Grades 6–8 · Everyday Life
How does a tankless water heater warm water as it flows?
An electric tankless water heater warms water on its way through a small heating passage. It does not keep a large tank of hot water ready. In a common design, sensing water flow starts electrical heating, and controls regulate the supply while water passes.
In the diagramFlowing water receives energy as it passesIdeal example: 8.4 kW into water · no storage tankIn: 15°COut: 35°CHeating passage6 L/min · rise of 20°CMore water each second shares the same power.Real controls vary power or flow; capacity is still limited.
Heating capacity still has a limit. Power measures how much energy can be transferred each second. If twice as much water passes each second while heating power stays the same, each kilogram receives half as much energy. Its temperature rise is therefore smaller.
The invented example sends 8,400 joules each second into the water, with no heat loss. At 6 liters per minute, an assumed density of 1 kilogram per liter gives 0.1 kilogram per second. Using 4,200 joules to warm 1 kilogram by 1°C, the rise is 8,400 ÷ (0.1 × 4,200) = 20°C. Water entering at 15°C therefore leaves at 35°C in this ideal account.
Real heaters can change electrical power, and some restrict flow to maintain a desired temperature. They cannot supply an unlimited temperature rise at an unlimited flow. Water already sitting in the connecting pipes can also take time to reach a tap; heating on demand does not remove that travel time.
About this explanation
Starting knowledge: Recognize power as energy per second, and use division and temperature differences.
Sources
- Stiebel Eltron — Water Heating Information & Options
- US Department of Energy — Water Heating: Technology Fact Sheet