Rocket mass heaters for Texas? and warm climates generally

We just got an inquiry from Texas, asking about who might install rocket mass heaters there and what it might cost.

Before trying to pin down a specific estimate, I'd want to consider what a rocket mass heater does, and how that might fit into an economical heating and cooling plan for such a warm climate.
We were recently in San Antonio for visits spanning late July through mid-October, and we were impressed with how hot and muggy the summer weather was. We understand that parts of Texas are much drier, and there may even be some parts with altitude. (San Antonio is so flat, generally speaking, that we had to drive over an hour for a field trip designed to test the new leg hardware on a rough-terrain climb. The "flat" parts of Washington look curvy to me coming back from that landscape. They have small hills and knolls, but not a lot of anything we would call mountains, in that part of Texas.)
But generally speaking, the inclement weather in Texas is going to involve an occasional snow event, which is dangerous mostly because the population is not accustomed to it, and a lot more of the arid-climate and flat-territory discomforts: extreme winds, storms, and (in dry regions) big differences between daytime heat and night-time cold. In the regions close enough to the Gulf to get a lot of humidity, there is a buffering effect where temperatures tend to be more stable over weeks at a time, without a big drop from day to night. The climate in San Antonio allows agave, cacti, rosemary, and (irrigated) sub-tropical plants that would never survive our hard winters up near the US/Canadian border. Their mission architecture reflects a lot of lessons learned from Spain, and even the classical architecture preferred by the Southern plantation culture takes advantage of some Mediterranean traditional designs that translate well to the climate.

We have worked with a few clients and students from climates like southern California and Israel, and the heaters they are building tend to be much smaller mass, shorter pipe runs, a hotter exhaust heat, and/or a quicker heat-up and cool-down so they can be run on demand instead of storing up heat for an extended winter season like we have up north.
Here is one example from Israel:


If you build a long, massive bench in a climate that doesn't have a steady cold season, you may run into problems getting the heater to fire up in conditions that cold-climate dwellers would never consider lighting a stove.
Like when you come into a 65F home after an 80F day outside, and the mass is only at 70F and you feel the evening's chill starting to dip down toward 60F.
For a person from Minnesota, that's a hot day, and the idea that you'd want your sofa hotter than 70F when you expect another hot day tomorrow sounds crazy. A Minnesota heater will be optimized to operate at around 0F (-15C) outdoor temps, and handle heat loads with the temperatures in the -30 range; maximum efficiency in this range may lead to a heater that doesn't want to start in warm summer weather, and has to be carefully started for the first time in fall if the outdoor temperatures are still above 60 F.
But to someone acclimated to warm weather, the evening's cool is a genuine discomfort, and they may not own the sort of woolen or synthetic garments a person from Minnesota would naturally consider as the first option for handling that temporary discomfort.

You may also live in an area with extreme variation between day and night; deserts do freeze. Or an area with altitude-related micro climates that result in cold snaps that are unusual for what most people would expect for your region.
We worked on a small mass heater with a family in Southern California, who need occasional heat through several months of the year due to their altitude, and wanted it to be as clean-burning and efficient as possible.


The first thing I would consider in any sub-tropical climate, however, is whether you can get your heating and cooling provided for free by using the natural energy already at play in these climates.

The climate around San Antonio, TX endures muggy hot summers, so thermal mass
is not quite as magical as it can be in arid summer climates where it can take advantage of night-time lows.
But it's still a darned good tool.
There are several basic principles that can be used as the basis for a no-energy heating and cooling design, or to augment heating and air conditioning and reduce their energy needs.

1) Cave Principle: Underground temperatures tend to stabilize around the 50's or 60's on average, across the earth. This varies near the surface due to local climate, and near warm geothermal features such as volcanic vents and hot springs.

