  We study in details a pumping mechanism for the lambda=1.35 cm maser
transition 6_16 -> 5_23 in ortho-water based on the difference between gas and
dust temperatures. The upper maser level is populated radiatively through 4_14
-> 5_05 and 5_05 -> 6_16 transitions. The heat sink is realized by absorbing
the 45 mum photons, corresponding to the 5_23 -> 4_14 transition, by cold dust.
We compute the inversion of maser level populations in the optically thick
medium as a function of the hydrogen concentration, the gas-to-dust mass ratio,
and the difference between the gas and the dust temperatures. The main results
of numerical simulations are interpreted in terms of a simplified four-level
model. We show that the maser strength depends mostly on the product of
hydrogen concentration and the dust-to-water mass ratio but not on the size
distribution of the dust particles or their type. We also suggest approximate
formulae that describe accurately the inversion and can be used for fast
calculations of the maser luminosity. Depending on the gas temperature, the
maximum maser luminosity is reached when the water concentration N_water ~
10^6-10^7 cm^-3 times the dust-to-hydrogen mass ratio, and the inversion
completely disappears at density just an order of magnitude larger. For the
dust temperature of 130 K, the 6_16 -> 5_23 transition becomes inverted already
at the temperature difference of Delta T ~1 K, while other possible masing
transitions require a larger Delta T > 30 K. We identify the region of the
parameter space where other ortho- and para-water masing transitions can
appear.
