Coolant boiling

Why is the coolant boiling out of the resavoir

I don't think it's a head gasket no white smoke no oil and water mix can drive about an hr before it need more coolant

May be radiator caps not holding pressure. make sure no coolant residue is around the neck of the radiator.
If this is not the problem may be cooling fan is not working.
Just check it out may be this will help.