Was opening the valve when I heard a snap noise. Now the knob and the stem spin as one piece. I can also move the knob and stem (as one piece) in and out by about a quarter inch. Valve is stuck open.

I don't see any markings on the valve body. I hesitate to remove the knob by holding the stem without further knowledge since the furnace is still working at this time.

Does anyone know why the stem spins? Is the problem only with the knob or do you think there is something internal broken?

Thanks for any info.