Can a fire do more than keep you warm? Yes: it can pump water, turn a saw, charge a phone and even make ice. This guide gives the honest numbers first, then covers stove-top thermoelectric generators, small steam engines and boiler safety, hot-air (Stirling) engines, refrigerators that run on a flame, and other ways to put heat to work. The books added for this guide are in Southern-Oregon\Off-Grid-Power\Heat-Engines (download list: local/library-files.txt). Wood gas, the best way to run a generator on wood, is in the Fuel guide and the Engines guide; storing heat in water and stone is in the Storing Energy guide. Where this guide works a number out, it says (arithmetic); check it with the Practical Math guide or the Calculator.
The short version
Use heat as heat first. A stove gets about half of the wood's heat into the room, the pot or the water tank; the heat engines in this guide turn only 1 to 10% of it into work or electricity (Aprovecho, p. 18; table below).
For real electricity from wood, run an engine on wood gas. About 3 lb of wood per kWh at best, against 10–25 lb for a small steam plant (arithmetic, below).
A stove-top thermoelectric generator is worth having for phones, LED lights and radios while you heat or cook anyway. Real stoves in a DOE contest made 14 to 268 watts (DOE stove report, p. 17).
Boilers explode. Never make one from a tank, a pipe, a water heater or a gas cylinder (rule of thumb). Test a boiler with water at 1½ times its working pressure, never with steam or air, and try the safety valve every morning (Maxims, p. 29, p. 121, p. 285).
If the water drops out of sight in the glass, kill the fire and never pump in water (Navy Boilerman, p. 324).
Hot-air (Stirling) engines are safe but weak: an antique one made about 50 watts (NASA Stirling Manual, p. 8). Use one to pump water slowly.
A gas or kerosene absorption refrigerator is the best heat-driven machine you can own. Keep it level: one stopped cooling when tipped just 1 to 2 degrees (NBS report, p. 33).
Every flame makes carbon monoxide (MSU Portable Kerosene Heaters, p. 3). Vent burners outdoors and keep a CO alarm with batteries (rule of thumb).
🧮 The honest numbers: what a pound of wood can do
A pound of dry wood holds about 8,600 BTU of heat, and a stove gets roughly 50% of it into the house (Aprovecho Capturing Heat Three, p. 18). That is about 2.5 kWh of heat in the wood and 1.25 kWh delivered (1 kWh = 3,412 BTU) (arithmetic). One horsepower is 746 watts (DOE Wind Power for Farms, p. 27), so one horsepower for an hour is 2,545 BTU of work (arithmetic).
Way of using the heat
Share of the wood's heat you get back
From 1 lb of dry wood
Worth building?
Heat used as heat (house, cooking, hot water, drying)
About 50% as useful heat
About 1.25 kWh of heat
Yes, always first
Wood gas in an engine generator
About 12% as electricity, at a good steady load
About 0.3 kWh (3 lb per kWh)
Yes, if you can build and tend a gasifier
Hot-air (Stirling) engine
7–10% for the best small factory engines; homemade ones far less
25–200 Wh
For slow pumping; homemade ones are teaching models
Small steam engine (no condenser)
About 2–4% as shaft work
50–100 Wh of work; 10–25 lb of wood per kWh
Only for heavy mechanical work, with a real boiler and a trained operator
Thermoelectric module on a stove
About 3% of the heat that passes through it; well under 1% of the stove's whole fire
A few watt-hours, but the heat still warms the room
Yes, as a free extra while you heat or cook
Where these come from (arithmetic, from the pages named):
Wood gas: "about 20 lb of wood have the energy equivalence of one gallon of gasoline" (FEMA Wood Gas Generator, p. 70). A gallon of gasoline is about 125,000 BTU (18,800 BTU a pound, 7½ gallons weigh 50 lb), and an average engine is "approximately 20 to 25 percent thermally efficient" (TM 9-8000, p. 74). At 20–25%, less about a fifth lost in the generator (rule of thumb), 20 lb of wood makes about 6–7 kWh. A generator running a light load wastes much more fuel per kWh (rule of thumb).
