had thermostat replaced, refilled antifreeze, got all air out, no heat. All shops say nothings wrong. Its 20 degrees outside. It appears that no fluid runs through core.just dont understand?

 

Sounds like your heater control valve is stuck in the closed position. Sometimes they break inside and you can still move the cable, but it's not turning the valve inside the hose.

OR - you have blockage in the heater hose(s) or in the heater core itself.

 

Year????????