2002 GMC Safari no heat when switch is turned on

I have a 1995 Safari and maybe it's about the same. The heater hose comes up to a valve on mine that is controlled by a vacume line. However on mine the valve itself broke off in the shut position and caused this same type of issue. The only time it would get the leaset warm is when I was sitting still. I replaced the valve itself and that solved the problem. Had to go to a GM dealer and order the part.