Why does Renault espace heaters only work on full speed?

If the heater is running normally on full speed then the likely cause is that the resistor block needs replacing. If you have a Mk 1 then it may be the switch on the fascia panel as it was used to switch the current to each resistor. The Mk 2 replaced the heavy current through the switch by using relays which are distributed around the car in various fuse boxes for some reason. Two will be found under the bonnet.

The resistor block itself is mounted adjacent to the blower motor and is almost totally inaccessible without some dismantling of the fascia.