Steam: "5 pounds is a common consumption of coal" per horsepower-hour in stationary engines (Hawkins New Catechism, p. 19), and coal holds about 11,000–12,700 BTU a pound (Univ. of Minnesota EB 436, p. 16): about 4%. A farm engine boiler is sized to boil off 7½ gallons of water an hour per horsepower (Maggard Rough and Tumble, p. 53); at about 1,146 BTU in each pound of steam (Maxims, p. 202) that is 3.6% of the heat in the steam. If the boiler gets about half the fire's heat into the water (Aprovecho's 50% rule of thumb, p. 18), that is about 2% of the wood's heat.
Stirling: a small Philips air engine made at most about 0.48 kW at 7.75%, and about 10% at its best setting (NASA Stirling Manual, p. 57).
Thermoelectric: a 25-watt Stirling generator, about 10% efficient, "uses only 27 percent of the fuel required by a 25 w (e) thermo-electric generator" (NASA, p. 405), which puts that thermoelectric generator near 3%. On a wood stove most of the fire's heat never passes through the modules. The best stove in the DOE contest made 276 Wh in 2½ hours (DOE stove report, p. 17); if it burned about 6 lb of wood an hour, like Aprovecho's example house stove (p. 18), that is under 1% of the fire.
What it means: a stove that heats the house and also charges the phones is a good deal. A fire built only to make electricity is not: a steam plant burns 10 to 25 lb of wood for each kWh, about what a fridge uses in a day (rule of thumb). If you need real power, see wood gas in the Fuel guide, and solar and creek hydro in the Off-Grid Power guide.
Heat one end of a joint between two different metals and a small voltage appears across it: the Seebeck effect (Kuphaldt Lessons in Electric Circuits vol. 1, p. 320). Many junctions in series make a thermopile: heat goes into one set and the other set is "bonded to a heat sink of some sort (air- or water-cooled)" (p. 323). A modern module (TEG, thermoelectric generator) is a flat square tile packed with such junctions. Power comes from the difference in temperature between its two faces, so a hot stove with a warm cold side makes almost nothing (rule of thumb). You can see the effect with a copper wire, an iron wire, a candle and a voltmeter (Kuphaldt vol. 6, p. 117).
What to expect in watts
Measured on real wood stoves: in a 2½-hour DOE-funded contest, five stoves fitted with thermoelectric generators peaked at 268, 139, 85, 32 and 14 watts and made 276, 235, 118, 47 and 28 watt-hours (DOE stove report, p. 17). The two that reached the market "were larger units closer to boilers" (p. 4).
What that runs: such stoves "can generate enough power to charge cell phones and light a home" in an outage (p. 2). A phone charge is roughly 10–15 Wh and an LED bulb 3–5 W (rule of thumb), so 28 Wh is two phone charges, and 276 Wh is a lit house for an evening plus every phone.
A single small module on a stove top makes a few watts; Wikipedia notes they are "put on top of a wood or coal burning stove" and "generate enough electricity to power an LED light or charge a mobile device" (Thermoelectric generator).
Stove fans (the little fans that sit on a stove and spin by themselves) run on the same modules (same article). They push warm air into the room with no wiring at all.
Choosing modules: TEG or salvaged Peltier
Modules sold as TEGs (generator modules) are built to take a hot side. Peltier modules from 12-volt camping coolers, wine coolers and computer coolers are built for cooling, but they also generate when heated on one face, usually less than a true TEG (rule of thumb).
Heat limits: most modules are bismuth telluride, which Wikipedia lists among the low-temperature materials, "up to around 450 K" (about 350 °F, 177 °C) (Thermoelectric generator). Cooling modules are often soldered for less. Never set a module straight on a stove top running at full heat; put a steel or aluminum plate between them, check it with a stove thermometer, and stay under the seller's rating, or about 250 °F (120 °C) for a salvaged Peltier you know nothing about (rule of thumb).
Testing a salvaged module: hold one face on a cup of hot water and the other in the air, and read the voltage on a meter; no voltage means a broken module (rule of thumb).
Keeping the cold side cold
The cold side is half the machine. Wikipedia: "The cold side must be cooled by air or water" (Thermoelectric generator).
Clamp the module firmly between the hot plate and a heat sink (a finned block from an old computer or amplifier) with a thin smear of heat-sink paste on both faces (rule of thumb).
