  We describe a method to compute renormalized coupling constants in a Monte
Carlo renormalization group calculation. It can be used, e.g., for lattice spin
or gauge models. The basic idea is to simulate a joint system of block spins
and canonical demons. Unlike the Microcanonical Renormalization Group of Creutz
et al. it avoids systematical errors in small volumes. We present numerical
results for the O(3) nonlinear sigma-model.
