Q: heater works for about 20 mins then starts to blow cold air

Check for low coolant level and, if low, there is a possibility of air trapped in the system. Check for collapsed (or soft) hoses. Check your engine operating temperature to see if it is in the normal range of 195 to 215 degrees, roughly. If the thermostat is sticking, in a too open position, that will cause engine temperature to be too low and will cause inadequate cabin heat. If this is the cause, the fix would be to replace the thermostat. If the heater core were plugged, or the water pump faulty, the problem you are having would not be intermittent. Indeed, if the water pump was not runnnig at adequate RPM, the engine would be operating on the hot side, if not overheating. If you desire a diagnostic to determine exactly why the level of cabin heat is fluctuating, please request a non functioning cabin heater diagnostic and the responding mechanic will come right to your location and get this resolved for you promptly. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.