Passat heater problem

I’ve got a 2004 Passat 1.8T, manual transmission, that has an odd heater problem. It will send out hot air only if the outside temperature is 50 degrees or above. Under 50, it blows whatever air temp is outside. AC is working fine. Anybody know what gives?

Is the 50 degree above condition the only condition connected with the loss of heat output? Any recent cooling system work done? Any cooling system maintiance that is due? Could you be incorrectly connecting these conditions?

Consider all the normal poor heat output causes first before you lable your condition odd. Have you done any diagnosis yet?

Scratching the bottom of the barrel for Ideas a few possibilities come to mind. Is coolant level full?

Does your car have a temp gauge, does it read normal. Possible thermostat stuck open and at the cooler temperature coolant does not heat enough to blow warm air.

Blend door sticking at cooler temperature and not allowing heat in.

No diagnosis done yet and no other issues with the heat output. Car had a full service 3,000 miles ago, and it hasn’t had any cooling system work done that I can remember. I included the information that the air conditioning worked fine simply because I thought there might be some connection. In any event, the only symptom is that the heater won’t blow hot air once the outside temp is below about 50.