  We present a survey of the formaldehyde emission in a sample of eight Class 0
protostars obtained with the IRAM and JCMT telescopes. The data have been
analyzed with three different methods with increasing level of sophistication.
We first analyze the observed emission in the LTE approximation, and derive
rotational temperatures between 11 and 40 K, and column densities between 1 and
20 x 10^13 cm^-2. Second, we use a LVG code and derive larger kinetic
temperatures, between 30 and 90 K, consistent with subthermally populated
levels and densities from 1 to 6 x 10^5 cm^-3. The column densities from the
LVG modeling are within a factor of 10 with respect to those derived in the LTE
approximation. Finally, we analyze the observations based upon detailed models
for the envelopes surrounding the protostars, using temperature and density
profiles previously derived from continuum observations. We approximate the
formaldehyde abundance across the envelope with a jump function, the jump
occurring when the dust temperature reaches 100 K, the evaporation temperature
of the grain mantles. The observed formaldehyde emission is well reproduced
only if there is a jump, more than two orders of magnitude, in four sources. In
the remaining four sources the data are consistent with a formaldehyde
abundance jump, but the evidence is more marginal (~2 sigma). The inferred
inner H2CO abundance varies between 1 x 10^-8 and 6 x 10^-6. We discuss the
implications of these jumps for our understanding of the origin and evolution
of ices in low mass star forming regions. Finally, we give predictions for the
submillimeter H2CO lines, which are particularly sensitive to the abundance
jumps.
