Heater issue

[Darth Menace]

Hey guys, I know this isn't Viper related or even cool vehicle related, but the generic dodge forum isn't too helpful when it comes to troubleshooting so I was hoping you guys might be able ot help me (and this isn't the alley, so I likely won't get criticised too much for the vehicle. Anyway, here is the issue. I have a problem with the heating that just started a few days. When I am driving the van, and the heat is on, it will blow hot (from the defrost, lower leg parts, etc.) but after about 20-30 minutes it will blow cold. The first time it happened I thought that the heater core had gone, but next day the exact same thing happened. There is no antifreeze leak at the firewall, or anywhere else for that matter so I don't think it is the heater core. I have done no work on the cooling system, so I don't know where to start. The antifreeze level is good.

[daytonprowler]

Sounds like a possible vacuum problem. There is usually a valve that is inline with one of the heater hoses. It opens when the temp settings are set to warm-hot. Valve is supposed to be closed when temp is set to cold. It maybe closing for some reason.......