Had this trouble since I bought it, the fans make loads and loads of noise, but sod all if anything comes through the vents. Today I had the fans on 2 but nope, I put my hand right up to the vents and felt absolutely NOTHING coming through. I put the fans on 3 and nope, not a puff! I understand this could be down to the pollen filters being blocked? If so how big a job is it and what sort of prices am I looking at?