In the face of the brunt of winter's cold, you can bet your double-insulated britches that heat is a prime commodity at any price. But when it comes to temporary shelters or occasionally used spaces, adequate warmth is often a luxury whose expense can't be easily justified.
Indeed, you may well have wondered if it would ever be possible to find a compact furnace that's reliable, inexpensive, and easy to operate . . . and if that's the case, you're looking at it. Believe it or not, this heater functions on the same principle as the pulse-jet engine, which was used in the earliest missiles. But instead of propelling an aircraft, the "thrust" from this pint-size model simply produces enough heat to take the chill off your bones on a cold winter's day.
Originally, we developed this stove to heat a homebuilt ice-fishing shelter. But after many hours of testing and modification, we realized that it'd be a shame to limit such a handy device to a single recreational use. Because it burns kerosene fuel, it's tidy and convenient . . . and since it's made of 1/8-inch plumbing fittings and galvanized downspout parts, it shouldn't cost more than $15 or $20 to put together if you make a point to use salvaged, rather than new, parts wherever possible.
Design of a Small, Pulse-Jet Heater
By referring to our illustration (in this story's image gallery), you can see that the heater has three main components: a burner, a fuel supply and control assembly, and a housing that also serves as a flue.
The burner, even though it's a thread-together proposition, requires a bit of work with a hacksaw and a small drill bit to make it function as designed. To allow fuel into the wick area, four 7/64-inch holes must be drilled through the wall of the pipe nipple that supports the wick. Additionally, the flat washer immediately above the fiberglass cloth should be bored to a 7/16-inch-center diameter, and six .055-inch-deep saw kerfs should be cut into each of its faces. Likewise, the fender washer directly above that should have ten .030-inch-deep slots sawed into its faces. To prevent excess fuel from running past the base washer, that disk should be sealed to its elbow with a layer of furnace cement.
The only other area of fabrication in the burner is the drip pan, which can be folded into shape from a 4-1/4 by 6-1/2-inch scrap of aluminum roofing. The completed pan should measure 1 by 1-1/4 by 2-1/4-inch, and its corner joints should be sealed with furnace cement. The rear wall of the pan is a hanger measuring about 2-1/4 by 4 inches. Fiberglass batting placed in the pan helps absorb any fuel overflow.
The fuel supply and its control plumbing are common hardware items. We used a one-quart fuel tank for our model to limit the burning time to several hours between refills; its vented cap can be sealed for storage. With the exception of the two 1/8-inch brass needle valves, the metal fittings can be either brass or iron, whichever you have or can purchase more inexpensively.
To reduce fuel pressure within the feed-control lines (which could cause a too-rich burn and subsequent overheating), we packed 1/4 by 1-inch twisted cotton wads into the cores (it's imperative that there be enough restriction to make it extremely difficult to blow through each of the tubes). Then, to guide each drop of kerosene down the copper burner feed line (rather than let it build up at the bottom), we inserted a length of 1/16-inch copper-coated welding wire into the tubing.
Tackling the fabrication of a sheet-metal burner housing is a task; instead, it's easier to use a 44-inch length of premade 26-gauge, 3-inch galvanized steel downspout as a "combustion chamber" and add to it a bread-pan wall spacer and some heat-dissipating fins. The dissipater is just a 17 by 38-inch section of aluminum roofing fan-folded to create 3-inch-deep fins. The spacer should have a 9/32-inch hole bored through each of its ends to allow the copper fuel feed line to pass through, and have six 1-1/2-inch openings in its sides to let heat escape. (The compression nuts and sleeves go on after the line is slipped in place.)
To complete the housing, the burner should be installed through a 7/16-inch hole drilled 5 inches from the lower back end of the spout. Then, with two 1-1/2-inch holes bored to the side of that, the spring-mounted sheetmetal lighting flap can be added. After this, the inlet elbow and the exhaust elbows and stack can be attached when the unit is permanently mounted.
Note that we've included a finned chimney cap in our design. Testing reinforced our belief that this addition helped induce the flow of flue gases out of the short stack; it's made in a similar manner to the heat dissipater, but its mounting edges are folded around the top of the flue spout and secured with sheet metal screws.
