Heater issue on '04 Silverado

[Crakums_GT]

A few weeks ago I got into some snow just a bit too deep and got it all packed up around the oil pan, radiator, and suspension. I put her in the garage and let all the snow melt but since then, the heater won't heat while the truck is idling. However, once the engine is warmed up all I have to do is rev the engine up to about 2000RPM and the heater blows nice hot air. I have checked a few things but can't figure out what might be causing this problem. Any help would be MUCH appreciated. Thanks.

-Greg

[Tunnelmotor99]

There are several things that could possibly cause this problem...low coolant, restricted heater hoses or heater core... I think the doors inside the HVAC case are vacuum controlled (but don't quote me on that). If there was a vacuum leak to the heat/cool door, when you are at idle there isn't enough vacuum in the system to keep the door open but if you rev the engine the vacuum increases, allowing the door to open.