Check that the two hoses going to the heater matrix are both hot - my guess is that they will be cool, indicating that the coolant isn't circulating. Some Renaults have a plastic bleed screws actually on the top of the hoses - undo them and let any air escape. If there aren't any then undo the top matrix hose, very gently prise it clear and let air escape that way.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix fan heaters.