Water cooling works best: a flat-bottomed pot or a closed metal box of water on top of the module. The water warms, so top it up or swap it; never let it boil dry (rule of thumb).
Air cooling: a big finned heat sink, ideally with a small fan run from the module's own output (rule of thumb).
A module whose two faces reach the same temperature makes nothing; if the output sags after an hour, the cold side has warmed up (rule of thumb).
Getting usable 5 V and 12 V out of it
A single module gives a few volts that rise and fall with the fire, so it needs a regulator between it and anything you care about (rule of thumb):
Phones and USB lights: wire two to four modules in series (plus of one to minus of the next) and feed a small DC-DC converter with a USB socket: a buck converter steps a higher voltage down to 5 V, a boost converter steps a lower voltage up (Buck converter; Boost converter). Never plug a phone straight into a module (rule of thumb).
A 12-volt battery: use a converter set to about 14 V, or a small charge controller, plus a blocking diode so the battery can't push current back through the modules at night (rule of thumb). Charging limits and controllers are in "Charge controllers and inverters" in the Off-Grid Power guide; diodes are in "Motors as generators" in the Engines guide.
Good loads: phones, LED lights, a radio receiver (see the Radio guide), a fan. Bad loads: anything that heats (kettles, heaters, hair dryers); use the stove for heat.
What steam is good at: steady, heavy mechanical work from any fuel that burns. Farm steam engines were belted to threshing machines, and a steam engine copes with a load that changes a lot: Maggard's farm engine might "do eight horse work one day and sixteen horse work the next day" (Maggard Rough and Tumble, p. 11).
What a small one needs:
Water: about 7½ gallons an hour for each horsepower (Maggard, p. 53). A 1-hp engine running an 8-hour day boils off 60 gallons unless you condense it (arithmetic).
Wood: at 2–4%, one horsepower-hour (746 watt-hours of work) takes about 7–15 lb of dry wood(arithmetic, from the table above). Someone has to cut, split and feed it.
A real boiler: a factory boiler for a ½-horsepower engine was "tested at 160 pounds cold water pressure" and rated "safe to run it at a steam pressure of 75 to 100 pounds" (Collins Boys' Book of Engine-Building, p. 267). Watson's book gives dimensioned drawings for engines and boilers "from five horse power down to model sizes" (Watson Small Engines and Boilers, p. 13), but a pressure boiler is skilled shop work in seamless or brazed tube (p. 118); a soldered tin boiler "is merely a makeshift" (p. 115).
A trained operator who stays with it the whole time it is under pressure. Hawkins: "Don't let any one talk to you when firing" (Maxims, p. 48).
Turning a pump, saw or generator
Belts carry the power from the engine's flywheel to the machine; a belt "ought to sag slightly as it hugs the pulley closer, and will last much longer" (Maggard, p. 138).
A pump or a saw suits steam best: both take steady power, and a saw can wait while the pressure comes back up (rule of thumb).
A generator: belt the flywheel up to a car alternator or a permanent-magnet DC motor and charge batteries; the speeds and wiring are in "Motors as generators" in the Engines guide. Steam engines slow down under load, so a governor or a charge controller is needed to keep the voltage steady (rule of thumb).
Condensing and reusing the water
Exhaust steam still holds most of its heat. At 212 °F each pound of exhaust steam "contains 1,146 heat units" (BTU) (Maxims, p. 202). Pass it through a feed-water heater to warm the water going into the boiler: "All the heat imparted to the feed water, before it enters the boiler, is so much saved" (p. 201).
Heat buildings with it: in most works "the exhaust steam produced will very nearly, if not quite supply sufficient exhaust steam to furnish all the heat required for" warming (p. 271).
Condensing it turns the steam back into water: the Navy's condensers "recover feed water by condensing exhaust steam" (Navy Boilerman, p. 245). Condensed steam is soft water and lays down little scale (rule of thumb). It takes a lot of cooling water: about seventy gallons an hour for each horsepower (Hawkins New Catechism, p. 361). A coil in a creek or a big tank is the homestead version (rule of thumb).
Use soft water. Hawkins wants "soft or rain water" even in a gas engine's water jacket, because hard water fills it with lime (New Catechism, p. 296); a boiler needs it more (rule of thumb). Scale on boiler plates is "a more or less perfect non-conductor of heat" (Maxims, p. 154), and the plate under it can be "heated to redness" (p. 152).
