Great site. Helped me several times. My heater blows cold air-sometimes, hot air- sometimes when I am calling for hot air all the time. Air blows out of the proper vents. I am assuming there is a vacuum operated valve. Does anyone know where it is?

thanks

 

What year / model is the rig? If it's fluctuating hot / cold would suspect you have a cooling issue, possibly t-stat, heater core, low coolant.