Field Guide
Rocket Stove Water Heater
A passive thermosiphon master build plan utilizing upcycled waste-stream materials for off-grid bathhouses.
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1
Core Concepts
The Rocket Stove
- J-Tube Design: Horizontal fuel feed, an elbow, and a vertical internal chimney (heat riser).
- Insulation is Key: Insulating the heat riser creates a powerful upward draft, sucking in air for complete, smokeless combustion.
- High Heat: Extracts maximum thermal energy from scrap twigs.
The Thermosiphon
- Density Differences: Water expands as it heats, becoming lighter than cold water.
- Natural Loop: Hot water rises out of the copper coil into the elevated tank.
- Passive Vacuum: Rising hot water pulls cold, dense water from the tank bottom into the coil. No electricity needed.
2
Materials & Tools Tracker
Waste-Stream Materials Checklist
Required Tools
3
Build Execution
CRITICAL SAFETY WARNINGS
- Never install a shut-off valve anywhere on the loop between the stove and the tank.
- Always purge old propane tanks professionally before cutting.
- Always fill the tank with water *before* lighting a fire.
If using an old electric water heater tank:
- Strip off the outer sheet metal jacket and old fiberglass insulation.
- Ensure interior glass lining is intact and wash out sediment via bottom drain.
- Keep existing plumbing ports: bottom port = cold feed out, upper port = hot return in.
[ Storage Tank ] <-- Must be elevated higher than stove
/ \
(Hot Return) (Cold Feed)
/ \
[Stove Coil] <------- [Bottom of Coil]
- Foundation: Lay flat, level base of concrete blocks/bricks.
- J-Tube Core: Build horizontal combustion chamber and elbow using firebricks or thick steel.
- Heat Riser: Stand 6"-8" steel pipe vertically on elbow (2.5 - 3 ft tall).
- Outer Drum: Place 55-gal drum over vertical pipe. Cut hole in bottom side for fuel feed throat.
- Coiling: Wrap 20-30ft of 1/2" soft copper around a mandrel slightly larger than inner heat riser. Space coils 1/4" to 1/2" apart.
- Placement: Slip coil over inner heat riser. Cold inlet emerges at bottom, hot outlet at top.
- Insulate: Pack space between copper coil/heat riser and outer drum with insulation (ash, perlite, clay mix).
Strict Geometry Rule: Any dips or high spots in the hot line will trap air, stalling the loop and boiling the water in the coil.
- Elevation: Tank bottom must be 12-18 inches higher than top of stove coil.
- Cold Line: Runs downward from lowest point of tank to bottom entry of coil.
- Hot Line: Runs from top exit of coil straight to upper portion of tank. Must slope continuously upward.
+-----------------------+
| ELEVATED WATER TANK |
| |
| (Hot Return) <=========[HOT]=========+
| | || (Continuous
| | || upward slope)
| (Cold Feed) | ||
+----------|------------+ ||
| ||
| (Downward ||
| slope) ||
v ||
+---------------+ ||
| ROCKET STOVE | ||
| | ||
| +---------+ | ||
| | Coil |==|=======================+
| +---------+ |
| | Fire | |
+---------------+
- Open Vent: Keep storage tank vented to air at the top (unless using commercial pressure setup).
- T&P Relief Valve: Install at highest point of hot loop. Vents steam if temps exceed 210°F.
- Bottom Drain: Install brass drain valve at absolute lowest point of loop to winterize/drain system before freezes.
4
Bathhouse Plumbing Layouts
Option A: Gravity-Fed (Off-Grid)
Simplest, most reliable passive option. Elevated cold tank supplies shower and stove loop.
[ Elevated Cold Tank ]
/ \
(Main Cold Supply) (Cold Feed to Stove)
/ \
| [ Hot Storage Tank ] <=== Thermosiphon
| | from Stove
| (Hot Supply)
\ /
+==> [Mix Valve] <=+
|
[Shower / Tub]
- Cold line from elevated tank to mixing valve.
- Hot line from storage tank to mixing valve.
- Use non-restrictive 2-handle valve.
- Cold tank bottom must be 6-8ft above showerhead.
Option B: Pressurized (On-Grid)
Safe for city water/well pumps. Uses an internal heat exchanger to isolate pressure from the stove.
(Open Vent)
|
+---------------------+
| Hot Storage Tank | <--- Unpressurized
| |
| +-------------+ |
[City] ====>| Copper Coil |====|====> [To Shower Hot]
(Pressure) | | (In Tank) | | (Pressurized)
| +-------------+ |
+---------------------+
^
| (Rocket Stove Loop Heat)
- Stove heats an unpressurized main tank.
- Submerge 50ft of 1/2" copper inside tank.
- Run city water through inner coil to absorb heat safely.
5
Local Missouri Sourcing
| Location | Region | Best For... |
|---|---|---|
| REFAB (Reclaimed Resale) | St. Louis (Gravois) | Salvaged copper pipe, brass valves, electric water heaters, vintage bath fixtures. |
| SA Recycling | St. Louis | Heavy-wall structural steel pipe (6"-8"), steel drums. Call ahead for yard picking. |
| Riverside Antiques | St. Louis (Cherokee) | Industrial metal, vintage copper fittings, aesthetic hardware. |
| Apex Recycling | Cape Girardeau | Clean industrial scrap, discarded HVAC copper coils, heavy pipe. |
| Satterfield Auto/Truck | Sikeston | Thick steel pipe, heavy frame scraps, fuel tanks for stoves. |
| Habitat ReStore | STL & Cape | Overstock plumbing fittings, PVC, mixing valves, insulation. |
Artist's Touch: Cob Finishing
Once the steel drum and coil are assembled, encase the exterior in Cob (clay, sand, straw). It acts as a massive thermal battery, absorbing the rocket stove's heat and slowly radiating it into the bathhouse. Sculpt it into an organic bench and finish with natural lime plaster for a stunning, water-resistant look.