no bleeding necessary, just run the engine and refill the radiator/reservoir back to capacity.
You need to burp the radiator and cooling system.
The cooling system is bled using the bleeder screw found on the thermostat housing. To find the thermostat housing on a 1998 Honda Civic follow the lower radiator hose to its outlet. While the cooling system is running loosen the bleed valve. When bubbles stop coming out the system has been bled.
A thermostat that was stuck then suddenly became unstuck might do that. If the coolant level and the rest of the cooling system seem OK, try replacing the thermostat.
Check the vans entire cooling system including the temp sending unit, cooling fan, thermostat an water pump. One of these is probably bad and needs replacing.
It probley has an AIR pocket in the cooling system. Bleed the air out of the system and make sure it is full of fluid.
Turn it on with thermostat
Did you bleed all air from the system? Did you check the operation of the water pump? Did you flush the system thoroughly? Did you accidentally install the thermostat upside down?
Replace the thermostat and service the cooling system.
The cooling system is the radiator, engine fan, thermostat, and hoses...
try the simple first: replace the thermostat, next flush the whole cooling system (not just radiator), if that doesn't work, you probably need a new heater core.
When you service the cooling system.
what is the year make & modle of it??