A method for evaluating a clogging level of an air filter of an HVAC unit, the method comprising the following steps: - acquiring a first plurality of parameters ΔP raw representative of a pressure loss between two sides (28A, 28B) of the air filter at a plurality of moments, - calculating a second plurality of parameters ΔP seg from the first plurality of parameters ΔP raw, each of the parameters ΔP seg respectively being representative of an average of at least some of the parameters ΔP raw over a past time segment, - obtaining a variety of scenarios Si representative of future operating conditions of the HVAC unit on a plurality of future time segments, - obtaining a parameter ΔP max representative of a maximum authorized pressure loss between the two sides (28A, 28B) of the air filter, - using a model, for each of the scenarios Si, calculating a third plurality of parameters (ΔP' seg) i representative of a future evolution of the pressure loss between the two sides (28A, 28B) of the air filter on at least some of the future time segments, characterised in that each of the scenarios respectively has a probability pi, the method further comprising the following steps: - using the third plurality of parameters (ΔP' seg) i, calculating the clogging level of the air filter in the form: - either of a likely residual useful lifetime of the air filter, representative of a likely length of time at the end of which the parameters (ΔP' seg) i will have reached the parameter ΔP max, - or of a likelihood so that, at the end of a given length of time H, the parameters (ΔP' seg) i have reached the parameter ΔP max, - for one of the past time segments, preferably the last in time, calculating a parameter K by using the parameter ΔP seg corresponding to said past time segment, the parameter K being representative of a hydraulic resistance of the air filter, the hydraulic resistance being defined as the ratio of said parameter ΔP seg divided by the square of an air flow rate Q air in the air filter, the air flow rate Q air being an air flow rate capable of creating, in the air filter, a pressure loss equal to said parameter ΔP seg; and - from the parameter K, calculating a parameter D representative of a difference in hydraulic resistance between the air filter and a new air filter, the step for calculating the likely residual useful lifetime of the air filter or the likelihood so that, at the end of a given length of time H, the parameters (ΔP' seg) i have reached the parameter ΔP max, is carried out only if the parameter D representative of a difference in hydraulic resistance between the air filter and a new air filter has exceeded a predetermined threshold D 0. - acquiring a first plurality of parameters ΔP raw representative of a pressure loss between two sides (28A, 28B) of the air filter at a plurality of moments, - calculating a second plurality of parameters ΔP seg from the first plurality of parameters ΔP raw, each of the parameters ΔP seg respectively being representative of an average of at least some of the parameters ΔP raw over a past time segment, - obtaining a variety of scenarios Si representative of future operating conditions of the HVAC unit on a plurality of future time segments, - obtaining a parameter ΔP max representative of a maximum authorized pressure loss between the two sides (28A, 28B) of the air filter, - using a model, for each of the scenarios Si, calculating a third plurality of parameters (ΔP' seg) i representative of a future evolution of the pressure loss between the two sides (28A, 28B) of the air filter on at least some of the future time segments, - using the third plurality of parameters (ΔP' seg) i, calculating the clogging level of the air filter in the form: - either of a likely residual useful lifetime of the air filter, representative of a likely length of time at the end of which the parameters (ΔP' seg) i will have reached the parameter ΔP max, - or of a likelihood so that, at the end of a given length of time H, the parameters (ΔP' seg) i have reached the parameter ΔP max, - either of a likely residual useful lifetime of the air filter, representative of a likely length of time at the end of which the parameters (ΔP' seg) i will have reached the parameter ΔP max, - or of a likelihood so that, at the end of a given length of time H, the parameters (ΔP' seg) i have reached the parameter ΔP max, - for one of the past time segments, preferably the last in time, calculating a parameter K by using the parameter ΔP seg corresponding to said past time segment, the parameter K being representative of a hydraulic resistance of the air filter, the hydraulic resistance being defined as the ratio of said parameter ΔP seg divided by the square of an air flow rate Q air in the air filter, the air flow rate Q air being an air flow rate capable of creating, in the air filter, a pressure loss equal to said parameter ΔP seg; and - from the parameter K, calculating a parameter D representative of a difference in hydraulic resistance between the air filter and a new air filter,