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Stratification & Draw-Off

4 min read

Hot water floats on cold, so a tall tank settles into layers, and drawing from the top and refilling at the bottom keeps the layers separate. The layers matter because the same heat is worth more when concentrated: a tank with 100 L at 60°C over 100 L at 15°C can supply 150 L of 45°C water, while the same energy stirred together at 37.5°C can supply none. Connect the heat source to the lower half, draw from the top, and let cold water enter gently at the bottom.

Part of Hot Water Tanks

Stratification is the layering of a tank by temperature, with the hottest water at the top, and draw-off is the way of taking hot water out without mixing it.

Layering Preserves Usefulness

Water at 60°C is lighter than water at 15°C (about 983 kg/m³ against 999 kg/m³, handbook values, recalled), so it floats. In a tall, narrow tank, the layers stay apart for hours if disturbed little. The parent page: draw hot water from the top for bathing and washing; feed cold water to the bottom; connect the heat source (stove coil, solar collector) to the lower half.

Worked example. Inputs: a 200 L tank; top 100 L at 60°C; bottom 100 L at 15°C; you want water at 45°C by mixing with cold feed at 15°C.

  1. Energy above the cold feed temperature in the hot layer = 100 × 4.18 × (60 − 15) = 18,810 kJ.
  2. Mixing fraction: x litres of hot per litre of tap water: 60x + 15(1 − x) = 45, so x = 30 ÷ 45 = 0.667.
  3. So 100 L of hot layer yields 100 ÷ 0.667 = 150 L of 45°C water.
  4. If the tank is stirred into one mixture: temperature = (100 × 60 + 100 × 15) ÷ 200 = 37.5°C. The energy is the same, but no water above 37.5°C can be drawn, so you get none at 45°C.

Stratification therefore preserves the heat’s usefulness, which is why the parent page says you get the hottest water first even if the whole tank is not fully heated.

How To Keep The Layers

  • Draw from the top, with an outlet pipe that takes water from the highest point.
  • Bring cold water in low and slowly. A fast cold jet mixes the tank. Fit a baffle plate or a downward elbow at the inlet so the water spreads gently.
  • Tall and narrow beats short and wide (parent page).
  • Put the heat source low: a heater near the bottom warms water that rises through the tank; a heater near the top heats only the top.
  • Avoid pumps and stirring unless you want to mix.

What You Need

  • A tall, narrow tank
  • A top outlet and a bottom inlet with a baffle
  • A heat source connection in the lower half
  • A thermometer on the side at several heights
  • A tempering mixer at the tap

Method: Check Your Layers

Step 1: After the tank has stood, read the temperature at the top, middle and bottom. A steady rise from bottom to top means it is stratified.

Step 2: Draw 20 L slowly and read again. The layers should move down, not blend.

Step 3: If a draw mixes the tank, slow the inlet flow or add a baffle.

Common Mistakes

  • A fast cold inlet jet.
  • A heat coil at the top.
  • Tempering at the tank. Mix at the tap, so the tank stays stratified and hot.

Safety

The top layer is the hottest water in the system

The top of a stratified tank is the part nearest 60°C and above, and it comes out first. It scalds in seconds, so temper it at the tap, especially for children and the elderly. The cool bottom layer, between about 25°C and 45°C, is where bacteria such as Legionella multiply (recalled from standard water-hygiene guidance), so heat the whole tank to 60°C or more regularly and do not leave it half-warm for days.

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