This question already has an answer here:
I have gas powered hot water heat in a home I purchased. When I tested it this morning I got a lot of whistling and other noises that subsided for the most part. After the heat had run for 20-30 minutes, I checked it radiator in the house. All the radiators were at least warm, with the exception of one of the radiators upstairs.
What could cause this single radiator to not get warm at all? I noticed slight temperature variances between the other radiators, but this one seemed to not be warm at all. I assume it should get warm if the hot water is flowing through it, and I also assume the hot water must be flowing through it to complete the circuit. So, what gives?