Resilience Through Functional Design


Moving Heat

In the Introduction to Appropriate Energy, energy is defined as the ability to do work. There are three jobs that energy typically performs: moving heat, moving things, and moving electricity. This article delves into the moving heat function that energy serves. Examples of various techniques and specific elements are provided to illustrate different ways to move heat.

The techniques and elements described below are context-specific, meaning they efficiently and sustainably provide for the desired function at the specific site that they were designed for. They are not one-size fits all techniques – and in fact, what may work well at one site may be a poor fit for another site right next door. Before any energy system is designed (ideally as part of a whole site design process), the intrinsic characteristics of a site should be thoroughly compiled and assessed so that the extent of the energy resources available are well understood.

Moving heat is typically done in two ways: first, releasing and concentrating heat in furnaces, heaters, and stoves so that it can be delivered to living spaces, for household water, or for cooking food, and second, removing heat via cooling equipment such as air conditioners and refrigerators. This work can be done with a wide array of fuels and forces, including burning liquid and gas fuels, wood, or coal; by electricity; by expansion and compression of gas; via friction; and using the sun.

We break the typical residential applications for moving heat into three categories: cooking, space conditioning, and water heating.


Cooking food in a timely manner requires a concentrated energy source. Wood, coal, liquid, and gas fuels are the primary concentrated energy sources for residential and small-scale use. Of those, wood renews the most quickly in the mediterranean- and temperate-climate forest ecologies where we primarily focus our design work, and is typically the best option in terms of sustainability in those areas. Biogas is a gas fuel that can be developed on a home- or local-scale and has the potential to be used in the large selection of turnkey residential appliances (like ranges and ovens) designed to run off of natural gas and propane.

At rural sites undergoing new development, initially purchasing or utilizing portable “camp-style” cookstoves that can be set on top of a table and configured with a portable propane tank will provide cooking capabilities within a new space very quickly while other, more sustainable systems can be designed for and built. Even after those more sustainable systems are built, having a propane cookstove as an easily-accessible backup will be helpful when there is the inevitable night of poor preparation as the new systems are being learned and new habits of slower and more intentional movement are developed. Onsite biogas production can eventually be investigated, and, if found to be feasible, any propane-fueled stoves converted to decrease their negative environmental impact.

Propane/Biogas-Fueled Cooking Systems

A portable propane “camp-style” cookstove set on top of a table and configured with a propane tank is recommended for new residential and communal kitchens to provide cooking capabilities within a new spaces very quickly and allow residents and guests to quickly cook with familiar systems when needed.  While many are familiar with the Coleman-style camp stoves, Partner Steel makes a far more durable, long-lasting, and domestically-made option.

Partner Steel portable propane cooktop

Solar Energy-Fueled Cooking Systems

During the warmer periods of the year at sites with an abundant solar resource, when standing around a hot oven or cooktop may not sound desirable, a solar oven is recommended for cooking. Solar ovens use reflective material to focus the radiant heat of the sun towards an enclosed pot.

Solar cookers have a bit of a reputation for being a novelty item that sounds good in principle but doesn’t work well, but they have come a long way in recent years.  Some, like the Hanes Solar Cooker 2.0 shown below, have a cooking sleeve that insulates and elevates the pot so sunlight can be reflected onto the bottom of the pot. The adjustable sleeve eliminates the need for plastic cooking bags. This model boils a liter of water in about 50 minutes, is adjustable for high and low sun, and is packable and lightweight.

The Hanes Solar Cooker 2.0 and Dutch Oven

Wood-Fueled Rocket Cooking Systems

Rocket stove technology is an important technology for maximizing the use of the energy contained in wood and making it a feasible replacement for the more modern conventions of gas-fired cooking in forest ecologies. Rocket stoves burn wood at very high efficiency (generally 93 – 95%, meaning that 93 – 95% of the energy contained in the wood is converted to heat and is thus transferrable to the cooking vessel, and only 5 – 7% is lost as heat or unburned gasses to the surrounding environment) as opposed to traditional wood burning stoves burning larger logs that generally operate at 50% efficiency (and this is under ideal laboratory conditions – if wood is wet and air flow is not managed properly, traditional wood stove systems can operate in the teens for efficiency). This high efficiency is the result of a design that creates both a high temperature burn and efficient air mixing and secondary combustion of wood gases (smoke). Rocket stoves are so efficient that they do not emit smoke when constructed and operated properly. Most of what leaves the chimney is CO2 and water, as all the smoke that would typically be visible coming out of a standard chimney has been combusted. Rocket stove technology can be utilized to replace the typical cooking appliances, including cooktops and ovens.

