A hot water tank stores heat in water, and 200 L warmed from 20°C to 60°C holds 33.4 MJ, or 9.3 kWh, enough for about seven baths of 40 L. Keep stored water hot, at 60°C or more, because lukewarm tank water between about 25°C and 45°C lets Legionella bacteria multiply, and temper the water at the tap, because water at 60°C scalds a child in seconds.
Part of Heat Storage Systems
A hot water tank is an insulated vessel that stores heat in water, drawn off from the top and refilled from the bottom.
What A Tank Holds
Heat stored = mass × 4.18 kJ/kg·K × temperature rise. Worked for the parent page’s 200 L tank (200 kg of water):
- Rise from 20°C to 60°C = 40 K.
- Heat = 200 × 4.18 × 40 = 33,440 kJ = 33.4 MJ.
- In kilowatt-hours: 33,440 ÷ 3,600 = 9.3 kWh.
- A 40 L bath at a 30 K rise needs 40 × 4.18 × 30 = 5,016 kJ, about 5 MJ (the same figure the bathhouse page uses, Shared Bathing & Laundry), so 33.4 ÷ 5.0 = 6.7 baths, about seven.
The parent page suggests 4-6 litres per square metre of living space as a starting point and 200 L for a small cabin, 500 L and more for a family home. For a 500 L tank over the same 40 K: 500 × 4.18 × 40 = 83,600 kJ = 23.2 kWh. Whether that is a night’s heating or a day’s depends on the house (Insulation vs Mass Trade-offs).
The Tank’s Four Needs
- Strong, watertight walls (Tank Construction & Materials).
- Heavy insulation (Super-Insulation).
- A layered fill: hot water at the top, cold at the bottom (Stratification & Draw-Off).
- A safe vent or relief so that heating cannot build pressure (Hot Water Systems).
Two Hazards Pull In Opposite Directions
Storing water hot prevents bacterial growth but raises the scald risk. Water kept at 60°C or above resists Legionella, the bacterium of a lung illness caught by breathing mist from showers and taps; it multiplies in lukewarm water between about 25°C and 45°C (recalled from standard water-hygiene guidance). Scalding: commonly cited exposure times for a deep burn in adult skin (Moritz and Henriques, 1947, recalled) are around 10 minutes at 49°C, about 30 seconds at 54°C, about 5 seconds at 60°C, and about 1 second at 71°C. Children’s thinner skin burns faster, as does the skin of the elderly.
The resolution is to store hot and deliver tempered: mix hot tank water with cold at the outlet, to something near 45-50°C at the tap, and lower for children and the elderly.
What You Need
- A tank with a vent, a drain, and connections for hot out (top), cold in (bottom) and the heat source
- Insulation
- A thermostatic mixing valve or a manual blend with a thermometer at the tap
- A thermometer on the tank
Method: Use A Tank Safely
Step 1: Charge the tank to 60°C or above at least once a day if you can, to prevent bacterial growth.
Step 2: Check the tap temperature with a thermometer before anyone washes.
Step 3: Drain and clean the tank yearly to remove sediment.
Common Mistakes
- Storing at 40°C to avoid scalds. It invites Legionella.
- Drawing from the bottom. The coolest water comes out first.
- Sealing the vent.
Safety
Tank water at 60°C scalds within seconds
Tank water hot enough to be safe from bacteria is hot enough to scald badly in seconds, far hotter than a person expects from a tap. Temper it with cold water at the point of use; tempering is a safety feature, not a comfort one. Children and the elderly are injured at lower temperatures and shorter exposures than adults. Never seal a tank that is heated; it must vent (Hot Water Systems). Cool burns at once with running water (Severity Assessment).
What’s Next
- Learn next: Tank Construction & Materials — what to build it from
- Combine with: Solar Thermal Collectors — the heat source