We investigate decay modes of spin-1 heavy vector bosons (V’) from the viewpoint of perturbative unitarity in a model independent manner. Perturbative unitarity requires some relations among couplings, which are called unitarity sum rules. We derive the relation between V’ couplings to the SM fermions (f) and V’ couplings to the SM gauge bosons (V). Using the coupling relations, we calculate partial decay widths for V’ decays into VV and ff. We show that Br(W’ → WZ)< 0.02 in the system that contains V’ and CP-even scalars as well as the SM particles. This result is independent of the number of the CP-even scalars. We also discuss what kind of interactions can make Br(V’ → V V ) larger than Br(V’ → ff). Our result is a useful guideline to construct models that predict Br(W’ → WZ) > 0.02.