💥 Boiler safety: read this before you light a fire under anything closed
Why boilers explode
"The one great cause of boiler explosion is the inability of the boiler to withstand the pressure to which it is subjected at the time" (Hawkins Maxims, p. 284). Hawkins lists the reasons (pp. 284–285):
Bad design (not enough stays and braces).
Bad workmanship.
Bad material: blisters and flaws in the plate.
Too much pressure: a reckless operator, a wrong gauge or a stuck safety valve.
Overheated plates from low water.
Scale and mud that keep the water off the iron.
The farm engineer's version: "A properly constructed boiler never explodes, except from low water or high pressure" (Maggard, p. 20). The damage comes from the hot water flashing into steam all at once when the shell lets go (Maxims, p. 284; Boiler explosion).
What never to use as a boiler
(Rule of thumb, from the failures in the books)
No propane or air tanks, fire extinguishers, pipe with caps on, kegs, milk cans or old water heaters as a steam boiler. They were never built or tested for fire on one side and steam on the other, and you can't see the corrosion inside.
No soft-soldered or tin boilers: a soldered tin boiler is good for about 10 pounds and "is merely a makeshift" (Watson, p. 115).
Even water heaters go bang: Aprovecho's "most carefully built wood fired showers … occasionally leaked or even exploded!" (Aprovecho, p. 9). Any closed tank over a fire needs a working relief valve (see "Hot water" in the Storing Energy guide).
Never repair pressure parts yourself: no welding, brazing, patching or plugging of a cracked, bulged, thinned or leaking shell, tube or firebox. Retire it from pressure use.
The safety valve
Try it every morning before lighting the fire, by lifting the lever or rod by hand (Maxims, p. 29). A valve that sticks can stick "even though tried every day", if it is only lifted and dropped back (p. 198).
Nothing between the valve and the boiler: any shut-off valve there "should be taken out at once" (p. 131).
Never add weight, wedge it or screw it down to hold more steam. Maggard warns that a weak gauge tempts you to screw the valve down, when you may already have 110 pounds or more on a boiler set for 100 (Maggard, p. 79).
Test the gauge against a known-good gauge before you set the safety valve by it (same page).
If the pressure is already far too high, don't suddenly lift the valve wide open: the rush can hit the shell like a water hammer (Maxims, p. 198). Pull or smother the fire and let it cool (rule of thumb).
Keep the water in the glass, at the same height all day (Maxims, p. 48). Above the top of the glass, water carried into a hard-working engine can "knock out a cylinder head"; below the lower gauge "you may get your own head knocked off" (Maggard, p. 19).
Don't trust the glass alone. Open the try-cocks (gauge cocks): the lower one should give water and the upper one steam. Use them every day or they lime up and become useless (Maggard, p. 20).
An empty glass looks like a full one. "The glass looks much the same when empty as it does when full" (Navy Boilerman, p. 323). Blow the glass through to be sure (p. 324; water gage column).
Blow down (open the blow-off cock to flush out mud) once or twice a day, more with dirty water, and close it again "when the water in the boiler has sunk to one and a half inches" (Maxims, p. 48): watch the glass the whole time (rule of thumb).
Check the water before lighting up each morning: "very many boilers have been ruined by neglecting this first simple precaution" (p. 29).
The fusible plug
A brass plug filled with soft metal sits in the crown sheet over the fire. If the water drops below it, the metal melts and steam blows into the fire to put it out (Maggard, p. 81; fusible plugs; Fusible plug). Some states required one by law (Maxims, p. 175); Maggard thought it made operators careless (p. 81). Keep the plug, but never count on it: it is a last warning, not a water gauge (rule of thumb).
If the water is low
The Navy's rule, for when the water goes below the bottom of the glass or you don't know whether it is high or low (Boilerman, p. 324):
Stop the fire. On a wood fire, smother it with dirt or ashes (Maggard, p. 162); never throw water into the firebox, which can crack the sheets (same page).
Shut the feed. "You must never feed water to a boiler in an attempt to restore the water level after a low-water casualty!" (Boilerman, p. 324). Hawkins agrees: "Don't pump cold water into a hot boiler" (Maxims, p. 48).
