Vents blowing hot air all the time

Most likely the heater control valve has failed, see if the is a manual valve on the heater hose line that can be turned off will confirm it.

 

Possibly a blend door issue

 

If your someplace still warm this time of the year. Nothing might be wrong. Their always seems to be some hot coolant flowing thru the heater core. Unless your truck has the manual shut off valves to cut all the coolant flow off you will get hot air at about 68F outside temp I usually find. It only a problem if you a/c is not working and you go south on the winter. That how freightliner is anyway.

It could be something like others already said if you don't have any control of heat and it's winter.

 

On my Freightliner C120, I have an electronic water control valve, which shuts the the coolant off from flowing to the heater core when on cold. It is controlled by a cab thermostat, which in turn is controlled by the the temp control dial that has a registered. I don't know about newer KWs, but I bet the system is electronically controlled. In which case, it could be a one of 3 parts that failed, like mine. Mine failed in the thermostat, and it defaulted to heat all of the time.

 

I meant to say resistor, not registered. Stupid autospell!

 

I drove a 2016 Kenworth T680 for awhile that had this same issue. Always blew hot air, even with the HVAC ‘off’. I got to driving around in winter, putting the windows down every few minutes to vent the cab.

The company shop didn’t fix it. I just remember them saying it didn’t throw any codes when hooking it up to diagnostics.

 

Did you press the defrost button, the one all the way to the left? Then it's functioning normally. Try to find a copy of the Owners Manual it's actually explained in there.

 

It's a winter, and I honestly envy you. You've got such a carrying truck, which decided to keep you warm without even your participation.