Build a no-pump thermosiphon solar water heater using junkyard radiators

The power goes out after an ice storm, the propane delivery is three weeks late, and morale sinks with every cold washcloth “bath.” Yet outside, even on a clear winter day, the sun drops about 1,000 watts per square meter on your roof—more heat in an afternoon than you’ll use for a day of showers and dishwashing. Water heating quietly eats roughly 18% of a typical home’s energy budget, and in a disruption it becomes more than a comfort—it’s hygiene, health, and hope. That’s why a no-pump thermosiphon solar water heater, built from scrap-yard radiators and basic plumbing, deserves a place in your resilience plan.

I’ve built and repaired off-grid water systems in mission clinics, farm outbuildings, and storm-hit neighborhoods, and what lasts is always the simple, field-repairable gear you can source locally. Thermosiphon systems have no moving parts—hot water rises, cold water falls—so gravity becomes your “circulator.” Pair that with a well-chosen radiator array and a safe storage tank, and you’ve got hot water without a single watt of grid power. It’s practical stewardship: redeeming materials others discard, serving your household and neighbors with something both useful and quietly dignifying.

In this guide we’ll map the whole build: how to select safe radiators (and avoid contaminated ones), the physics of thermosiphon flow and why heights and pipe diameters matter, and a proven plumbing layout with pipe sizes and slopes that actually work. We’ll cover tank options, insulation, relief and tempering valves, stagnation control, and freeze protection. You’ll get a shopping list, junkyard inspection tips, and a step-by-step assembly with troubleshooting for air locks, weak flow, and lukewarm output.

If you’re ready for a hot shower powered by sunlight and ingenuity—not an electric pump—you’re in the right place.

Thermosiphon 101: Passive Solar Hot Water Without Pumps—Principles, Layout Rules, and Performance Targets

Picture a cold morning after a storm: grid down, no pumps humming, yet your tap still runs warm because the sun did the work yesterday. That’s the promise of a thermosiphon—simple physics, no moving parts. For those of us who see preparedness as wise stewardship, a passive solar heater is a quiet way to turn daily sunlight into dependable comfort and hygiene for family and neighbors.

How Thermosiphon Works (And Why It’s Reliable)

Principle: As water heats, its density drops and it rises; cooler, denser water sinks. Put a solar collector below a storage tank and you get a self-starting circulation loop whenever the collector is hotter than the tank bottom.
No power required: The “pump” is buoyancy. Fewer failure points means year-round resilience.

Layout Rules That Make or Break Flow

Elevation: The tank outlet/inlet must be higher than the top of the collector. Aim for 24–72 inches of vertical rise; 12 inches is a bare minimum. More rise = stronger flow.
Piping: Keep runs short and fat. Use 3/4″ or 1″ lines. Minimize elbows; every 90° adds resistance. Maintain a continuous upward slope from collector to tank: 1/2″ per foot is a good target. No dips that trap air.
Orientation: Hot outlet from the top of the collector to the top/upper third of the tank; cold return from the bottom of the tank to the bottom of the collector.
Night cooling control: Add a heat trap (a “gooseneck” up-and-over loop 8–12″ above the tank outlet) or a light-cracking-pressure swing check oriented vertically. These prevent reverse thermosiphon after sunset without choking daytime flow.
Air management: Include a high-point manual air vent near the tank or a vacuum breaker to purge air on startup.

Common mistakes: Mounting the collector level with or above the tank (flow stalls), long skinny piping (3/8″–1/2″), too many fittings, or any downhill section on the hot line that becomes an air lock.

Performance Targets You Can Plan Around

Water temps: Expect 110–140°F (43–60°C) on sunny days with proper glazing and insulation. Always temper to ~120°F at the tap for safety.
Output: A 20–30 ft² collector footprint can typically deliver 15–30 gallons/day of hot water in mild seasons; double that area for 30–60 gallons/day. Winter yields drop with sun angle and ambient temps.
Energy math: On a clear day, 1 m² (~10.8 ft²) of collector can capture 1–2 kWh of usable heat; size for your climate and needs.

Troubleshooting early builds: If you can touch the hot riser and it’s only warm while the collector is baking, you likely have air trapped, insufficient rise, or excessive head loss. Fix the slope, purge air, and simplify routes.

