HG failures like that one, where the gasket itself is blown through are usually the ones that will cause you to lose heat at idle. Basically because its such a big blow out, it doesnt take heavy engine load to pressurize the cooling system. The compression fills the heater core when at idle, when coolant flow is minimal. When you get back going the waterpump then pushes the air out and then you have heat again since coolant has pushed the air out.