© 2018 C. Chuang et al. We investigate the energy relaxation of hot carriers in a CVD-grown graphene device with a top h-BN layer by driving the devices into the nonequilibrium regime. By using the magnetic field dependent conductance fluctuations of our graphene device as a self-thermometer, we can determine the effective carrier temperature T e at various driving currents I while keeping the lattice temperature T L fixed. Interestingly, it is found that T e is proportional to I, indicating little electron-phonon scattering in our device. Furthermore the average rate of energy loss per carrier P e is proportional to (T e 2 - T L 2), suggesting the heat diffusion rather than acoustic phonon processes in our system. The long energy relaxation times due to the weak electron-phonon coupling in CVD graphene capped with h-BN layer as well as in exfoliated multilayer graphene can find applications in hot carrier graphene-based devices.