1997 dodge ram B2500 van heating issue

heater blows cold. under the hood there is an inline valve in one of the heater hoses, the one that goes to the top of the core box iirc. it is mechanically switched on/off by it looks like a very small vacuum line. I tried to move the valve by hand with no luck. I could get a plastic coupler line & replace this valve. does it appear like this might be the problem (the eng warms up fine). thank you for your time. RR