A copper–water closed flat pulsating heat pipe (PHP), 3.4 m long and of inner diameter 4 mm, was manufactured and tested. This PHP aims both at homogenizing the temperature of a large aluminum plate, of surface area 150 × 420 mm 2 , and at lowering its mean temperature by transferring the heat to an adjacent heat sink of same dimensions. The PHP thermal resistance is measured at various heat transfer rates, condenser temperatures, and inclination angles. It decreases when the heat transfer rate increases or when the PHP is progressively tilted from the vertical unfavorable orientation to a favorable one. Resistance values as low as 0.04 K/W are measured. Whatever the conditions, a minimum heat input of 200 W is necessary for the correct start-up of the PHP. A map of the operating regimes—no-flow, oscillating, intermittent, and stable behavior—is proposed. Nonreproducibility effects are highlighted in tilted position, leading to different operating regimes at increasing and decreasing heat loads, but also more broadly depending on the history of the working operating conditions of the PHP. However, the concept proposed in this work is very promising for applications involving large heat source and heat sink.