Clio heater - no other thread found - Steve Somers
Other daughters Clio has a cool heater
I've bled it,checked for exhaust gases (recent new h/gasket), visually inspected controls, both in and out are HOT (???) but little hot air.
Forced flushing with a farmsize hose improved things slightly.
Rad flush emptied my wallet of £1.80 and little else.
A local radiator refurb company says to try bleach!?
If its iron rust - what can 'dissolve' rust?
If its alloy heater matrix corrosion why is there apparently good water flow?
If thine heater offends thee - RIP IT OUT? (entry wound via dashboard!)
SS :0(

Clio heater - no other thread found - Phil I
As both in and out are hot your trouble is on the controls .
Common fault here is the clip which holds the bowden cable (on the right hand side of the box) allowing the cable to slip slightly which then means the flap doesnt close correctly.
Try holding the flap shut with the end of the lever and see if you then get hot air to vents.

hth
Phil I