Minimum activation flow
Minimum activation flow is the smallest water flow (typically 0.25–0.6 GPM) that triggers a tankless water heater to fire. Below it, the unit stays off and the tap runs cold.
A trickle of hot water — shaving, rinsing a razor, a low-flow faucet barely open — can sit below the activation threshold, producing the classic tankless complaint of lukewarm water at low flows.
Lower is better. Models with 0.25–0.4 GPM thresholds handle low-flow fixtures gracefully; units with high thresholds pair badly with water-saving aerators.