Do a little more troubleshooting. Does the top radiator hose get hot? Do both heater hoses get hot? If so, your thermostat is working properly and the water pump is circulating coolant. The lack of pressure in the system may be a bad radiator cap. In warmer weather, this would eventually lead to your system overheating and blowing all the coolant out. If your hoses are getting hot, then you need to investigate whether the blend door in the HVAC system is working. Water circulates through the heater core all the time, but the blend door determines whether air is being routed through the heater core. I think your HVAC system may be controlled through vacuum lines. Check to see that you don't have any broken or leaking vacuum lines.