Shut off the steam to the engine and get people away from the boiler (rule of thumb).
Let it cool slowly, then look inside for bulged or scorched plates before you use it again (rule of thumb).
Testing with water, never with steam or air
"Testing new boilers under steam pressure is both dangerous and unwise"; a water test does the same job safely (Maxims, p. 285). Water barely compresses, so a leak just drips; air or steam stores energy and a failure flies apart (rule of thumb).
Test pressure: 150 pounds of water pressure for every 100 pounds of working pressure (1½ times) (p. 121).
How: fill the boiler "with water to safety-valve" (Maxims, p. 134), pump it up with a hand force pump, and look for leaks, weeping seams and bulges (Maggard, p. 87; Collins, p. 123).
Even water tests hurt people: a blown rivet in one test scalded five men (Maxims, p. 285). Stand to the side.
Low-pressure and no-pressure choices
Don't need shaft power? Don't make steam. Hot water and space heat need no pressure at all; an open, vented tank can't build pressure (rule of thumb).
A hot-air engine uses the fire's heat directly with no boiler, and is "by all odds the safest to use" (Collins, p. 215). See the next section.
A thermoelectric module has no moving parts and no pressure (above).
Model engines: a copper boiler 4 inches across and 12 inches long will run an engine with a 1-inch cylinder "all day long at 1,000 revolutions per minute" at 50 pounds (Watson, pp. 115–117); even a model needs a safety valve, a water gauge and a water test (rule of thumb).
A hot-air engine (today called a Stirling engine) keeps the same air shut inside. One end is heated by a fire and the other is kept cool. A loose piston called the displacer (or transfer piston) shuffles the air between the hot end, where it expands and pushes the power piston, and the cold end, where it shrinks again; a heavy flywheel carries it round (Collins, pp. 215–216; how it works; Stirling engine). Better engines add a regenerator, a mesh or plates between the hot and cold ends that soaks up heat from the air going one way and gives it back on the way back (Hawkins New Catechism, p. 324).
Why bother: they can "use any source of heat", run quietly and last a long time (NASA Stirling Manual, p. 7); there is no boiler to burst. Why they lost out: the old air engines had "high reliability and safety, but low specific power" (same page): big and heavy for the work they do.
What they really deliver
Antique hot-air engines: about 50 watts for an old machine; a modern air engine of the same kind made 850 W (NASA, p. 8).
The best small factory engines: a Philips air engine in a generator set built for 200 watts (p. 55) made at most about 0.48 kW, at 7.75 to 10% efficiency (p. 57). Factory Stirling generators of the early 1980s made 25 to 70 watts, and one cost $5,000 (p. 405).
Homemade engines from cans and pipe fittings are teaching models: they spin themselves and little more (rule of thumb). Getting the fire's heat into the hot end and out of the cold end is the hard part, and the books' factory numbers came from engineers with test rigs.
Best uses: pumping water and slow work
"The old hot air engines were used almost entirely for pumping water" (NASA, p. 406). A modern fluid-piston engine pumped water 10 feet high at 7% efficiency on propane (same page).
The pumped water cools the engine. In the Ericsson hot-air pumping engine "all the water passes" through a jacket around the cool end on its way from the well to the tank, while the bottom sits in the fire (New Catechism, p. 319; the pumping engine; in section). Heat the bottom "a very few minutes", then start it with a turn or two of the flywheel by hand (p. 321).
Other slow jobs: turning a grindstone, a churn or a fan, where a few dozen watts all day is enough (rule of thumb). For electricity, a thermoelectric module is simpler for the same few watts.
Hot cylinder: a 4-inch length of iron pipe a little over 2 inches across, a threaded cap on each end, the top cap drilled for the displacer rod (p. 216). (The old scan blurs the fractions; check the drawing.)
Displacer: a smaller capped pipe with a hardwood plug, deliberately loose so air can pass around it, on a rod through the top cap (pp. 217–218).
Power cylinder: "half as large as the expansion cylinder", with a piston of the same metal that fits "pretty accurately, as it is not packed" (pp. 219–220).
Connecting pipe from the top of the hot cylinder to the power cylinder, and a crankshaft with two cranks, one for each piston (pp. 221–223).
