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Bread Box / ICS Collector

4 min read

A bread box or integral collector storage heater puts a 40-80 L black tank inside an insulated, glazed box tilted to the sun, and it can deliver 50-70°C water on a good day. A 60 L tank behind 1 m² of glass gains about 29 K in a day, but through the same glass it loses most of that overnight unless a cover or shutter closes the glass face at sunset.

Part of Batch Water Heaters

A bread box heater is a painted tank mounted inside a glazed, reflective, insulated box, collector and storage in one.

Day Gain, Then Night Loss

Inputs, assumed: 60 L tank behind 1 m² of glazed face; 5 kWh/m² per day; 40% reaches the water. Parent page: tank 40-80 L.

Day gain: 1 m² × 5 kWh × 0.40 = 2.0 kWh = 7,200 kJ. Water heat capacity = 60 × 4.18 = 251 kJ/K. Rise = 7,200 ÷ 251 = 28.7 K, about 29 K. From a 20°C start, 49°C.

Night loss: the glass face is the weak point. With a single-glazed face of assumed U = 6 W/m²·K and 1 m² of face, UA = 6 W/K. At the end of the day the tank is at 49°C and the night air at 15°C (assumed), a difference of 34 K, so the initial loss is 6 × 34 = 204 W. The tank’s time constant is the heat capacity divided by UA: 251,000 J/K ÷ 6 W/K = 41,800 s = 11.6 h. After 12 h, the excess above air falls by a factor e^(−12 ÷ 11.6) = 0.355: the temperature drops from 49°C to 15 + 34 × 0.355 = 27°C. Most of the day’s gain is gone by morning.

A night cover that cuts the face U to 1.5 W/m²·K gives UA = 1.5 W/K, a time constant of 46 h, and after 12 h the excess falls by e^(−12 ÷ 46) = 0.77, leaving 15 + 34 × 0.77 = 41°C.

The parent page says the insulated box retains heat for hours after sunset; the numbers confirm that, and also show why “hours” does not mean “all night” without a cover.

What You Need

  • A tank of 40-80 L — old water heater tank, pressure vessel or welded drum (parent page)
  • An insulated box with a glass front, tilted toward the equator
  • Reflective sides to bounce light onto the tank
  • A night shutter or quilt for the glass face
  • A vent, relief and draw-off connections at the correct ends (cold in at the bottom, hot out at the top)

Method: Build The Box

Step 1: Build the box around the tank with heavy insulation at the back and sides, leaving the front glazed.

Step 2: Paint the tank black; line the box with reflective material so light off the walls strikes the tank.

Step 3: Tilt it per Insolation & Angle; cold water enters at the bottom, hot water leaves at the top (parent page).

Step 4: Close the shutter at sunset and open it in the morning; the shutter is worth more than extra back insulation.

Common Mistakes

  • No night cover. The glass face drains the tank by morning.
  • Assuming 50-70°C every day. That is a good-sun figure; poor sun gives 30s.
  • A tank rated for lower pressure than the plumbing. Check the tank’s pressure rating or leave it open-vented.

Safety

A glazed tank can stagnate near boiling and scalds on release

When no water is drawn for days, a glazed tank can sit at or above 70°C and can reach boiling in long hot spells; the stagnation steam must have a safe vent or relief path (Plumbing Connections). Water at 60°C scalds deeply in seconds, so mix with cold water before it reaches a tap, especially for children and the elderly. Bacteria multiply in lukewarm water between about 25°C and 45°C (recalled from standard water-hygiene guidance), so do not leave tank water standing lukewarm for days.

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