There is a huge lag between seasons the deeper you go underground, and the temperature also tends to approach the average. Buried heat pumps take advantage of this to provide low-energy heating and cooling.

http://terratek.ca/services/thermal/category/geothermal/

Buried homes, thermal coils, and deep basements can be expensive or unworkable in areas where there are deep frost/drainage issues or excavating is expensive (for example if you're on top of limestone or caliche or bedrock or permafrost). You can also berm up around the sides of the home, moving the ground level upwards. There are definitely issues to consider in doing this safely and effectively, but I'm working on an article on what some neighbors have done with a series of earth-sheltered buildings and will post that link on permies.com when it's available.
Paul Wheaton's wofati design is another along these lines. A wofati might work even better in a warmer climate than Montana, where the mass would only need to store heat or cool for a few months at a time rather than for an extended, very cold winter.


In desert climates, where daily extremes far outweigh any impact of "average" seasonal temperatures, cave dwellings are traditionally popular. they are also found in some places where extreme wind, hostile neighbors, or other factors make a solid-walled dwelling attractive enough to be worth the effort.
http://www.popularmechanics.com/home/improvement/outdoor-buildings/cave-homes-461109


It's worth noting that if you do build your home to resemble (or within) a genuine cave, with all thermal mass and no insulation, your temperature will stabilize at roughly the cave's temperature. Caves, like basements and castles, make great root cellars but are often too cool to be ideally comfortable for the naked human body. Maybe there's a reason that cave-dwellers are often shown wearing fur. Warmth from heating, appliances, and occupants will supplement this heat, but if you really need to control indoor temperatures separate from the ambient earth temperature, you'd want insulation somewhere to preserve that temperature difference.



2) Adobe/Mass Wall Principle: simple, massive walls can absorb heat all day, and remain warm well into the night. Making the walls 8" thick or more* allows the sun's heat to finally penetrate inward just as the wind is cooling down at night, allowing the interior to experience offset and comfortable temperatures due to thermal lag. If you are too cold indoors during the day, or too warm at night, you open a window, because the interior walls are 8 to 12 hours offset from the outside weather. (*up to 14" thick in modern adobe, and even thicker in some traditional villages with taller construction).

The idea that heat moves only one direction, or on a fixed timeline, is an incorrect oversimplification, but the comfort adobe provides in warm desert climates is completely genuine.

In colder climates, or in very hot deserts where much more heat is delivered during the day than can be shed at night, insulation and heat-reflecting surfaces on the outside will allow better control of internal temperatures. A combination of air movement (when outside air does happen to hit a temperature appropriate for improving the indoor comfort) and artificial climate controls are used to achieve the desired indoor temperature, and the adobe or thermal mass simply stores it and makes it last longer.


3) Passive Solar Storage: Many people know that "passive solar" means capturing sun energy without using electrical gizmos to store it, but you can also use passive-solar design for cooling. The idea is to notice the way the sun's angle changes throughout the year, and orient the house and landscaping features in order to create the most pleasant comfort environment for any season. Using as much of the natural solar energy as possible can reduce or eliminate your HVAC bills, and in some cases make a home or vacation cabin that stays comfortable with zero occupant effort - like when you're gone at work or on vacation.

In the summer months, the sun is higher overhead. In the winter, it's lower. So by having a thermal mass (masonry wall, bench, etc) across from a sun-facing window (south-facing, in the northern hemisphere) you can arrange for it to get hit by the sun on winter days and early spring, but not in the summer or early fall. It's a matter of calculating the angles.
One trick I like for directly observing the angles is to note that the full moon is opposite the sun. So around winter or summer solstice, you can track the sun's movements for the actual sun angles, and then you can track the nearest full moon's movements (and shadows) to estimate the opposite solstice. Also note the spring and fall angles, because these will become important when deciding when to exclude sunlight.

There is a temperature lag throughout the Earth, because it takes time for added solar energy to bring the earth up to a higher temperature, or for the earth to cool down once the sun's energy delivery has diminished. This means that the Sun is highest at summer solstice (June 21 or so in the north), but the hottest months aren't until July, August, and even into early September in some climates. By late September the sun hits equinox (its middle setting), but the weather is only just starting to cool down again. By Dec 21, the sun is at its lowest point, but the winter tends to continue getting colder through January and early February. And by the spring equinox in March, we still have frost and snow, but the sun is back to the same place it was in September's "Indian summer" weather.

