a bit of a headwind to overcome, but at least three made the final cut. http://www.inspirationgreen.com/wood-stove-decathlon.html


BTW: I've never seen a house with a 3000 BTU/hr heat load at the 99% outside design temp, but would be very interested in seeing the specs on that house.

"The output rate of the BURNER of a 20lb burn that is done in one hour is 100K+ (independently of how much it's buffered by the thermal mass of the appliance), which means it can't be continuously fired in a low-load (or even moderate load) house."

That's the whole point Dana...it is NOT continuously fired! It is only fired once in 24 hours and it only generates on average 5400 BTU/hour over the entire 24 hour period.

True, if there isn't any indoor/outdoor air exchange, the relative humidity should not change at all. However, sinuses don't lie so there must be something that causes the building ventilation rate to increase when using wood stoves (i.e., exchange higher humidity indoor air for dryer outdoor air). And please don't tell the folks who sell steam kettles for wood stoves about this...

A good winter coat has about a 500 BTU/hour heat load at the 99% design conditions. It's all about size and eliminating doors and windows!

Having lived in the same house both with & without wood stove heat, a house where the indoor air humidity is constantly being monitored, the notion that the wood stove affects the indoor humidity is complete bunk.

So, you're saying you've seen houses that had the same heat load as six good winter coats? I've not seen that house! (Even PassiveHouses in my neighborhood have design heat loads a bit over 10,000 BTU/hr.)

The whole point about the firing rate being a large multiple of the peak heat loads is that it necessarily forces cold-firing, which is more polluting than many hours of continuous burn. In not superinsulated homes it's possible to size wood stoves so they can be fired continuously, which I'm suggesting has lower particulate emissions. The same 20 lbs of wood can be fired in the ~30K max-fire Tula example over the course of many hours, and reloaded while the stove is still hot. Most homes in my neighborhood have average mid-winter heat loads above 12KBTU/hr, and continous firing of a stove like that does not result in overheating.

This is sort of like the differences between very high-mass boilers with thermal buffering vs. modulating low mass boilers. There's ways to screw it up with either, but that doesn't mean you have to. But unlike an oil or gas fired high mass boiler that hits close to it's steady state combustion efficiency within seconds, high mass wood burners don't (not really.) Lower mass rocket stoves can come up pretty fast- maybe even in under 5 minutes (though I have yet to see convincing test data that it's that short). Masonry wood burners aren't that fast.

Snooping around the web for test data, the Texas company Dragon Heaters has a series of bell-chambered masonry clad rocket stove designs that look like they comes up pretty fast, but it's not entirely clear if their test methodologies are fully up to snuff, ( or exactly when they started the clock on the burns.) http://www.dragonheaters.com/ (see their blog section for some graphs of burn data.)

No, I am not saying anything of the sort. My point is that you can achieve very low heat loads if you aggressively compromise on building size and doors/windows. The problem is most people don’t want to live in a building like that or pay for the cost of accomplishing the required building envelope…which is the problem with PassiveHaus too.

Our masonry heater is still quite warm when it gets fired once each evening. There is no way I would want to continuously feed our 20 lbs of wood into a wood stove multiple times per day. I also wouldn’t want to lose my baking oven either. And finally, I don’t see my cats napping on a wood stove like they do on the masonry heater. Nope, I won’t be trading my masonry heater for a wood stove anytime soon!

Multiple loads of 20lbs would cook you (and the cats!), not just the bread. Smaller masonry heaters taking smaller loads still works.

I too am a big fan of the comfort & convenience aspects of masonry heaters relative to woodstoves, but right-sized woodstoves really aren't bad. Only oversized woodstoves have the truly negative characteristics mentioned in prior posts. Soabstone woodstoves & small ceramic kachelofen type stoves have some (but not all) of the comfort characteristics of large masonry heaters, when sized correctly for the loads.

Yes, I agree with your perspective Dana and I can certainly see how a smaller soapstone EPA approved wood stove could be a more attractive option from a cost standpoint. Even if we call the performance a stalemate, I still think the convenience of only having to accomplish a single firing per day, having a baking oven, and having a surface that never gets too hot for the cats is worth the additional cost to me. If you have cats, you know how important it is to keep them happy… I recognize that cost to comfort is a personally subjective thing. Yes, whatever one selects, it certainly needs to be properly sized and loaded for the heat load or you will not be happy...

I like to think of a masonry heater as being a battery that gets charged up with heat rather than electricity. Sometimes, you might not be able to build a heater battery big enough, or responsive enough, to keep up with the demand. In those cases you would be much more comfortable with a woodstove, that you could ( with a little practice ) more easily balance the charge rate to the discharge rate. I would think that this would very rarely happen in an ICF structure ! Unless of course one were unfortunate enough to be cajoled into installing some very leaky windows and doors, especially doors ! I don't want to mention brand names here ( rhymes with " Heather Field " ) !