Walker Stove Riserless Core Rocket Cooktop

Matt Walker’s riserless core rocket stove designs have enabled a flat metal cooktop to be the main radiative surface for the rocket stove while still maintaining the high efficiency burn characteristic of rocket stoves. The metal cooktop, on which pots, pans and kettles can be placed and moved around to find exactly the right temperature for the culinary task at hand, makes for a very flexible and easy to intuit cooking set up. Units can be quite small, and could be a good fit for some of the smaller spaces, and can also be quite large and capable of cooking for larger groups. Small and large versions are illustrated below.  The portable camp-style propane stove previously discussed can also be placed on top and used whenever a quicker, more conventional cooktop is desired.

Left: Tiny Masonry Cookstove. Image: Matt Walker, Walker Stoves
Full-size Masonry Cookstove. Image: Matt Walker, Walker Stoves

 For an in-depth dive into Matt’s riserless core technology, performance reviews and DIY tips, visit Matt’s YouTube video roll.

Dual-Chamber Cob Oven

Dual-Chamber Cob Ovens like the one illustrated below eliminate the smokiness typically associated with these types of ovens, allowing the chef to breathe clean air while tending the fire. Essentially, these ovens employ a two-door system linked with a strategically placed chimney that improves draw and airflow and makes for a cleaner, hotter, faster burn. These ovens get up to temperature faster and have a four times cleaner burn that traditional cob ovens. Plans for making a dual chamber cob oven are available through Watch Ernie Wisner’s video walkthrough of this system to see it in action. 

These ovens store heat in their massive walls, and are well suited to cooking pizzas and calzones, baking bread, and various types of slow cooking where the fire is lit, the mass brought up to temperature, and then the item to be slow cooked is set inside and the door shut for however long it needs to cook. Slow-cooked beans and yogurt making are good examples, and when not in use, the dry, dark, and potentially cool inside can be used for curing ferments when outside temperatures fluctuate too wildly. Pizza ovens are also an excellent community building tool and have a gravity all their own when organizing a work party with the promise of wood-fired pizzas at day’s end.

Dual-Chamber Cob Oven 3D View. Designed and drawn by Ernie and Erica Weisner.
Dual-Chamber Cob Oven Profile View. Designed and drawn by Ernie and Erica Weisner.

Rocket Powered Barrel Oven

Barrel ovens that utilize rocket designs, like the one shown below, are quick to heat (unlike traditional cob ovens), entirely wood fired, excellent for baking, and cheap and inexpensive to build. These systems can be used indoors as well as outdoors. They rely on a continuously burning fire for heat and do not have thermal mass storage like a cob oven.

Path of hot gases around the baking chamber. Design and drawing by Tim Barker
Sketch of rocket-powered barrel oven. Design and drawing by Tim Barker

Tim Barker’s plans for constructing both a barrel style oven and converting a more typical oven to take advantage of a wood-burning rocket engine are excellent and very approachable for the DIY enthusiast. For the more visual DIYer, the Rocket Ovens video manual will walk through all the steps to building one of these ovens start to finish.

Champion ND-TLUD Biochar Cookstove

The “Champion” Biochar Cookstove was developed by Paul S. Anderson in 2005, and later updated in 2008, as a wood burning cookstove that also produces charcoal as a byproduct of the cooking process. Ideal for sites with an abundant fuelwood resource (or when sites develop one), the Champion is classified as an ND-TLUD stove, ND standing for natural draft (the stove design creates its own draft) as opposed to forced air (a secondary power source and fan push air through the system), and TLUD standing for Top-Lit UpDraft (the manner in which the fire is lit, from the top of the fuel column and burning downwards).

The Champion Biochar Cookstove does not actually support the weight of the pots or cooking implements – those are instead supported by a pot stand or other cooking surface. This is critical as it allows the cookstove to be removed from underneath the pot or cooking surface once pyrolysis has completed and combustion of the charcoal has begun, enabling the user to preserve the charcoal. To allow for continuous cooking Champion stoves are used in tandem, so that a second fuel cylinder can be lit and swapped out with the first once pyrolysis is complete. The fuel cylinders act like batteries that can be quickly changed in and out to cook for however long is needed, and each cylinder yields charcoal at about 25-40% the original dry fuel volume.

