The heat quenching factor Q (the ratio of the heat signals produced by nuclear and electron recoils of equal energy) of the heat-and-ionization germanium bolometers used by the EDELWEISS collaboration has been measured. It is explained how this factor affects the energy scale and the effective quenching factor observed in calibrations with neutron sources. This effective quenching effect is found to be equal to Q/Q, where Q is the quenching factor of the ionization yield. To measure Q, a precise EDELWEISS measurement of Q/Q is combined with values of Q obtained from a review of all available measurements of this quantity in tagged neutron beam experiments. The systematic uncertainties associated with this method to evaluate Q are discussed in detail. For recoil energies between 20 and 100 keV, the resulting heat quenching factor is Q = 0.91+-0.03+-0.04, where the two errors are the contributions from the Q and Q/Q measurements, respectively. The present compilation of Q values and evaluation of Q represent one of the most precise determinations of the absolute energy scale for any detector used in direct searches for dark matter.
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