I'm pretty sure that there is no valve controling fluid flow. Coolant runs thru the core full time. Temp is controlled by the blend door. When you say no heat, I assume you mean no warm air in any switch position, vent, defrost, heat, etc...

If that's the case, try swapping the **** from the temp selector and fan. The temp **** is know to crack.

If you're getting warm air from the defrost, but you can't get the airflow to move down to the heat output below the dash, you have either a bad blend door motor or blend door.