Top: temperature measured by two sensors mounted on the HFI 4-K shield at the level of 30 and 44GHz (bottom curve) and 70GHz (top curve) reference loads. The rms variation of this temperature over the full mission is σ_30,44 = 1.55mK and σ₇₀ = 80μK. Middle: temperature of the 20-K focal plane measured by a temperature sensor placed on the flange of the LFI28 30GHz feedhorn. The consecutive temperature steps after day 500 reflect changes to the set point of the temperature control system. We applied these changes to control the level of temperature fluctuations. Bottom: temperature of the 300-K back-end unit.