My 1995 Dodge Caravan, 3.0 V6 engine. I get no heat at idle, but once I am on the road, or if I rev up the engine the heater is fine, Had thermostate replaced and Heater control valve. I hear what sounds like water rushing into the heater when I first rev up the engine but the sound goes away after a minute of two. If I let the van idle for a while and then rev up the engine again I again get this rushing water sound. I have try filling up the radiator several times with antifreeze but can't get heat at idle.

 

Possibility you have an air bubble in the system near the heater core. If there isn't an air bleeder provision on your engine (it's often labeled) you can try parking on as steep an uphill slope as you can find and take the radiator cap off. Run engine until it warms up and the thermostat opens. Before it does, you'll get some overflow as the coolant heats up and expands. When the thermostat opens hopefully the system will burp out the air bubble. If it does you'll see the level in the radiator drop. Immediately top off with coolant or water and put the cap on. See if there's any improvement.