  In this paper we present results obtained with the new grain code in Cloudy
which underline the strong effect of photo-electric heating by grains in
photo-ionized regions. We study the effect that the distribution of grain sizes
has on the magnitude of the effect, and show that this effect is nothing short
of dramatic. This makes the grain size distribution an important parameter in
modeling of photo-ionized regions such as H II regions and planetary nebulae.
