We study in details a pumping mechanism for the λ = 1.35 cm maser transition 616 → 523 in ortho-H2O based on the difference between gas and dust temperatures. The upper maser level is populated radiatively through 414 → 505 and 505 → 616 transitions. The heat sink is realized by absorbing the 45 μm photons, corresponding to the 523 → 414 transition, by cold dust. We compute the inversion of mas...