The heat quenching factor Q ^ { \prime } ( 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 ^ { \prime } , where Q is the quenching factor of the ionization yield .
To measure Q ^ { \prime } , a precise EDELWEISS measurement of Q / Q ^ { \prime } 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 ^ { \prime } are discussed in detail .
For recoil energies between 20 and 100 keV , the resulting heat quenching factor is Q ^ { \prime } = 0.91 \pm  0.03 \pm  0.04 , where the two errors are the contributions from the Q and Q / Q ^ { \prime } measurements , respectively .
The present compilation of Q values and evaluation of Q ^ { \prime } Ê represent one of the most precise determinations of the absolute energy scale for any detector used in direct searches for dark matter .