Airbound. Eliminate that as a cause and your should be fine. May just take a bleeding at the convectors, otherwise a good purging.
For the future and FYI, see if your circulator is pumping away from the expansion tank connection point. You may want to install an air separator.
Sounds like an adorable little place. Ought not be hard to correct.
I have a small hydronic system in a 722 sq foot single room guest house. The four 4' convectors in the main room are cold, the small convectors in the bath and kitchen are hot. Circulator is running fine, water is hot leaving the boiler and still pretty hot returning.
Unit was just serviced...a new domestic hot water coil was installed Thursday, and my tenant noticed no main room heat this morning.
What could cause a single circulator system to "split?" Why am I getting great heat from two convectors and nothing (the supply pipes coming from the wall are cold) from the others?