Key takeaway: Gravity will circulate your water if you respect elevation, slope, and friction. With those rules in place, you can turn “junk” into dependable hot water. Next, we’ll translate these principles into hardware—choosing and plumbing junkyard radiators for real-world performance.

Sourcing Smart: Picking Junkyard Radiators, Metals, and Glazing That Can Handle Heat, Pressure, and Weather

You’re standing between two rows of sun-faded sedans and work-worn pickups, looking for the heart of your collector. Stewardship starts here—choosing materials that last, work safely, and make the most of what’s already in the waste stream.

Radiators: What to Grab—and What to Leave

Go for older copper/brass radiators or heavy-duty truck radiators with metal end tanks. They handle heat well, are repairable (solder/braze), and play nicely with copper piping. Avoid aluminum radiators with plastic end tanks; they’re hard to modify, prone to UV cracking, and mixing aluminum with copper in a wet system invites galvanic corrosion.

Potable warning: don’t run drinking water through a junkyard radiator. Many older radiators contain lead-bearing solder and residual glycol. Use radiators in a closed collector loop only, with a heat exchanger on your storage tank.

How to evaluate:
– Size and fin density: A 20×26 in radiator offers ~3.6 sq ft collector area; plan on 20–30 sq ft per small household. Two to three truck radiators is a solid start.
– Condition: Check solder seams for green crust (dezincification), look for crushed fins, and inspect inlet/outlet necks for cracks.
– Pressure test: Cap one port, add a Schrader valve to the other, submerge, and air-test to 20–25 psi. Bubbles mean walk away or be ready to braze.

Metals and Fittings: Build for Heat and Corrosion

Use Type L copper for headers and manifolds; it balances strength and solderability. Brass fittings are ideal; if you must transition to steel (mounts), isolate with dielectric unions and anti-corrosion paste. Stick with lead-free solder or 15% silver brazing rod for high-heat joints. Plan your loop pressure: 12–18 psi for a closed loop with a 2–4 gal expansion tank and a 30 psi relief valve; open, vented thermosiphon systems prevent overpressure but must be protected from freezing with a glycol mix.

Glazing: Light In, Weather Out

Choose clear tempered glass—salvaged patio door panels are perfect. Single-pane tempered maximizes heat gain; avoid low-E coatings (they reflect IR). If you’re unsure, shine a small flame near the glass—low-E shows a colored tint in one reflection. Polycarbonate twin-wall (6–10 mm, UV-stabilized) is a tough alternative in hail zones, though it insulates slightly more and can trap moisture without venting. Leave a 1/8 in expansion gap in frames and use high-temp silicone or EPDM gaskets.

Common Mistakes

Mixing aluminum radiators with copper piping—galvanic corrosion kills the system.
Using untempered window glass—one hailstorm and you’re rebuilding.
Forgetting venting/relief—heat plus trapped pressure is a bad witness and a worse repair bill.

Key takeaway: Select copper/brass radiators, pair them with Type L copper and brass fittings, and glaze with tempered glass. Test, isolate metals, and design for pressure and weather. Next up, we’ll lay out the geometry that makes thermosiphon flow reliable without a pump.

Build the Collector: Converting Radiators into a Glazed Absorber Box, Manifolding for Low Restriction, and High-Temp Finishing

A neighbor once asked why my shop smelled like a body shop and a barbecue at the same time. Truth is, building a good collector feels a bit like both. We’re reclaiming old hardware—giving it a second life—and shaping it into something that quietly serves every day. That’s stewardship you can stand behind.

Frame and Glazing: A Hot Box That Lasts

Build a shallow, insulated box to house the radiators. A simple frame from 2x4s (3.5″ cavity) with a 1/2″ exterior-grade plywood back works well. Line the back with 1.5–2″ mineral wool (Rockwool) for high-temp resilience; skip foam near the hot face—most foams slump above 200°F. Paint the interior matte black.

Size the box to leave a 1–1.5″ air gap between the radiator face and glazing. For glazing, 3/16″ tempered low-iron glass is best for clarity and heat tolerance; twinwall polycarbonate can work, but plan for expansion and expect some long-term yellowing. Set the glass on EPDM glazing tape with a wood or aluminum stop, and drill tiny weep holes at the lowest edge to drain condensation. A 5–10° inward pitch on the glass frame helps shed water.

