A crack in the heating plant wall was caused by the boiler explosion. ((CBC))

Three workers were injured, one seriously, when a boiler exploded at a heating plant near the Supreme Court of Canada in Ottawa Monday.

Paramedics at the scene said one man has second-degree burns to 50 per cent of his body and a severe head laceration. He is reported in serious condition in hospital.

Another man has first-degree burns and is in stable condition while the third has minor injuries, paramedics said.

Firefighters evacuated 27 people from the heating plant, which supplies heat to buildings on Parliament Hill.

They said it wasn't yet clear when people could return to work in the building because the explosion was so strong it cracked one wall. They are also worried about the presence of asbestos.

"We see exterior damage to the building," said Kim Ayotte, chief of special operations for the Ottawa Fire Service. "There is damage to the facia, explosive damage to the vent louvres."

A hazardous materials team has been brought in to make sure no toxic substances were released by the blast.