My favorite simple trick for dealing with this lag, theoretically, is to plant some deciduous shade trees or vines on a trellis to the south and west, where they can block the late-summer sun but leave that aspect open for early-spring heat.
You can also use removable awnings outside the house, or use a reflecting pond with deciduous plantings around it to beam some extra heat your way in early spring.

We cover some of these things in our booklet "Shelter," which is in fact available for a very reasonable price at scubbly.com. There are also some excellent websites out there - I suspect "builditsolar.com" is a good one, because I haven't manage to read all their resources pages yet, and I recommend/use their heat-loss calculator almost exclusively.

I know more about temperate-climate passive solar than subtropical, but the subtropical and tropical tricks are even more astonishing.
You can, for one thing, create cool-generation areas around the building, and then use the sun's energy to draw cool air from these areas into the rest of the building.
The basement of any of the traditional Colonnial, Cape Cod, or Sears Catalog prairie-style home with a central stairwell and a basement can produce a lot of passive cooling in summer, if you open all the stairwell doors to the central stairway to draw cool air from the basement.

Moorish and Mission architecture often have towers that catch the sun, and serve as ventilation shafts to pull air through the rest of the building. (image, next post)

Instead of basements, cool-generating features like elegant shade collonades are used along the sides of the building. This has the double function of keeping some sun off the walls themselves, and providing cool air to circulate into the rest of the building. Some classical Mediterranean and Middle Eastern architecture even has a tunnel-like entrance with a constantly-flowing fountain, or fountains in the central courtyard of a family manor. The courtyard itself serves as both a light-well for growing a small elegant garden with fruit trees for added coolth, and a chimney moving warm air up and out of the living quarters to be replaced with fountain-cooled air.

Simple courtyard dwellings are common in these climates even for poorer people:


For the traditional American wood-framed homes above, shade porches can give a similar effect, though they don't store the nights' cool like a masonry collonade.

Shaded, reflective, and/or insulated exteriors on those upper walls are critical to keep the solar load down in summer for the hotter climates, and to preserve your ability to trap heat indoors in winter.


So the bottom line for any climate is to orient and structure the building appropriately for the climate.

4) Usage matters:
Thermal mass is like a giant battery or flywheel, stabilizing your temperature so it doesn't swing through those uncomfortable extremes.
There are some situations where you might not want to have to charge up a big battery just to briefly use the space, or waste all that charge after you've left the space.

For a building that will only be occupied occasionally, such as a workshop or guest cottage, using insulation alone with minimal thermal mass makes it easier to bring the building to the correct temperature when you are using it, without sacrificing energy to keep it at temperature when you're not.

For any building that's used for a week or more at a time, that's where people sleep, or that's used or occupied for most hours of the day (like a greenhouse or climate-controlled storage facility), you want:
Insulation on the outside, thermal mass on the inside, to hold on to those comfort temperatures and keep them stable.

A concrete floor slab counts for a lot of thermal mass, but can be difficult to heat. Perimeter insulation makes a much bigger difference than under-slab insulation in most situations, and is easier to do as well. If you are pouring a new floor slab as thermal mass for a passive-solar structure, by all means insulate underneath it, you don't have the option to do it later.

5) Wind capture:
Wind is generally better at removing heat than bringing it, due to evaporation.
As long as there is any moisture available, the evaporative cooling effect is likely to be bigger than the heating effect, and air will have to be substantially hotter than the desired temperature to deliver enough heat to bring thermal mass or human bodies up to comfort temperatures.
The exception might be areas with consistent warm winds or very hot, drying winds from a particular direction summer (such as afternoon upslope winds, or winds from the big inland deserts in Australia and Africa and the western USA). Hot winds represent a potential directional source for wildfire hazard, as well as a potential heating energy.