Manifolding for Low Restriction: Let Gravity Win

Thermosiphon systems thrive on low head loss. If you’re using two or three junkyard radiators, plumb them in parallel. Run 1–1/4″ Type L copper headers along the top (hot out) and bottom (cold in), with short 1–1/2″ silicone radiator hose couplers to each unit’s necks. Use long-sweep elbows, keep runs short, and maintain a gentle rise toward the hot outlet (about 1″ per 4 ft) so air can escape.

Prefer all-copper/brass radiators if you can find them; they’re easier to solder and play nicer with copper manifolds. If you use aluminum/plastic radiators, isolate dissimilar metals with hose couplers and plan on an inhibited glycol blend to control corrosion. Add a manual bleeder or auto air vent at the high point, a drain cock at the low point, and a 30 psi relief valve for safety. The goal: palm-warm bottom header, hot-but-not-boiling top header under sun—steady, quiet flow without a pump.

High-Temp Prep and Finish: Make Heat, Not Smoke

Degrease radiators thoroughly (hot water + detergent, then solvent wipe). Scuff to dull. For aluminum, use an etch primer; for copper/brass, a high-temp primer. Finish with flat black high-heat paint (rated 1000°F). Apply thin coats and “bake” the coating in the sun before glazing to off-gas. Black paint increases absorptivity; flat finishes minimize reflection and improve heat gain.

Common Mistakes and Fixes

Choked flow: undersized headers or sharp elbows. Upsize to 1–1/4″ and use sweeps.
Peeling paint: poor prep. Degrease, scuff, and cure before glazing.
Foggy glazing: no weep holes or too little air gap. Add drains and ensure 1″+ spacing.
Galvanic corrosion: mixed metals without isolation. Use silicone couplers and inhibited fluid.

Key takeaway: build a tight, insulated, glazed box, then manifold for easy, low-restriction flow. Next, we’ll mount and tie the collector into the storage loop so gravity can do the pumping.

Piping for Gravity Flow: Tank Height, Line Sizing, Heat Exchangers, and Code-Safe Safeties (PRV, Expansion, Mixing)

Picture a crisp morning: the first sun hits your junkyard radiator collectors, water inside warms and wants to rise. Whether it actually moves depends on your piping. Thermosiphon is a gentle engine—give it height, wide roads, and no dead-ends, and it will run for decades without a pump.

Height and Slope: Let Buoyancy Do the Work

Tank higher than collectors: Aim for 3–6 ft of vertical separation from collector outlet to tank inlet; 18 in is a bare minimum. More head = more driving force.
Continuous rise and fall: The hot line must rise continuously from the collector’s high outlet to the top of the tank; the cold return must fall continuously from the tank bottom to the collector inlet. Pitch at least 1/4 in per foot. Any dip can trap air and stop flow.
Keep it short and smooth: Place the tank as close as practical. Use sweeping 45s or long-radius 90s to reduce friction.

Common mistake: Spring check valves. If you need backflow prevention at night, use a gravity/swing check with near-zero cracking pressure, or create a heat trap (8–12 in vertical U) near the tank.

Line Sizing and Materials: Big, Simple, Durable

Size: 3/4 in copper Type L is the minimum; 1 in for runs over 10 ft or for multiple radiators. Large diameter lowers friction so thermosiphon can carry 1–2 gpm without a pump.
Materials: Use copper near the collectors; avoid PEX in the hot zone (stagnation can exceed 250°F). Transition to oxygen-barrier PEX only in cooler, well-insulated sections.
Insulation: Use 1 in wall, high-temp foam (EPDM/Armaflex HT) and UV-wrap it outdoors.

Troubleshoot: Gurgling or “stuck” flow = air pocket. Add an automatic air vent at the highest point and a drain/purge tee at the lowest.

Heat Exchangers: Safe Separation from Junkyard Metals

Automotive radiators can contain lead solder and traces of ethylene glycol—keep potable water separate. Run a closed collector loop (radiators) to a heat exchanger that warms your domestic tank:
– Coil-in-tank: 50–100 ft of 1/2 in soft copper, double-wound low in the tank. Easy, efficient, and gravity-friendly.
– Sidearm/double-wall HX: Code-friendly where toxic fluids are involved; mount vertically so hot rises on the potable side.

