A thermosiphon water heater circulates water with no pump because heated water is lighter than cold: with the storage tank 30 cm above the heater and a loop of 3 m of 20 mm pipe, 1 kW of heat drives about 0.023 L/s with the water leaving the heater about 10 K hotter than it returns. The loop must stay open to the air at the storage tank, with no valve between the heater and the tank, because a fire-heated vessel that is closed can burst like a boiler.
A thermosiphon hot water system heats water in a vessel or pipe coil near a fire and lets the difference in weight between hot and cold water move it to a storage tank, using only heat and gravity. It needs no pump, but it needs careful venting and care with scalding and bacterial growth.
What The Parent Page Describes
- Mount a dark-coloured metal tank or a coil of black pipe above or at your wood stove or fireplace.
- Connect the bottom of the heating vessel to the bottom of a storage tank positioned higher.
- Connect the top of the heating vessel to the top of the storage tank.
- As water heats in the vessel it rises to the storage tank, and cooler water sinks from the storage tank to the heater, which makes continuous circulation with no pump.
The storage tank must be above the heating point, and a height difference of 30 cm is sufficient, the parent page says.
A Short Rise Can Turn A Loop Over But Gives No Tap Pressure
The driving force is the difference in weight between the hot column and the cold column.
Inputs: water density 998.2 kg/m³ at 20°C and 983.2 kg/m³ at 60°C (standard handbook values I recall, not figures from this wiki), height difference h = 0.3 m.
- Density difference = 998.2 − 983.2 = 15.0 kg/m³.
- Driving pressure = g × h × Δρ = 9.81 × 0.3 × 15.0 = 44 Pa.
- For comparison, one metre of static head is 9,790 Pa. The thermosiphon force of 44 Pa is the pressure of about 4.5 mm of water head.
So 30 cm of rise is enough to turn over a loop of short, wide pipe, and no more. It gives no pressure to push water to a tap; that comes from the cold supply feeding the storage tank (Calculating Head Pressure).
The expansion coefficient from the same two densities is β = 15.0 ÷ (998.2 × 40) = 3.76 × 10⁻⁴ per kelvin, averaged over 20-60°C.
Worked Example: How Fast The Loop Flows With One Kilowatt
The flow adjusts itself until three things balance: the heat the water carries, the buoyancy that drives it, and the friction that resists it.
Inputs (assumed for the example): heat delivered to the water P = 1,000 W; vertical height h = 0.3 m; loop length L = 3 m; bore d = 0.020 m; water at about 40°C with density 990 kg/m³, specific heat c = 4,180 J/(kg·K) and viscosity 0.65 mPa·s (all recalled handbook values).
- Heat carried: P = ρ × Q × c × ΔT, so Q = 1,000 ÷ (990 × 4,180 × ΔT).
- Buoyancy: Δp = g × h × ρ × β × ΔT = 9.81 × 0.3 × 990 × 3.76 × 10⁻⁴ × ΔT = 1.095 × ΔT pascals.
- Friction for smooth laminar flow (Hagen-Poiseuille): Δp = 128 × μ × L × Q ÷ (π × d⁴).
Solving by trial: at ΔT = 10.5 K:
- Q = 1,000 ÷ (990 × 4,180 × 10.5) = 2.31 × 10⁻⁵ m³/s = 0.023 L/s.
- Buoyancy = 1.095 × 10.5 = 11.5 Pa.
- Friction = 128 × 0.00065 × 3 × 2.31 × 10⁻⁵ ÷ (π × 1.6 × 10⁻⁷) = 11.5 Pa.
The two match, so the loop settles at ΔT ≈ 10.5 K and 0.023 L/s. The speed is 0.023 × 10⁻³ ÷ (π × 0.01²) = 0.073 m/s, and the Reynolds number is 0.073 × 0.020 ÷ (0.66 × 10⁻⁶) ≈ 2,200, just inside laminar flow, so the formula holds only roughly. Treat the answer as the scale of the effect.
What the arithmetic teaches:
- Heat input sets the temperature rise. More heat means a larger ΔT, a modest rise in flow.
- Friction falls as the fourth power of the bore. A 32 mm pipe has (32 ÷ 20)⁴ = 6.6 times less laminar friction than a 20 mm pipe over the same flow. Short, wide, smooth pipes matter most.
- Fittings and bends add resistance that this smooth-pipe calculation leaves out. Keep elbows few.
Time To Heat A Tank
Heating 100 L from 15°C to 60°C takes 100 × 4.18 × 45 = 18,800 kJ = 5.2 kWh. At 1 kW into the water that is 5.2 hours, and at 2 kW 2.6 hours. Not all stove heat reaches the water, so real times are longer.
Layout That Works
- Heater lowest, storage above, by at least 30 cm and preferably more.
- Hot pipe from the top of the heater to the top of the storage tank; cold return from the bottom of the storage tank to the bottom of the heater.
- Every pipe rising continuously from heater to tank, with no sags or crests that trap air.
- Short, wide pipes, insulated on the hot side.
- Hot water drawn from the top of the tank; cold feed enters at the bottom.
What You Need
- A heat exchanger: a copper or stainless coil or a metal vessel in or against the fire
- A storage tank above it with a vent
- Pipes of copper or stainless steel; avoid PVC for hot water (Salvaged Modern Pipe)
- A cold feed from the header tank that is always open to the heater
Common Mistakes
- A valve or cap between the heater and the open vent. The vessel becomes a closed boiler.
- Firing the heater when it is empty. The metal overheats and the next water to enter flashes to steam (Water Level Gauge).
- A horizontal or downhill run that stalls circulation.
- Using a car radiator, which may have held antifreeze.
- Leaving warm water standing for days.
Safety
A closed, fire-heated water vessel can explode, hot water scalds, and lukewarm stored water breeds bacteria
Water that turns to steam expands to roughly 1,600 times its volume at atmospheric pressure, as I recall, so a vessel with no escape route over a fire can burst violently. Keep the system open to the air at the storage tank, never fit a valve between the heater, the tank and the vent, and never fire the heater empty. Water hotter than about 50°C scalds quickly and at 60°C in seconds, as I recall; mix hot with cold at the tap and warn children. Warm stagnant water between about 20°C and 45°C is where Legionella grows, as I recall from public-health guidance: keep stored hot water hot, or use it quickly. Keep the drinking supply apart from any non-drinking line (Plumbing Separation).
What’s Next
- Learn next: Rocket Stove Design — a clean-burning fire to heat the coil
- Combine with: Indoor Plumbing Layout — fit the heater into the house plan