Swamp coolers use evaporation to cool and humidify the air. They work best in dry climates. They serve a similar purpose to the fountains and gardens in Moorish and Persian architecture, except not nearly so elegant.
One of my mother's tricks from the maritime Pacific Northwest, where summer's heat is brief enough that nobody bothers much with air conditioners, is to hang up a bedsheet as a shade curtain or awning on the sunny side of the house. On very hot days, she'd spray the sheet with water from the hose, getting dual benefits of shade and evaporative cooling to prevent overheating of that south-facing wall. Damp laundry works as well, and may be a great way to take advantage of sun-generated winds near the hot side of a building.

Symmetrical traditional building plans also offer good through-flow for collecting a cross-breeze. This lets you take advantage of the local climate anytime it's more comfortable than your building happens to be. Cross-breezes are great for cooling, and for helping to cool you as you sweat in hot weather. In climates where humidity is a concern, good ventilation also helps reduce problems like mold and mildew.

Greenhouses can use artificial air pumps to move too-hot upper air down into tubes through the ground, warming the earth for later and cooling the upper reaches for now. Air is not a great mechanism for heat transfer because it has very little heat capacity, and it tends to deposit moisture when it cools and dry it back up as it heats. Moist air has more heat capacity than dry air, but may also cause more problems as it's heated and cooled.

Solar chimneys let you generate your own wind.

Prevailing local winds in winter and summer may affect how you want to orient the buildings in a room, such as putting a pantry toward the winter winds rather than trying to heat a room with a picture-window or door facing that direction.

I'm starting to get more generic, and there are great sources online for most of this info.
Specifically, passive-solar home design is a great term to look up, along with traditional or historic architecture or traditional dwellings for any region with a climate like yours. Don't just go by USDA zones (that only tells you the lowest likely temperature, not the humidity or high-temperature issues). Look at seasonal rainfall patterns and temperature patterns, and find out what your climate's generally called (for example, climates with a hot dry summer and cooler-weather rainfall are called "Mediterranean," while climates with more summer rainfall and less winter precipitation may be considered "Monsoon" climates). areas that complain about muggy summer heat should share tips, including the whole Eastern USA, and areas with arid drought problems should share tips, including most of the Western USA. A tip from Arizona about how to cool your home in summer may be more useful in our dry Okanogan valley than it is in Maine.

When it comes to rocket stoves specifically, I would suggest that

1) Mass heating is mostly developed for temperate and sub-arctic areas, where there is a long, constant cold season and the mass can be heated up and held at temperature for months at a time. The larger the home and the colder the climate, the more mass and fuel is considered in the masonry heater's design.
Rocket mass heaters are a subset of this masonry heating tradition that are mostly used in small, well-sheltered buildings (insulated and/or passive-solar cottages), and are optimized for fuel efficiency, low environmental impact, and low-cost owner-builder construction. They were originally invented and prototyped in coastal Oregon, a mild maritime climate with winter temperatures averaging 30s to 50s F. They have subsequently been tested and proven out in colder areas, such as the inland/sub-alpine Western states, New England, Alaska, and the Great Lakes region.

2) Mass heaters in sub-tropical or hot desert climates are a specialized option. We have consulted on designs for small on-demand heaters, tropical wood-drying kilns, and food dehydration systems for areas whose ambient moisture is too high for successful dehydration without added heat.
I would first consider good passive-solar design, thermal mass walls, or active capture strategies for using the climate's natural warmth for off-season heating.
In this climate, a mass heater bench may serve as a heat-sink through much of the year, soaking up ambient daytime warmth and reducing the daytime high temperatures inside the building.

I would rarely recommend installing a mass heater in an existing building in any climate before insulating the space - insulation will generally be cheaper and easier, and will reduce or possibly even eliminate the need for heat. If possible, insulate outside any thermal mass such as masonry walls.

