The ESA CMB Planck mission will require an accurate control and removal of instrumental systematics below a level of few μK. Although instruments and spacecraft are designed to he instrinsically very stable, the extremely tight requirements concerning signal stability call for the development of software strategies to remove the residual spurious effects during data analysis. A particularly important class of systematic effects is represented by low-frequency periodic fluctuations that can be removed using properly designed ‘Blind’ filters applied to the scientific data streams. However, telemetry information monitoring the instrument status may improve the effectiveness of removal procedures. A good example is given by thermal instabilities of the 20 K stage of the PLANCK cooling chain. A successful, yet simple approach to remove such fluctuations has been developed; the method combines thermal housekeeping data with scientific data to reconstruct the transfer functions connecting the 20 K stage temperature fluctuations with the perturbations detected by each radiometer. The work has been carried out using temperature measurements of the prototype of the Planck 20K Sorption Cooler combined with a thermal model of the LFI instrument and a software specifically developed to simulate long term-variations which are not available from the Cooler experimental data. This code (named GLISSANDO) is also discussed at the end of this paper.