Use propylene glycol in the collector loop where possible; if using ethylene glycol, a double-wall HX is wise. Include an air vent at the collector high point and a fill/backflow preventer on the closed loop.

Code-Safe Safeties: Protect People and Plumbing

T&P relief valve: 150 psi/210°F on the tank’s top hot port; pipe 3/4 in discharge to a safe drain, never capped.
Expansion control: If there’s any check/backflow on the cold feed, install a 2 gal potable expansion tank, precharged to house pressure, on the cold inlet.
Mixing valve: Thermostatic (ASSE 1017/1016). Set distribution to 120°F even if the tank reaches 160–180°F for storage efficiency.
Vacuum relief on cold inlet to prevent tank collapse when cooling/draining.

Key takeaway: Honor the physics—height, slope, and wide pipes—and honor safety—T&P, expansion, and mixing. Good stewardship means no shortcuts. Next, we’ll tie the collectors into the tank and walk through commissioning and seasonal tuning.

Cold Nights and Hot Days: Freeze Protection, Scald Control, Stagnation Management, and Nighttime Heat Traps—All Passive

Cold nights and blazing afternoons are where passive design earns its keep. A well-built thermosiphon should protect itself—and your family—without a switch to flip. Think stewardship: safeguard the system, the water, and the people who use it.

Freeze Protection That Needs No Power

Closed-loop glycol: Run the collector/radiator on a separate loop with 30–40% propylene glycol (not ethylene). Circulate through a coil or sidearm heat exchanger on the storage tank. Why: Glycol prevents freeze damage down to roughly -10 to -20°F, depending on mix, and tolerates stagnation better than plain water. Add a 1–2 gallon diaphragm expansion tank sized to at least 125% of the collector-loop fluid volume, a 30 psi relief valve, and an automatic air vent at the high point. Charge the loop to 10–15 psi cold. Use brass/copper components; avoid mixing aluminum radiators with copper unless your glycol includes specific aluminum inhibitors.
Passive drainback: If you run water directly, mount the tank above the collector and pitch all lines 1/4 inch per foot back to the tank or a drainback reservoir. Put a vacuum breaker at the collector high point so it can gulp air and drain when the sun fades. Why: Dry collectors don’t freeze. Common mistake: flexible hoses or sags that trap water.

Scald Control, Automatically

Store hot; deliver safe. Fit an ASSE 1017 thermostatic mixing valve on the tank outlet, set to 120–125°F. Run unions for service and check valves per the valve’s diagram. Why: You can store at 140°F for Legionella control and still bathe kids safely. Test with a kitchen thermometer at the tap; if it creeps above 125°F during long draws, your mixing valve needs adjustment or descaling.

Stagnation Management on Hot Days

Oversized summer sun can push collector temps over 250°F. Favor winter by tilting the collector to your latitude +10–15°. Size storage at roughly 1.5–2 gallons per square foot of collector. Use high-temp materials (silver braze on manifolds, EPDM gaskets). In glycol loops, allow “steam-back”: route the collector top to the expansion tank so fluid can boil out of the panel into the tank without bursting seals. Common mistake: undersized expansion tank—measure loop volume when filling.

Stop Nighttime Heat Traps

Reverse thermosiphon will chill the tank by dawn. Add heat traps: create 8–12 inch up-and-over loops on both supply and return near the tank, and install a low-cracking-pressure swing check on the return oriented for daytime flow. Insulate everything except the last 2–3 inches around the check to avoid sticking. Night test: if pipes feel warm hours after sunset, your trap isn’t working.

Key takeaway: with a few passive safeguards, your “junkyard” heater quietly protects life, limb, and hardware. Next, we’ll commission it and set a maintenance rhythm that keeps it honest for years.

Start-Up and Upkeep: Filling with Glycol, Air Purge, Leak Testing, Flow Verification, and Troubleshooting Common Failures

A crisp fall morning is the best time to bring a new thermosiphon online—cool ambient temps help you spot leaks, and low sun gives you a gentler first heat cycle. Think of start-up as stewardship: a rhythm of care that protects your work and serves your household.