A heavy 5- or 6-inch flywheel and a firebox with air holes at top and bottom (pp. 224–226).
To run it: a flame centered under the bottom of the hot cylinder, then "Give the flywheel a couple of turns" (p. 226).
The same layout scaled up, with a water-cooled top and a regenerator, is the old pumping engine (rule of thumb).
❄️ Heat into cold: absorption refrigerators and ice makers
How a flame makes cold
Gas and kerosene refrigerators use the absorption system: a small flame drives ammonia out of water under pressure until it condenses to a liquid, and the liquid boils off again in the cold box, soaking up heat. There are no moving parts; "The Servel is used in RV's now. Modern kerosene and propane refrigerators use this system" (Aprovecho, p. 37; Absorption refrigerator). The sealed system is under real pressure: the tested unit held 1.52 lb of ammonia and was pressure-tested to 717 psi (NBS report, p. 9). Never drill, cut, bend or heat its tubing(rule of thumb).
Keeping an absorption refrigerator running
From the National Bureau of Standards tests, for the Army, of a propane-fired household-type absorption refrigerator:
Level it carefully. It failed when tipped just 1 degree forward or to the left, and 2 degrees backward or to the right (NBS report, p. 33). One degree is under ½ inch across a 2-foot-wide fridge (arithmetic); use a spirit level on the freezer floor, and re-level after moving it (rule of thumb).
Keep it out of the heat. It worked at about 103 °F, slipped at 110 °F, and failed at 118 °F (p. 23). Once it failed it did not recover until the room was cooled well below that (p. 24). Its limit was about 105 °F (p. 33). In summer give it the coolest, shadiest spot and free air around the back (rule of thumb); the testers set it about 6 inches from the wall (p. 15; the back, with condenser and heater box).
Right gas pressure: it ran best at 11 inches of water column, the maker's figure (p. 23, p. 33). That needs a proper low-pressure propane regulator, never a hose straight from the tank (rule of thumb).
Reset the thermostat with the seasons: no single setting was right for both a 70 °F and a 90 °F room (p. 33).
Choosing one: a freezer compartment "will use much more fuel"; top-opening boxes are more efficient; units that run on both gas and electricity are "of especial value" (WHO field manual, pp. 271–272).
Fuel: how long a tank lasts
The tested fridge was rated for 1,300–1,600 BTU an hour (NBS report, p. 9). With the burner on all day that is 31,000–38,000 BTU, or about ⅓ to ⅖ of a gallon of propane a day at 91,000 BTU a gallon (Univ. of Minnesota EB 436, p. 16) (arithmetic). The thermostat turns the flame down much of the time, so real use is less. A kerosene model burning the same heat would use about ¼ gallon a day (kerosene: 135,000 BTU a gallon, same page) (arithmetic).
Burner care and carbon monoxide
(Rule of thumb unless cited)
Clean flame: a steady blue gas flame, or a kerosene flame with no smoke. Soot in the flue or a yellow, smoky flame means a dirty burner, a wrong wick height or a blocked flue: fix it before you use the fridge again.
Clean the flue (the tube above the burner, with a baffle hanging inside) with a long brush, and keep spiders and wasps out of the burner's air holes.
Kerosene models: only clean 1-K kerosene, never gasoline (CPSC Kerosene Heater Safety, p. 1). Trim and clean the wick like a lamp wick (see "Kerosene lamps while the kerosene lasts" in the Lighting guide), and don't let the tank run dry.
Carbon monoxide: "Any heating system that burns fuel, and does not have a chimney, will give off some gases into the room" (MSU Portable Kerosene Heaters, p. 3). Vent the fridge's flue outdoors as RV fridges are, and keep a CO alarm in the room.
Solar and fire-driven ice makers
How they work: a heated vessel drives ammonia over to a condenser by day; at night one valve is turned and the liquid boils off in coils in an insulated box, making ice. The DOE-funded ISAAC ice maker had a solar collector, two pressure vessels, a condenser and piping, with "no moving parts or electrical components" (ISAAC report, pp. 3–4). On cloudy days a "burner coil, fueled by kerosene, propane or biomass" heats it instead (p. 5).
Hard to build well: the first two prototypes fell short, and the third worked well but "reliability was marginal and it was too expensive" (same page).
