It could be a leaking T&P but it could be something more serious. Is the burner running continuously or does it shut down normally? Have you checked the water pressure? Is there a PRV valve on the system and is there an expansion tank? Basically the folks here will need a complete description of the system and it's operation. Then they will still tell you to get it checked by a plumber which isn't a bad ideal if you don't deal with this stuff every day.