Charge the Loop (The Right Mix, The Right Way)

Fluid choice: Use inhibited propylene glycol (solar/hydronic grade), not automotive ethylene glycol. Aim for 30–35% PG by volume with distilled water. This balances freeze protection (burst protection near -20°F) with low viscosity for natural circulation. Avoid mixes over 40%—thicker fluid kills thermosiphon flow.
Pre-checks: Expansion tank precharge equals your cold fill pressure (typically 5–8 psi for a low-stress closed loop). Relief valve set ~15 psi.
Fill from the lowest point with a drill pump or garden sprayer. Open all high-point vents. Fill until fluid runs bubble-free at the highest vent; then close vents and top off to reach cold fill pressure.

Why: Thermosiphons rely on density differences; too much glycol or trapped air can stall flow that a pump would muscle through.

Air Purge and Leak Testing

Purge: With sun warming the collector, crack high bleeders again. Gentle “burps” are normal as microbubbles release. Lightly tapping vertical risers and rocking the collector a few degrees helps stubborn pockets.
Leak test: Hold at 8–10 psi for 30 minutes. Walk each joint with dry paper towels; any dampness is a fail. Retorque hose clamps after the first heat cycle; elastomers relax when hot.

Common mistake: Pressure-testing junkyard radiators at automotive levels (15–20 psi). Don’t. Keep stress low to extend life.

Verify Flow (Trust but Verify)

Touch test: In sun, collector outlet should be hottest; return line 5–15°F cooler. A 30°F+ drop screams low flow or air.
Instruments: Install two dial thermometers (supply/return). Optional rotameter: target 0.5–1.0 gpm for a small 1–2 m² array. No movement by midday? You’re air-locked or too viscous.
Night losses: If the storage cools overnight, add a low-cracking swing check (<0.2 psi) or a heat-trap loop to stop reverse thermosiphon.

Troubleshooting Common Failures

No flow at noon: Reversed connections, air pockets at the top, or too much glycol. Reconfirm slopes (at least 1 inch rise per 10 feet), bleed again, dilute to 30–35%.
Gurgling/steam smells: Stagnation. Shade the collector, verify relief valve operation, consider a heat dump fin coil.
Brown/black fluid, sour smell, low pH (<7.5): Glycol breakdown. Drain, flush, refill with fresh inhibited PG; check for mixed metals and add corrosion inhibitor as specified.
Scale in HX/lines: If potable side runs hard water, add a softener or periodic descaling protocol.
Mixed metals corrosion (aluminum radiator + copper lines): Use a quality inhibitor, dielectric unions, and a sacrificial anode where practical.

Key takeaways: Keep glycol between 30–35%, purge air thoroughly, verify a modest delta-T under sun, and inspect after the first heat cycle. Record baseline temps and pressures; recheck before first frost and first heat wave. Faithful, small checks now prevent big failures later.

You’ve now got a clear path to turn scrap metal and sunshine into quiet, dependable hot water. The physics are simple—hot rises, cold falls—but the craft is in the details: a collector that drinks light without choking flow, big smooth piping climbs, a tank perched with purpose, and safeties that never take a day off. Redeeming a set of old radiators into a serviceable thermosiphon isn’t just frugal; it’s stewardship—less grid, less waste, more resilience for your family and neighbors.

Next steps: walk your site at solar noon and mark your winter sun window; confirm you can place the tank at least 18–24 inches higher than the top of the collector with a continuous rise back. Source two copper/brass-core radiators or a single large fin-tube unit; pressure-test them to 1.5× your intended operating pressure. Sketch the loop with 3/4–1 inch supply/return lines, a heat exchanger if you’re on potable, a 150 psi/210°F PRV, an expansion tank matched to your glycol volume, and a mixing valve set around 120°F. Build one collector first, glaze with tempered glass, paint flat black high-temp, then test on sawhorses with 30–40% propylene glycol. Add thermometer wells at collector out and tank in, and a simple flow indicator. Log temps for a week, then scale.

Expect small course corrections—bleeding a stubborn air bubble, bumping pitch, adding a heat trap—and you’ll gain a system that serves for years. Share what you learn with your circle; resilience multiplies in community. Do the next right piece of work, and let steady, faithful steps carry you the rest of the way.

Build a no-pump thermosiphon solar water heater using junkyard radiators

Thermosiphon 101: Passive Solar Hot Water Without Pumps—Principles, Layout Rules, and Performance Targets