The Icy Ball was a 1930s fire-charged version; more than 100,000 were sold (Aprovecho, p. 37; Icyball). Aprovecho rebuilt one from small propane cylinders, then stopped because of "the potential for explosion" when its soldered safety plug proved unreliable (same page). Don't build one: use an absorption fridge you can buy or salvage, or store winter ice (see "Cold: storing winter for summer" in the Storing Energy guide).
What a 12-volt "thermoelectric cooler" can do
The plug-in camping coolers use Peltier modules run backwards. They cool only about 20 °C (36 °F) below the room and are much less efficient than a compressor fridge (Thermoelectric heat pump). Fine for a drive, poor for living off a battery (rule of thumb). Their modules are worth saving for stove-top power (above).
🔧 Other ways to put heat to work
Engine waste heat: only about 25% of an engine's fuel turns into work; "What happens to the remaining 75 percent of the heat? It is largely absorbed by the engine parts or passes out with the exhaust gases." An ordinary six-cylinder engine "generates sufficient heat to warm a six-room house in freezing weather" (Navy Construction Mechanic, p. 133). When a liquid-cooled engine runs (a tractor, a truck, a wood-gas generator), its hot coolant can warm a tank of wash water through a coil; never bring the exhaust indoors (rule of thumb).
Exhaust steam for heating and hot water (above, "Condensing and reusing the water").
Stove fans that run on a thermoelectric module (above).
Hot water by thermosiphon with no pump, and heat stored in water and stone: see the Storing Energy guide.
Charcoal and wood gas for engines: see the Fuel guide.
🧯 Safety: burns, steam, carbon monoxide and fire
Scalds: water at 140 °F (60 °C) takes about 5 seconds to cause a severe burn, at 158 °F (70 °C) about 1 second (Ciochetti & Metcalf, p. 5). Steam and boiler water are far hotter.
Steam leaks: a hissing joint on a boiler means shut down and let it cool; never tighten a fitting under pressure (rule of thumb). A blown rivet during one water test "struck a man in the face, and … parboiled him from head to foot" (Maxims, p. 285).
Boiler explosions: stay out of line with the ends and the firebox door of a boiler under pressure, and keep children and visitors away entirely (rule of thumb).
Fire: an unshielded single-wall stovepipe needs 18 inches of clearance to anything that burns (CMHC, p. 15). Boilers, engines and fridge burners are hot too: kerosene heater surfaces run 320 °F to more than 500 °F (MSU, p. 3).
Gasoline burners under a boiler: Watson calls gasoline "treacherous stuff" (Watson, p. 115); use wood, charcoal or a proper kerosene or alcohol burner (rule of thumb).
Ammonia from a cracked refrigerator: it smells strongly and burns the eyes and lungs; get out, air the room, and take the unit outside (rule of thumb).
⚠️ Old advice in the library to ignore
Book says
Do this instead
On low water out on the road, run the engine's front end onto high ground "so that the water would raise over the crown sheet and thus make it safe to pump up the water" (Maggard, p. 162).
Never feed water after a low-water scare: kill the fire, shut the feed, let it cool, then inspect (Navy Boilerman, p. 324).
A cork in the filling hole of a toy boiler "will act as a safety valve, for it will blow out before the boiler blows up" (Collins, p. 74).
A cork can swell, stick or be pushed in too hard. Any boiler, even a toy, gets a real weighted or spring safety valve (rule of thumb).
Pump the boiler up with a bicycle pump and run the engine on compressed air "without steam and, naturally, without fire" (Collins, p. 125).
Never put air pressure in a boiler: compressed air stores energy and bursts violently. Test with water only (rule of thumb).
"If you find a thin plate, what would you do? Put a patch on" (Maxims, pp. 131–132).
That was a licensed boilermaker's job. A thinned, cracked or patched boiler is retired from pressure use (rule of thumb).
Calk stack leaks "with a mixture of asbestos wicking and red lead" (Boilerman, p. 359).
No asbestos and no red lead: use high-temperature stove cement or furnace sealant (rule of thumb).
Refill a melted fusible plug with babbitt by pouring it in (Maggard, p. 83).
Fit a proper replacement plug of the right melting point, and find out why the water got low (rule of thumb).