I have my heater fan on full, but it blows allmost no air. It's as if there is some thing blocking the air intake under the dashbord. Having it set to recycled or air from outside the car makes no difference.
If your Megane has air con, it has a pollen filter which could well be blocked....lift the bonnet, below windscreen passenger side there is a plastic flap with the filter inside.
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Try fixfault.com to fix fan heaters.