3) When mass heaters are built in a climate that does not experience routine snowfall, or where there is a pattern of occupants wanting a fire on days/evenings when the daytime high has been higher than 60 F, the design will need to be modified for better draft. (There are climates like this where mass heating is a good idea, for example in clammy maritime climates, or deserts where there is a brief daytime high followed by cold or sub-freezing night-time lows.)
Depending on whether the occupants want to be responsible for considering the outside temperature before lighting a fire, a heater that puts more heat out the chimney may be needed (less efficient, but more reliable draft in these conditions). Benches should be shorter - up to about 20 feet is usually workable - and all elements that resist draft flow such as changes of direction and bottlenecks or slot-like openings should be reconsidered in light of the need for draft under poor conditions.

4) Cooling with thermal mass: In a climate where you are more likely to use AC than heat for most of the year, it is worth considering options to chill the thermal mass, and building it with that function in mind. Just making sure it's in the shade during summer is a good first step; it will naturally act as a heat sink as noted above.

If there are cool night-time temperatures during most heat waves, you could consider making ways to run cool outside air through the heater at night to chill it for daytime comfort.
You could make the bench cleanouts accessible from the outdoors, for example, to use the heating channels directly as cooling channels. (This may put smoky smells into the house unless you manage to close off the firebox and run the air-exchange from exterior to exterior, e.g. up the chimney. You'd also want to clean the stove thoroughly in any case, as fly ash may block heat transfer on the bottom surfaces of the channels where the coolest air will flow.)
You could also use exterior air channels built separately from the heater's mass, for example an outside air port that runs along the back of the heater before entering the room.

I would not recommend spraying water directly onto the heater except in purpose-built greenhouse designs (it could cause problems with mold, mineral bloom, or rotting and structural destabilization depending on the type of construction). You could create a cool-air generation space outside the building using shade porches, collonades, shade plans, ponds or fountains, or a swamp cooler, and use the heater's separate air channels and/or chimney to pull cool air into the building.

You could also run a small AC unit or heat pump on its cooling cycle in the same room as the heater, and point the vents at the heater so it's storing the chill when nobody is sitting there.

In any climate, a thermal mass is most useful if it's located in the main occupied rooms of the home, and is soaking up that ambient comfort temperature and stabilizing it for you. If you're thinking of an emergency heater in the basement, you are pretty much going to be better off with a short-cycle radiant heater such as a woodstove or radiant space-heater, because that thermal mass will not be the right temperature when you need to start using it, and it may therefore be difficult to start.

Another rocket novella from yours thoughtfully,
Erica W

Edit: I should have credited the architecture blog "Misfits" for most of those Middle Eastern design diagrams and photos. Excellent reading. Misfits Architecture: subcategory, It's Not Rocket Science

Here's some images for those Middle Eastern (Persian) designs:
"windcatcher" seems to be a good word to search, as there's more info linked to that term than to "solar chimney".
The solar chimney designs are mostly non-traditional buildings in modern, asymmetrical architectural style, which suggests to me that they're excited about the theory but haven't looked at the history of how it's already been integrated into traditional buildings.
http://en.wikipedia.org/wiki/Windcatcher



http://www.greenprophet.com/2010/02/hassn-fathy-sustainable-architecture/

http://misfitsarchitecture.com/tag/building-performance/page/2/
Tons of good ideas and pictures at this link, too many to share here. Excellence in architecture both ancient and modern.

Here's another incredibly rich site:

this particular image shows three ways to use a "sunspace" to move heat around. the heat naturally moves when the right vents are opened, but you have to open them (or arrange for automatic controls like for a greenhouse, if you prefer). So it's passive collection and distribution, with active controls.

http://www.dennisrhollowayarchitect.com/simpledesignmethodology.html
It is no surprise that our mass heaters have been most popular in the areas least amenable to passive solar heating: those labeled "fair" in this diagram:


If you live in the areas marked "Good" or "Excellent," above, consider passive solar heating/cooling first.
There may be many home improvement projects that would give you greater long-term benefit by capturing free solar energy, rather than just allowing you to burn wood efficiently. If you do decide a mass heater is needed, try to locate it within the building according to passive-solar design principles so it doubles as the storage mass for some free solar heating and/or cooling potential.

and here's the basic diagram that I remember from first-year architecture class, showing the virtual sun's path at different times of year.

