Heater doesn't blow warm air
 

I have a '94 Beretta 2.2l and am having problems with the heater. I can't get it to blow warm air. It will switch from vent to defroster and the blower speed all works fine but it only blows cold air. I have tried replacing both the thermostat and temperature control on the control panel with no luck. Cooling system is full with coolant as well so this shouldn't be part of the problem and car does not overheat at all either. There is a sort of whistling/humming sound coming from the defroster when I have the blower motor on low. Anyone have any suggestions?

See if both hoses going to the heater are hot. If both are hot then the heater core is good. If only one is hot then the core is clogged. If the heater core is good then you have a bad blend door actuator.

Where exactly are these hoses located?

Coming from the engine going to the heater connection at the firewall.

I checked the heater hoses and they seem fine. How sure are you about it being the blend door actuator? What about a vacuum leak somewhere, is that a possibility?

You said the heater hoses are both hot. That means that there is heat in the heater core for the fan to circulate. If the hot air is not getting to the vents then it has to be the blend door not moving. The blend door actuator is electric and does not need vacuum to operate correctly.

I would tend to agree with oldchevy, many items in vehicles built in the 70-80's did have vacuum operated items, but by the 90's, and yours being a 94, its most likely electronically driven... with the key on, engine off, try selecting different options of AC/Heat and floor vent/face vents and listen for the doors actuating / transitioning to each position...

Thanks for all your help guys! Turns out it was the air door actuator. There was a broken tab which wasn't allow it to actually turn the door open.