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How Thermosiphon Works

5 min read

A thermosiphon loop circulates because water in the hot column is less dense than water in the cold column, and the weight difference over the height between them is the driving pressure: with a 0.5 m height and water at 20°C and 60°C, that pressure is about 74 Pa, a tiny force that still turns a short, wide loop. The flow settles where the heat carried away matches the heat gained, which for a collector delivering 500 W with a 10 K rise across it is about 43 L/h.

Part of Thermosiphon Systems

Thermosiphon circulation is flow driven by density differences in a loop of water heated at one point and cooled at another.

The Driving Pressure Is Small, And That Sets The Rules

The driving pressure is Δp = g × h × Δρ, where h is the height of the hot-and-cold columns, Δρ is the density difference between them, and g = 9.81 m/s². Inputs: water density 998.2 kg/m³ at 20°C and 983.2 kg/m³ at 60°C (standard handbook values, recalled); h = 0.5 m.

  1. Δρ = 998.2 − 983.2 = 15.0 kg/m³.
  2. Δp = 9.81 × 0.5 × 15.0 = 73.6 Pa.

For comparison, one metre of water head is about 9,800 Pa, so 74 Pa is under 1% of that. The loop can only drive flow through short, wide, smooth pipes; every elbow, valve and sag takes a share of the little pressure there is. That is the reason for the rules on pipe layout: short runs, wide bore, continuous rise, no valves (Plumbing Connections). The wiki’s hot-water page balances this driving force against pipe friction for a fire-heated loop and finds friction falls with the fourth power of the bore (Hot Water Systems).

How Fast Does The Water Move?

The flow adjusts until the heat carried by the water equals the heat the collector delivers. Heat carried = flow × 4.18 kJ/kg·K × temperature rise across the collector.

Inputs, assumed for the example: the collector delivers P = 500 W to the water; temperature rise across the collector ΔT = 10 K.

  1. Mass flow = P ÷ (c × ΔT) = 500 W ÷ (4,180 J/kg·K × 10 K) = 0.01196 kg/s.
  2. In litres per hour: 0.01196 × 3,600 = 43 kg/h = about 43 L/h.

A stronger sun delivers more power, which raises the temperature rise and the flow together, and the circulation speeds up as the day gets sunnier. Over six hours at 43 L/h, 258 L passes through the collector, so a 200 L tank is turned over about 1.3 times and ends up reasonably mixed, though stratification at the top may leave the hottest water there (Stratification & Draw-Off).

Why The Tank Must Be Above The Collector

At night the collector cools. Its water becomes denser than the water in the tank above it, so it stays at the bottom of the loop and does not rise. The loop shuts itself off. If the tank were level with or below the collector, the heavy cold water in the collector could sink into the tank and push warm tank water up into the collector, where the glass would cool it: a reverse flow that drains the tank’s heat overnight.

The parent page sets the minimum at 30 cm for the tank bottom above the collector top; a taller column increases Δp in proportion to the extra height.

What You Need

  • Wide, short pipe with a continuous rise from collector top to tank
  • A tank with its bottom above the collector top
  • A vent at the high point and a drain at the low point
  • A thermometer or hand on both pipes to diagnose circulation

Method: Diagnose A Loop That Will Not Turn

Step 1: On a clear noon, feel the top pipe and the return. If both are cold, the collector is not heating; check the glass and paint. If the top is hot and the return is hot, the flow is slow or stalled.

Step 2: Check for air: open the vent. Air in a high point blocks the loop.

Step 3: Check for sags: a dip holds water at the wrong height.

Step 4: If the layout is right but flow is weak, increase pipe bore or raise the tank.

Common Mistakes

  • Treating 74 Pa like normal plumbing pressure. Taps need the water-supply head, not the loop’s buoyancy (Calculating Head Pressure).
  • Narrow pipe or many elbows. They absorb the whole driving pressure.
  • A closed valve at the vent. It makes the loop a boiler.

Safety

The loop is only safe if it can vent steam

Circulation stops if the tank is full of hot water and nothing is drawn, and the collector keeps heating: it then stagnates above boiling. A closed loop with no pressure relief is a pressure vessel heated by the sun, and the vent or relief path is not optional. The tank water also scalds; temper the delivery and keep children away from taps.

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