Note that although the paths are parallel, the sun's rays spread out in all directions, and you will get low, slanting sunlight from east to west at sunrise, the opposite at sunset, and a similar, comparatively low south-to-north angle (in the northern hemisphere) in winter even during the middle of the day.

http://en.wikipedia.org/wiki/Passive_solar_building_design

These basic overviews show simple concepts that are useful, but I believe traditional design as mentioned in the upper links has far more sophisticated systems to offer. I personally am not aesthetically drawn to modern architecture's extruded beams and plate glass curtains, and I cannot imagine trying to re-invent a house design from scratch and coming up with something significantly better than the traditional compounds in time-tested ancient villages. I guess that makes me anti-modern; I don't think even a well-educated architect is going to beat the most brilliant of the ancestrally-trained Persian kings on his home turf, though he may achieve greater comfort through the use of historically unobtainable devices like AC units. But the historically unobtainable is also likely to be future-historically unobtainable, hard to repair, and irreplaceable, that is, unsustainable.

There's a lot more discussion of passive solar design here on permies.com in this forum:
https://permies.com/forums/f-82/passive-solar

This thread is posted to "rocket stoves" because of the impulse that started it. If you're looking for efficient heat and you are not familiar with passive solar design, you might have ended up here at the Rocket Stoves threads when passive solar is a more efficient, lower-cost option that will meet your needs better.

-Erica W.

This is a really amazing write-up covering an extremely broad amount of material.

I'm in San Antonio as well, and I heat my house probably 4 days a year. I can attest to awnings and window shades being incredible tools here that can be done on the cheap. With the humidity and temperatures being too high at night (84 degrees) during the summer months (april-october), evaporative cooling and "cold temperature storage" is not very effective. But blocking the sun from hitting your windows and walls *is* very effective! Same with growing big old shade trees in front of your east and west facing walls, but that takes many years to reap the benefit. With 7 solid months of hot weather, I would suspect any building (even adobe/cob) that isn't built with some sort of sun shade to be quite uncomfortable in peak summer.

I am interested in rocket stoves as a way to cool my house though, not to heat it. I was thinking about what you said about rocket stoves being used in these types of climates for specialized "purpose-built" uses. I think a rocket stove outdoor cooking thingy would be very useful here, especially if it had a hot box oven area by it. I think 50 years from now, people will look back at us and think, "Wow... People used to cook inside their own houses while simultaneously running their A/C units to cool down the heat it produced...." It's madness I tell you! But there isn't a real energy efficient option right now other than not cooking for 6 months because grills burn so much fuel for so little output.

Anyway, if there's ever a workshop on outdoor cooking with rocket stove tech, sign me up! The high temperatures and amount of experimentation to make a stove that was usable as a primary cooking implement is pretty far outside my knowledge and abilities.

Paul Sims wrote:Same with growing big old shade trees in front of your east and west facing walls, but that takes many years to reap the benefit.

FWIW, if you're in a spot where southern catalpa trees will be happy, those huge leaves provide effective shade a lot sooner than some of the smaller leaf trees. Along Hamilton's riverwalk, there are a few of them that don't look to be more than 15-20 years old, but have such a shady canopy that it's even better than being under a 100+ year old pecan.

But there isn't a real energy efficient option right now other than not cooking for 6 months because grills burn so much fuel for so little output.

Efficiency is less of a concern when there are people out there that will pay you to take the fuel. I could cut a lifetime worth of mesquite this month, and have the same people looking for a way to get rid of more within a couple years.

Jus found this searching around. Brilliant! Thank you, really tweaked my brain.

I have been trying to see (don't have book(s) yet) if anyone has come up with a really workable way to use a RMH in a small, very well insulated space. My house in the works will be 300 sq ft and super insulated, in Western Rockies at 5000 ft. Cold winters but often sunny and warm summers. Makes my wonder a bit if the real optimum solution for my location would be, either find a way to incorporate a lot more thermal mass inside the insulated earthbag envelope and add more windows etc (passive solar) or not even use earthbags and go with cob or something and figure out a way to insulate the outside of the envelope?