Biochar cookstoves are still relatively unknown, and cannot yet be purchased, however they are easy to make. For construction plans detailing four different methods for construction a Champion-style ND-TLUD biochar cookstove see Paul S. Anderson’s Construction Plans for the “Champion-2008” TLUD Gasifier Cookstove PDF on the topic on his website.

Mass production Champion model cookstove assembled in Chennai, India.
Champion style biochar cookstove built by the 7th Generation Design team using scavenged materials.

Space Heating and Cooling

Space temperature moderation (“conditioning”) can be provided passively and actively. Passive conditioning (heating or cooling that does not require the ongoing use of fuel obtained by human energy, typically achieved by gathering or deflecting the sun’s energy) can be provided through the intelligent siting of and climate-appropriate design of spaces, structures, and their surrounding environment. In some contexts, passive heating and cooling strategies will be sufficient in themselves to maintain comfort; however, for many sites, some level of active heating or cooling will still be required to maintain reasonable levels of comfort during the heights of summer or depths of winter. Additionally, there are typically existing structures onsite that were not designed and built to maximize passive conditioning strategies, or there are time constraints that require the purchase of pre-fabricated or more conventionally-built structures as opposed to the often cheaper but more time intensive building methods used to maximize passive conditioning.

Conventional active space-heating systems include wood stoves, liquid and gas fuel-fired heaters, and electric heaters. In forest ecologies, wood is often available (or fuelwood-producing systems can be designed at a site to make it so), renews more quickly than liquid or gas fuel, and as with cooking, can be maximized using rocket technology to improve combustion efficiency. Of these options, electric heating is the most inefficient, and should not be used at a site with sustainability goals except in the most unique of circumstances. 

Conventional active space-cooling systems include electricity-powered fans, electricity-powered and water-fueled evaporative coolers (“swamp” coolers) and electricity-powered direct expansion (“DX”) units. 

Passive Strategies for Space Conditioning

Passive strategies for space conditioning should be thoroughly considered during the design phase of any new structures at a site. Passive strategies for influencing the temperature of an interior space are those that don’t consume fuel. The main strategies for this include 1) keeping heat where it’s wanted via insulation and leakproofing, 2) storing heat with thermal mass, and 3) controlling solar gain (how much sunlight gets inside).

Keeping Heat Where It’s Wanted

Ensuring that any existing and future structures are properly insulated and leak-proofed (except where air intake or exhaust vents are intentionally placed) is hands down the biggest bang-for-the-buck strategy for controlling the temperature of indoor spaces. In modern construction methods, this is typically done using batts of insulative material held against the roof, floor, and exterior walls. In natural construction, this involves selecting natural construction materials and methods that are insulative. Earthbag, cob, and clay-straw slip wall construction are both highly-insulative natural building techniques, while polewood wall construction (log-cabin style, etc) is among the least. Double-pane windows should be utilized, especially in new construction, where the payback is much more attractive than upgrading from existing single pane windows to double pane. In terms of leak-proofing, doors and windows are the primary offenders. The small cracks around the windows and under the doors add up – older structures often have enough leaks to amount to several square feet, which is the same as having a fair-sized window open all of the time. Windows should be properly sealed, and weather-stripping used to seal any door leaks.

Storing Solar Heat with Thermal Mass

A material that has thermal mass is one that has the capacity to absorb, store and release heat energy. Its density and levels of conductivity help to keep the internal temperature of a building stable. They are generally dense materials, such as concrete, stone, brick, cob, ceramics, and water.

This image has an empty alt attribute; its file name is Sun-Warmed-Cob-Bench-And-Earthen-Floor.jpg
A sun-warmed cob bench and earthen floor make excellent thermal mass heat storage during the cold months by capturing energy from the low-angle sun rays entering through the windows. During the warmer months when the sun is higher they will act as heat sinks and keep the home cooler. Image credit: unknown.

