Check the cables on my 97 +4 the cable moved a flap to allow hot or cold air in, you. As discussed flush the core. I had a Renault 5 TS once that was full of sludge in the heater core.

It should be a very effective heater.

JohnV6
2022 CX Plus Four
2025 Renault Scenic Alpine etech "Yvette"