Stop in for a cup of coffee

Except it can’t. Each radiator is individually vented with a pressure relief valve in addition to the one on the boiler, so overpressure on the main boiler will never occur. Steam radiator systems are low pressure for heating..not like a high pressure steam locomotive or ship boiler for generating power for movement.

Worst it will do is run out of water for steam.
Only steam heating I know about was in the Navy. And believe me, those could blow sky high!!!