GM-GMC: not blowing hot air, coolant level, chevy silverado

Question
Hi, I have a 2004 Chevy Silverado 1500. If I start my truck to let it warm up, it only blows cold air and that's after about 20 min. As soon as I press the gas down and start to drive the heat starts to blow. It will be fine even when stopped for a period of time as long as I leave it running. Hope you can help. Thank You, Jon

Answer
Assuming that your truck reaches operating temperature and you don't have a faulty thermostat.  You should check your coolant level, what you are describing sounds like a low coolant level condition.  When the coolant level is low, the heater core is bypassed, I guess they figured they would rather freeze you out than cook your engine.  After you start driving the pressure and circulation in the system increase and coolant will begin to flow through the heater core.
Hope this helps.