In the paper we consider the Bessel differential operator L^(μ)=\dfrac{d^2}{dx^2}+\dfrac{2μ+1}{x}\dfrac{d}{dx} in half-line (a,\infty), a>0, and its Dirichlet heat kernel p_a^(μ)(t,x,y). For μ=0, by combining analytical and probabilistic methods, we provide sharp two-sided estimates of the heat kernel for the whole range of the space parameters x,y>a and every t>0, which complements the recent results given in [1], where the case μ\neq 0 was considered.