Keeps shooting coolant out overflow cap

Combustion gases sneaking into the cooling system often pool under the thermostat and prevent it from opening. Thermostats open in response to hot liquid, not hot air. Coolant circulation will stop including flow through the heater core, so while "heater is not working" can mean lots of different things, it sounds like you have a leaking cylinder head gasket and there is cold air coming from the heater. What has you convinced it is not leaking when all symptoms point to that? What else will cause coolant to spray out of the reservoir? Does that even occur before the engine is warmed up?