  Several dedicated surveys focusing on early-type galaxies (ETGs) reveal that
significant fractions of them are detectable in all interstellar medium phases
studied to date. We select ETGs from the Herschel Reference Survey that have
both far-infrared Herschel and either HI or CO detection (or both). We derive
their star formation rates (SFR), stellar masses and dust masses via modelling
their spectral energy distributions. We combine these with literature
information on their atomic and molecular gas properties, in order to relate
their star formation, total gas mass and dust mass on global scales. The ETGs
deviate from the dust mass-SFR relation and the Schmidt-Kennicutt relation that
SDSS star forming galaxies define: compared to SDSS galaxies, ETGs have more
dust at the same SFR, or less SFR at the same dust mass. When placing them in
the M*-SFR plane, ETGs show a much lower specific SFR as compared to normal
star-forming galaxies. ETGs show a large scatter compared to the
Schmidt-Kennicutt relation found locally within our Galaxy, extending to lower
SFRs and gas mass surface densities. Using an ETG's SFR and the
Schmidt-Kennicutt law to predict its gas mass leads to an underestimate. ETGs
have similar observed-gas-to-modelled-dust mass ratios to star forming-galaxies
of the same stellar mass, as well as they exhibit a similar scatter.
