Check for air vents, usually located at the radiators and other “high points” in the system. If they are the manual type then carefully open one. DON’T DO THIS WHEN THE WATER IS HOT! Turn of the power to the boiler and ensure the water supply to the boiler is opened.
Have a damp rag and a small cup or something to catch the water handy before attempting this. If the vents sputter then bleed them until the sputtering stops (steady flow of water). Do this at each vent working from the top down and back again until all air is purged from the system.
The contractor who installed the system may be willing, for a small fee, to show you as the homeowner the intricacies of the system that you need to know and he/she may be able to adjust and inspect it for proper operation.
Could be money well spent for peace of mind and future comfort.