I have three heating zones (forced hot water in baseboards) in my house. On the top floor, one zone controls all the baseboard heaters on that floor. None of the baseboards on one side of the house in that zone work, while the baseboards on the other three walls does work. None of the baseboards on the non-working side have bleeder-valves that I can find. I have tried bleeding the entire system from the boiler, but still those baseboards do not throw any heat. Any ideas on the cause and possible solutions? Thanks!
It sounds like the second floor system is split (one zone, but not a single loop of pipe) so that water is circulating in half (or three-quarters), and not in the other half (or quarter).
You either need to look harder (pull the covers off if you have typical fin-tube baseboard) for bleeder valves you are not finding, or cut the system open and add some. You have classic symptoms of air in the system, and there should be bleeders at the high points of the system, since you really can't get all the air out by bleeding down in the basement. If you have any bleeders on the working sections, certainly give them a try to see if there's any trapped air there, even though they are working.
The small-percentages possibility other than that is that the system has actually frozen on that side. Air-lock is far more likely.