I do have friend in Durango who built an earthship almost 25 years ago, with a lot of personal modifications to actually tone down the heat gathering properties. They put in a small wood stove as back-up to the passive solar structure. They fired it up once to test it and have never used it even once since then. There have been a few times of week or 10 days overcast when it was in low 60's ? indoors and they were wearing sweaters etc, but fairly comfortable and they just felt it was too much bother to gather and carry the wood and clean the ashes after, and then the sun came back out.

Hi
Im from israel and i build the stove up there.
I didnt read it all but i want to stongly recomand a bypass that is conecting the manifold and the chimney. It is very hard here sometimes to create a stack effect and its the only why i found that works.
After that we do long benches and they hold the heat amazingly!
Good days
Adiel

hhh
that weekend israel should get 0 Celsius and snow and all the country in panic...

Hello:
Would it be possible to run a rocket stove with the bypass valve so it can run the heat directly to the flu when we have cold nights but warm days, then, run it through the mass when the weather turns cold for longer periods (I see the stove can have a bypass valve to get it started but can it also operate for short-term heating bypassing the mass?). We are in Dallas, TX which is north of Israel. During the 2-3 months when it actually does get cold here (it's 20’s & 30’s this week) we could subsidize our nat gas heat 24/7… The house will be approximately 4500 sf (large family) and I have an unlimited resource of spring storm-fallen limbs that I could put to use. I THINK I have an idea to keep it comfortable: The heater would go on a porch that has sliding doors to the inside of the house and sliding doors to the outside (to be added along with the rocket stove AFTER the house is completed to avoid problems with building inspectors who might claim it's an inside space now). By opening and closing the doors when it’s either not needed, or, if we've accidentally over-heated the mass, we could perhaps maintain a comfortable environment throughout the winter. Am I off my rocker?
Thanks for your consideration.

4500sf is a very large space, and while a RMH is certainly capable of heating that much when the whole layout is properly designed, having it in an annex room and not at the center will make it much more difficult to heat evenly. The devil is in the details, and with thoughtful design you may even be able to make this idea work.

If the house sort of wraps around the porch so three sides can open to the house while one side opens outside, that may be a practical plan, especially if the porch is a step or two down from the rest so heat naturally circulates to surrounding areas.

Carlton: Your region doesn't show in your ID, but we do have at least one rocket heater (Bonny 8") that's been an effective heater for most of a 5000-sf shared home. I believe they have a little woodstove at the far ends of both wings so the folks there can have a little parlor heat-boost if needed, but the overall effect is they use 1/4 the firewood as previously (when it was the same two woodstoves plus a fireplace insert instead of the RMH).

The Daybed 6" plans show a bypass, and we have used them on other heaters.

For those wondering whether to do a mass heater, or just passive-solar mass: You might see about running the calcs for solar gain and what amount of mass could store heat for how long. builditsolar.com is one site with some nice calculators, and there are others.

There is nothing to stop an RMH from serving as passive-solar storage 90% of the time, and you can fire it up during a cold or cloudy snap as needed. The bypass, or a very short duct run, would be helpful in that case as you'd be more likely to have cold starts than in a constantly-used mass heater. I think this is part of why the fuel savings is so impressive in milder climates: the mass really evens out the daytime and nighttime temperature spikes, allowing you to 'coast' for much longer on surprisingly little energy.

-Erica

So here I am, two years later ...

interesting thought in this thread of using a rmh to cool a house instead of heat it. In the southern US homes use to have a cuppola above the stairs with windows in it they could open to pull air through the house coupled with floor to ceiling double hung windows that could be opened both top and bottom to let out hot air and bring in cooler air. But we took a tour of an old plantation house one time where the owners had put gas jets into their cuppola that they would light to draw air through the house better, effectively creating an air conditioner system that kept the house cool in summer. They used heat to draw heat up and out of the house.

So now I'm wondering how to modify a rmh to do the same thing in summer while still providing heat in winter when needed. Hmmmm. Great food for thought here.