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The Way of the Warming

This fun and hopefully not-too-confusing article talks about how a heater heats a space and why it's better for even tight and "passive" homes than any other natural heating system.  

     I don't talk about the radiant aspect of the heat or how good it feels here, but that's one of the best reasons to install a Masonry Heater.  Here we focus on BTUS.

    The Way of the Warming

    Warmth is constantly expressed from the surface of a charged stove, and that alone is hard to wrap one's mind around.  We're so used to baseboards that cycle on and off, or woodstoves that run out of fuel and cool off it's hard to imagine the feeling of a constant emitter.  Luckily, we all have experience with the sun. 

         Think of the way a sun warms a well-placed patio space in the spring or fall.  The sun rises and it's warmer, warmer, warmer, until the wind blows or the sun sets.  

         Now think of the home's walls as objects that slow the wind and the sun as something that reaches a balanced output with the wind.  It's like that Spring day when the sun is shining and you feel like you should need a sweater, but the sun keeps you comfy in your shirtsleeves.  Confused?  Let me try another analogy. 

         Think of the home as a bathtub.  The tub's drain size represents the heat loss of the home, and the faucet's Gallons-Per-Minute flow rate represnts the heat coming from the heater.  Properly matching the drain and faucet allows even a large bathtub to be filled to the right level (temperature).  The smaller the drain (heat-loss) the larger the space that can be kept wet (warmed).  

         Masonry Heaters are not dragsters.  We don't go 0-60 very fast (in HVAC world that's like heating from 50 to 70 degrees F), but what we do incomprably well is to maintain a super-comfy atmosphere in the heated space with only one or two simple and quick firings per day.  


    Sizing and Output

         There are different ways to approach making a stove to the right output, but the one I feel is most accurate at any given moment is surface area.  A given material at a given temperature gives off a given amount of heat (which I might express as watts per square meter or BTUs per hour per square foot). 

         Whether that surface area is a simple box seven feet tall in the middle of the room or a bench running around the perimeter of a sunken living room, each charged square foot that's exposed and radiating into the space being warmed will put out a fixed quantity of heat energy.  

         So when we design a stove, we think about surface area, firebox size, and channel length or bell size.  In this way, the stove is a cohesive unit designed to meet the range of heating need the home will experience throughout the heating season.  

         A given length of channel will absorb a given amount of energy, so if the channel is too short for the amount of wood you're burning you'll be heating the air above the chimney, and if it's too long you'll be condensing water vapor out of the exhaust and losing draft.  While the latter is possible I've never seen it (but then, I've never tried either).  Both spreadsheets and experience play a role in design of heat-exchangers.

         This is how we make a heater the right size and wall-thickness for any home.  

    Heaters in Superinsulated Homes

         Homes that are designed to minimize heating need run into a particular "sustainable" heating.  Once you've taken pains to eliminate thermal bridges and mazimize insulation, paid top dollar for thiple-paned windows and designed the home to maximize solar gain, you're left with such a small heat load that many will tell you that an electric heat pump is about your only option to keep warm.  The good news though, is that you don't have to give up on heating with wood.  Moreover, "passive" homes give a great opportunity to make a super Architectural Grade Heater.  Read on to learn what makes our heaters the best way to build sustainability in to your next Green home project.  


        Let's think of a 1,000 square foot room.  If you want to maintain around 72 degrees in that room, the radiating surface needs to be warmer than the room or heat won't flow into the room.  Outupt of any radiant surface depends, as mentioned above, on surface area and temperature.  Most radiant floors operate at a minimum of 80 degrees.  80 degrees x 1,000 square feet = what I'll call 80,000 Magic Heat Outputs (MHOs).  I call it this to avoid actual math and use instead the simple conceptual math that always produces nice round numbers.  

         In a super insulated 1,000 square foot home, even 80,000 MHOs will raise the temperature too high, and because it's thermal mass based and 4" of concrete is a typical thickness for a radiant floor, you're stuck with this output for a long time, so the system will shut down and the house will cool off until the floor is well-below active temperature, at which point the boiler will go "click" and the floor will becgin to charge again, and the house will slowly transition from too cold for the last six hours to too warm for the next six hours.

         Let's compare that equation with a Masonry Heater for that same space.  Our theoretical heater is a 24"diameter cylinder about 6' high, which gives it a surface area of around 36 square feet.  Let's put a fire in that stove and get the surface up to 130 degrees.

           At 130 degreees our 36 square feet puts out, say, 8,000 Magic Heat Outputs, and let's say the wall thickness is balanced to give that output for 12 hours.  Assuming 80% combustion efficiency that means we're burning 10,000 MHOs of wood in our firebox all at once.  That happens to be about 9"x 12" worth of sticks laid upright.  Stack it, light it, and when it's done, shut the damper.  It's easy, and you get convenience and that sense of security that wood burning brings, without the long-heating cycle of the radiant floor and gives you the enjoyment of a real wood fire and the security of locally available renewable fuel.


       Where the Radiant Floor fails be being too massive, the woodstove fails by being to... un-massive.   Light a fire and you boil yourself out in a short time, because wood burns cleanly only at high temps, and a woodstove built for an 8,000 MHO need doesn't actually exist.  If you made one it would have a very small firebox that would hard to lay, light, and tend, to the max.  This is why pellet stoves exist, and they make noise and tend to clog and break down.  


          Masonry Heaters with appropriate wall thicknesses, fireboxes, and surface areas solve both these problems at once.   The first by shrinking surface area, and the second by buffering the heat from a reasonable size fire (that is, a fire that is convenient to lay and light - no need to tend it). 

         Rather than an impossibly small fire impossibly well tended for a 8 hours you have an easy, right-saized fire that burns for 30-45 minutes and charges the right-sized mass for 8-12 hours of output.  The space is kept warm without blowers or augers, and most importantly, the experience of burning real wood is provided.   

         Larger stoves for less "passive" spaces have thicker walls and longer channels, so there's a bigger fire and a longer firing cycle.    


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