Rheem Professional Classic RTEX electric tankless water heaters: quick sheet

When a hot tap opens and flow through the unit reaches the turn-on rate (0.3 GPM on the RTEX line), the flow sensor signals the control board. The board reads the inlet and outlet temperatures and fires the elements through solid-state switching, adjusting power continuously (self-modulation) to hit the setpoint. When flow stops, heating stops; there is no stored hot water.

Capacity is a temperature-rise limit, not a gallon number. A given kW can raise a given flow by a fixed number of degrees, so in winter (cold inlet) the same unit delivers far less GPM at shower temperature than in summer. If the draw is above what the unit can heat, the outlet temperature simply drops; this is the most common 'it's broken' complaint and is not a defect.

Power is split into circuits: RTEX-13 runs on one 60 A double-pole breaker; RTEX-18 on two 40 A, RTEX-24 and RTEX-27 on three 40 A, and RTEX-36 on four 40 A. Each breaker feeds its own chamber/element group. If one breaker trips or one leg is loose, the unit still runs but delivers lukewarm water. On 208 V the same heater produces noticeably less power (the 208 V Performance listings show about 13 kW for the RTEX-18 frame).

Setpoint is adjustable in 1-degree steps from about 80 F to 140 F on the front control. Low supply voltage, scale on the elements, and a restricted inlet screen all reduce output or cause the safety limit to open.