Boiler does not circulate water
I live in an old house from 1930's with a boiler (22 years old) and hydronic radiator. The radiator has three zones (2nd, 1st floors and basement). I was away from the house for a few days and turned the boiler off. When I came back, I found that water does not circulate the second floor. I initially thought there must be air pocket created but opening the breeder on the 2nd floor did not do anything. Then I figured that a section of the pipe may have frozen. I am planning to wait until the spring comes. But could there be another reason? I started to double because it was warm for a few days (>40F) last week and that did not do anything.