Renault Scenic Megane 1997 heater blowing cold air only

The heater is not working in the car it only blows cold air even when the engine is hot. Could this be something simple or is it a garage repair job?

I don’t know the car, but likely it is the blend door. On some cars it is vacuum driven so any vacuum problem can cause the problem. Find a vacuum hose map and start checking for cracked or disconnected hoses. It may also be electric controlled or mechanically controlled. Other than that you could have a bad heater hose or even a bad thermostat.