no heat

I had to move my 70 superbee today. Its 5 degrees out. I had the car running a good 15 minutes at idle. No heat. I checked the supply and return hoses to and from the heater core, One was cold the other was hot. Not extremely hot though. I also noticed that the temperature guage did not move in the 15 minutes the engine was running. No heat.

Should I flush the system?
Cant be the tstat right?

Ideas?
Thanks

Might be the T stat. Check temperature of top radiator hose verses temperature of bottom radiator hose. Careful removing rad cap but watch in the tank for circulation while running. Or frozen slush...

I hope you have the proper mix of antifreeze/water for your climate.