The boiler at St. Mary's Hospital in Passaic apparently split in two early Saturday, exploding and sending debris flying over 30 feet, The New York Times reported.
Patients were immediately evacuated by ambulances to nearby St. Joseph's Hospital in Paterson, with no injuries or casualties reported.
"Don't ask me how, but no one was hurt," Chief T.N. Zayatz of the Passaic Fire Department said.
The boiler is inspected regularly and investigators have been unable to discover a cause for the explosion so far, the newspaper said.
The Times said the hospital, which lost its air-conditioning and hot water in the blast, would not be allowed to reopen until it passes a state inspection.