What happens after you open a hot tap
Flow begins
Opening a hot fixture lowers pressure on the hot side and moves cold supply water through the heater. The unit must see at least the model's activation flow before it starts heating. [1]
The controller performs checks
The control reads sensors and, on a gas model, begins its approved fan, ignition, and flame-proving sequence. Electric models stage or modulate elements according to their design.
Heat crosses the exchanger
The burner or elements heat a metal heat exchanger, and moving water absorbs that energy. Faster flow or colder inlet water requires more input to reach the same outlet temperature.
Output modulates
Temperature and flow sensors provide feedback so the controller can adjust firing rate or electrical input within the model's operating range. It cannot exceed the unit's maximum capacity.
The call ends
When the fixture closes, the flow signal disappears, heat stops, and a model-specific post-purge or pump sequence may continue briefly. Exact sequences and displayed codes belong to the installed model's documentation. [4]
The parts involved in a tankless heating cycle
- The inlet screen or filter protects internal water passages and may require model-directed inspection.
- The flow sensor tells the controller that a hot-water draw has begun.
- The heat exchanger transfers energy to moving water without storing a full tank.
- Gas models include a gas valve, burner, ignition and flame-sensing controls, fan, combustion-air path, and exhaust vent. [2]
- Electric models use high-current heating elements and electrical controls sized to the equipment.
- Temperature sensors and the controller regulate operation and stop the unit when a monitored condition falls outside its approved range.
Normal operation and signs of a problem
Normal sequence
A sign to investigate
A brief startup delay followed by stable hot water while flow remains within the rated range.
Repeated hot-cold cycling, ignition attempts, shutdown codes, or temperature that cannot stabilize at ordinary demand.
A fan or burner sound during a gas heating call and a short post-purge after the tap closes.
Soot, exhaust odor, damaged venting, abnormal flame sound, or a carbon-monoxide alarm.
The display returns to standby after flow stops.
A persistent error that returns after the model's approved basic checks.
Tankless output is a heat-transfer calculation. Required temperature rise equals the desired outlet temperature minus incoming-water temperature. Simultaneous flow is the combined hot-water flow through every active fixture. As either flow or temperature rise increases, the heater must add more energy. Published performance tables show how much flow a particular model can deliver at different rises. [1]
This is why a unit can support one shower but lose temperature when a second high-flow use begins, or perform differently as incoming water changes seasonally. The selected temperature on the display is a target within the equipment's capacity, not a guarantee at any possible flow.