Objects that have thermal mass have inherent qualities for both heating and cooling. When thermal mass is used for its heating capacity, when exposed to sunlight, the thermal mass absorbs the sun’s heat energy. The heat energy that is absorbed will slowly spread (conductivity) through the mass and then radiate or release the heat energy throughout the evening and night – particularly nice during the winter months. If dense objects with thermal mass are kept out of the sun, they will absorb the warmth from the surrounding air, helping to cool a space. Getting sunlight onto thermal masses during the winter months and keeping it off of them during the summer – “controlling solar gain” – are described below.

Controlling Solar Gain

In order to get sun on interior thermal masses during the winter but keep it off during the summer, roof overhangs can be utilized to take advantage of the sun’s seasonal angles. This is shown in the figure below.

Utilizing architecture to provide summer cooling and winter heating. Image Credit: unknown

One design challenge here is that the sun is at the same angle in the spring and fall, but the fall is warmer in many areas than the spring. Deciduous trees on the south side of the structure, or a trellis over the south-facing windows with a deciduous vining plant, can be utilized here to provide sun during the spring and shade during the fall, as shown below.

Another design approach here is to control the size and placement of windows. Most passive solar experts recommend that the glassed area equal no more than 8 to 12% of structure’s total floor area, with 70-85% of that on the south-facing wall, 10-15% on the east wall to catch morning sun, and 5-10% on the west wall to avoid overheating on hot afternoons. 

More extensive resources on passive solar design are available, including the National Institute of Building Sciences web page on passive solar design.

Earth Tubes

In addition to properly siting structures to minimize cooling requirements and employing structural design strategies to maximize passive cooling and heating, subterranean earth tubes can be utilized for additional passive cooling.

This method utilizes 4” conventional, thin wall plastic sewer drain vent pipe to passively heat or cool a home’s fresh air intake with zero-energy consumption. Fresh air enters the series of non-porous pipes embedded around a home’s foundation, where it is then piped underground and heat is either transferred to it from the surrounding soil (in the case of heating) or transferred from it to the surrounding soil (in the case of cooling). 

Earthtube function is maximized in a high-thermal mass home, like those built using earthbag, cob, or strawbale construction.

Passive cooling using subterranean tubes. Image Credit: unknown

The exhaust of warm air from the home is necessary to ensure that suction is created at the fresh air intake and air is pulled through the system correctly.  This can be achieved through the use of a solar chimney or exhaust fan, both discussed in our Moving Things blog post. Incoming air can also be humidified and filtered by placing trays or screens of hydrated charcoal just prior to the tube’s entry into the living space (this type of design has a long history in Middle Eastern countries). 

Active Strategies for Space Conditioning

Wood-Fired Rocket Systems for Space Heating

Just as for cooking, wood rocket technology is an important technology for maximizing the use of the energy contained in wood and making it a feasible replacement for the more modern conventions of gas-fired heating in forest ecologies. 

Rocket Mass Heaters for Supplemental Interior Space Heating and Cooling

This style of wood-fired rocket system helps to drastically lessen the inherent inefficiency of using high temperature flames to heat a relatively low-temperature space by employing an earthen mass in the form of a bench to both store and radiate heat. For spaces that are likely to be occupied frequently and for long durations, these mass benches provide an excellent thermal battery that can be used to moderate indoor temperatures for long periods of time, as explained wonderfully by Paul Wheaton on YouTube. They also help to keep a space cool – a bench thermal mass will act as a heat sink in the space on hot days.

Paul Wheaton gives Justin Rhodes a rocket mass heater thermodynamics crash course.

Rocket mass heaters that feed into a mass like the one pictured below typically employ a J-Tube design, but can also employ the Batch Box design as detailed further below. The wood pieces used to fuel a rocket mass heater can be relatively small, typically the size shown in the image below, and thus can be served well by scrap wood, sticks and small branches, and even some brushy fuels.

Passive Strategies for Space Conditioning. Image: Quail Springs Permaculture

The 55 gallon steel barrel seen commonly in this style of rocket mass heater design is known as the “bell”, and provides a way to quickly heat a space in addition to functioning as part of the powerful draw mechanism of the chimney. The bench mass can take longer to warm up to a point where it is radiating heat, especially if it has not been utilized recently and has reached ambient temperatures, but will also continue radiating heat long after the fire has intentionally or unintentionally been extinguished. If a design like this is agreeable and a good fit within the recommended buildings, the Rocket Mass Heater Builder’s Guide by Erica and Ernie Wisner is an excellent manual for DIY construction.

