Heater not blowing at idle??

[Jsposito]

I'm hoping someone can lend some insight or advice. My heater blows hot and hard when I'm cruising down the highway but doesn't blow at all when sitting at idle. This is a new truck that I've just started working on so I don't know much of it's history. Does anyone have any ideas what would be causing this issue? It's going to start snowing soon and a fully functional heater sure would be nice!!