I assume you set the temperature to hot, e.g. 25C/90F and higher, but after engine warmed up, you don't get any hot air.
There are several possibilities:
1. Low coolant - open up filler cap and fill in fresh coolant, or distill water (temporary only for troubleshooting)
2. Coolant flow control valve that is called "heater control valve" stuck/rusted - follows the hot coolant hose (1/2" to 5/8" hose) towards the firewall, and you will see a rectangular valve. See YouTube
. You should be able to manually move the valve to confirm it is broken/stuck
3. Thermostat failure - stuck thermostat. See YouTube
4. Coolant temperature sensor failure - there are 2 temp sensors called ECT that are identical. Swap them to see whether you will get hot air on hot engine. After confirmed broken, replaced them by follow YouTube
. This is a common old car "maintenance"
Hope this list is complete