Batch Box Rocket Heater

The Batch Box style of rocket heater is recommended for spaces that need to be heated quickly where aesthetics are not a concern – such as workshops or garages. Batch box rocket heaters allow for a larger load of wood to be set into the combustion chamber at a single given time, thus providing a longer total combustion window before the stove must again be fed. This design is very efficient and burns very clean and hot. The shop heater application detailed on YouTube with the inventor Peter Vandenberg shows the double bell system capable of heating a large space quickly. These systems can be engineered to release heat quickly into a space, as shown below, or to charge a thermal mass for long term temperature moderation.

Peter van den Berg explains the operation of his Batch Rocket Mass Heater.
Schematic of the batch box rocket stove system by Peter Vandenberg.
A double bell, massless batch box shop heater designed and built by Peter Vandenberg.

Critical to the Batch Box design’s efficiency is the use of a secondary air injection channel that draws in and mixes pre-heated, oxygen rich air into a low-pressure zone at the exit from the burn chamber to the heat riser. Typically a metal tube called a P-Channel is used, although fire bricks can be utilized as well. This injection of oxygen creates a secondary combustion so complete that there is zero smoke when operating at design temperatures. Designs for Batch Box Rocket Heaters are available online for free as open source plans. Dimensions and ratios must be kept very exact in building a batch box chamber, lest the burn dynamics within the chamber be degraded.

DC Solar PV and Battery Bank-Powered Compression Refrigerators/Freezers

Refrigerators and freezers remove heat from a small, enclosed space for the preservation of organic materials like food or medicine. For off-grid structures in an area with a high solar resource, refrigerators with DC-powered compressors that can be hardwired to the battery banks are recommended. Further information on these units that move refrigerant to provide cooling is provided in our Moving Things blog post.

Water Heating

Hot water for bathing, clothes- and dish-washing, while not a necessity, is a luxury most would have a hard time sacrificing. In modern home design, hot water systems are often very energy intensive in order to ensure that hot water is available 24/7/365 in all locations throughout the home with no input on the part of the landowner besides a monthly payment to the gas company and replacement of a water heater every 10 years.  If there is a need to have hot water available in this way, there isn’t a whole lot of opportunity for reducing energetic and environmental footprint besides utilizing on-demand water heaters. However, if expectations and habits can shift in such a way that a resident can plant ahead a bit and bundle all of the hot water needs into a narrower time block, it becomes possible to not only have hot water with a small fraction of the energy used in a conventional system, but to utilize local energy sources in obtaining it.

Wood-Fired Rocket Mass Water Heater with Supplemental Solar Heating

Recommended for sites with a large supply of fuelwood, rocket mass water heaters function by transferring heat from the hot exhaust gasses to a large mass of unpressurized water. The heat from this water mass is then transferred to pressurized water contained within an internal copper coil. The image below details the schematic for such a system. Geoff Lawton’s video walkthrough of the system in use at Zaytuna Farm provides an excellent overview of how the system works.

Geoff Lawton’s video walkthrough of Tim Barker’s Rocket Stove Hot Water System built at Zaytuna Farm.
Rocket stove hot water heater schematic. Note how the heated water is contained in a non-pressurized container, thereby eliminating the risk of explosion. Credit to Tim Barker for the design and Good Life Permaculture for the drawing.

At sites with an abundant solar resource, the unpressurized water mass can be preheated by circulating water from the unpressurized water mass through a solar hot water heating panel using a solar PV-powered electric pump. This will certainly shorten the wait time until hot water is available once the fire is lit, and may even be able to provide warm or hot water without the need for a wood fire during certain times of the year. Maximizing this system’s effectiveness will require that a resident bundle all of the hot water needs into a block every evening, and plan ahead enough during the overcast days enough to gather a few small sticks, light a fire, and wait 30  minutes for that hot water.

Integration of solar hot water heating system with rocket-heated unpressurized water mass. Image Credit unknown.

That’s a wrap on our overview of some of the different methods of Moving Heat on a homestead! We’ll be continually updating this with new information. Please leave a comment below, and make sure you check out our Introduction to Appropriate Energy if you haven’t already and the remaining parts of this series, Part 2: Moving Things and Part 3: Moving Electrons!

And if you would like help designing for the energy needs of your site, please reach out to us!

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