The global asymptotic stability of the unique steady state of a nonlinear scalar parabolic equation with a nonlocal boundary condition is studied. The equation describes the evolution of the temperature profile that is subject to a feedback control loop. It can be viewed as a model of a rudimentary thermostat, where a parameter controls the intensity of the heat flow in response to the magnitude of the deviation from the reference temperature at a boundary point. The system is known to undergo a Hopf bifurcation when the parameter exceeds a critical value. Results on the characterization of the maximal parameter range where the reference steady state is globally asymptotically stable are obtained by analyzing a closely related nonlinear Volterra integral equation. Its kernel is derived from the trace of a fundamental solution of a linear heat equation. A version of the Popov criterion is adapted and applied to the Volterra integral equation to obtain a sufficient condition for the asymptotic decay of its solutions. Following the terminology used in [17] we need to distinguish between the concept of the attractor Âβ and the B-attractor A β when formulating our main results in Theorem 5.1. We point out that, in general, the attractor Âβ is a proper subset of the B-attractor A β . We prove that the continuous semiflow Φ β induced on H 1 (0, π) by the system (1.1) has a global attractor Âβ = {0} for β ∈ (0, β 0 ) and that for β ∈ (0, 4 π ) the B-attractor and the attractor coincide, i.e. A β = Âβ = {0} . In [7], the authors prove that the global B-attractor A β exists for β ∈ (0, ∞) and that A β = {0} for β ∈ (0, 1 π ) . We thus extend previous results by determining larger parameter ranges where the B-attractor and the attractor are shown to be equal to {0} . The existence of the B-attractor A β shown in [7] for β ∈ (0, ∞) implies, in particular, that all orbits are bounded in the underlying Banach space. Since H 1 (0, π) → C [0, π] , the orbits satisfy Φ β (t, u 0 ) ∞ ≤ c(u 0 ) < ∞, t ≥ 0.