Installing the Heater in a Small Space
To install the heater in an ice-fishing shelter or in any other single-walled structure, it's necessary to make an inlet opening in the sheathing slightly larger than the outer dimensions of the elbow. The exhaust opening, since it's exposed to considerably more heat, should measure about 6 by 6 inches, and its four edges should be protected with sections of polished aluminum bent to create a 1-inch lip on both sides of the wood. For drip flashing, we chose to use two steel pie plates; we cut 2-1/4 by 3-1/2-inch openings in the bottoms and mounted the inlet plate outside, facing outward.
Conversely, the remaining pan should be attached inside, with its bottom facing inward. Use No. 6 by 3/8-inch panhead screws to fasten the plates to the wall.
Since the stove must be mounted close to the wall to allow the structure to fold up, a two-layer heat shield must be used behind the stack, as shown in our illustration. It consists of a polished aluminum sheet cut to fit around the bread pan; it's held away from a second plate on a 1-3/8-inch ceramic electric fence insulators. This provides a free-air space between the sheets to encourage convection.
If you plan to use the heater on a conventional stud-framed wall or on one with metal sheathing and insulation, it's necessary to use an approved ventilated thimble where the flue pipe passes through the structure. An adapter can be made of sheet metal to mate the rectangular spout to the round stovepipe opening, and short extensions can be used to bring the stove housing further away from the wall to comply with the National Fire Protection Association's bulletin 89M, or with your local fire prevention code. A sheet metal or polished aluminum surface mounted on 1-inch noncombustible spacers should be utilized as a heat shield.
It's important not to mount the fuel tank too high above the copper fuel-line inlet, or you'll waste kerosene and the stove may overheat. As an extra safety precaution, locate the control valve assembly a reasonable distance from the burner so as not to expose it to excessive warmth. After the initial lighting (which requires opening the shutoff valve just enough to moisten the wick, then touching an ignited match to the burner through the flap hole), give the space to be heated ample ventilation and allow the galvanized flue coating to burn off. Open the other needle valve, if necessary, to increase the heat output.
With this done, the compact furnace can be operated safely, though it should never be left entirely unattended. If your heater "huffs" noticeably and repeatedly, produces black smoke, or tends to burn fuel in the drip pan, it's running too rich and may well overheat. Reduce the fuel flow until the symptoms cease; don't continue to operate the stove under these conditions!
You'll find that a quart of kerosene will last about four hours on the low setting, half that at "full throttle." After approximately 16 accumulated hours of use, the burner surface should be cleaned through the flap opening with a toothbrush, and the feed line should be purged with a short dose of air pressure.
For a small price, then, and some occasional maintenance, you can enjoy heat where you thought you couldn't afford to . . . and that's a warm feeling.
How It Works
Our downspout heater is a scaled-down and considerably simplified version of the earliest form of jet engine, which was merely a reinforced cylinder with a flap-equipped inlet, a combustion chamber, and an exhaust duct. Like the jet, our stove relies upon a continuous airflow through its corrugated steel cylinder to function; cool combustion air is drawn through the outside inlet and past the burner, where it mixes with droplets of fuel introduced to the combustion area by way of the small channels cut into the wick washers. The ignition of this mixture heats and rapidly expands the gases, which are forced out the flue stack, entraining a fresh supply of air behind them to continue the process.
Because a precise fuel/air ratio is difficult to maintain with this design, combustion is intermittent, resulting in a rhythmic pulsation of thrust . . . hence the name pulse jet. The engine's drawbacks — inconsistent fuel consumption, noise, and a cogent vibration — make it impractical for aviation but perfectly acceptable for other purposes, such as a space heater . . . especially since it takes its combustion air from outside the living area. The fuel-control question is eased somewhat with our restricted double feed design: With the auxilliary needle valve closed, only a small amount of fluid is absorbed by the burner wick; with both needles open, the full amount of fuel is available to the burner. The main shutoff valve blocks the fuel flow entirely to turn the heater off.