Gas and Oil Home Heating Furnaces - Same Zone 2Rooms Hot 2 Rooms Cold

I have gas fired hot water baseboard heating system.There are 2 Zones. Zone 1 is the 1st floor and zone 2 is the 2nd floor. The baseboard in two rooms just got cold while the other 2 rooms are ok. I noticed that on Zone 2 there are two pipes going up thru the floor. One pipe is a branch of the first pipe. the first pipe is hot all the way to the floor while the branch is hot only to a point and cold towards the floor.I think my system is like that of "redbeard266"

There are no exposed pipes and the 4 rooms had heat for the past 15 years until early January 2003. One of the room that have heat is so hot that it could serve for 3 rooms. Does that mean that this room is pulling all the heat from the others. thanks

The first thing to check is to see if there is any section of pipe that runs through an outside wall and is un insulated. This will cause the pipe to cool off faster than it should. let us Know the results. Also check for drafts coming in through the wall under the baseboards.