Why the heater does not blow hot air?

My best guess is that the heater control valve is not opening, it could be the cable that controls it or the heater valve itself. When the car is up to normal temperature with the heater on full hot, fan on lowest setting, both heater hoses should be hot to the touch (about the same temp as the upper radiator hose) if one or both are not there is no coolant circulating through the heater core.

This can also be the symptom of a bad heater core or head gasket failure. Check for both of those issues if the control valve does not fix your problem.