There are two main target gases in a Boiler Room. Carbon Monoxide from the gas fired heaters and natural gas leaks.
Most typical monitoring is for carbon monoxide monitoring and your local regulations may have a code requirement addressing this. The EC-Gold Dual can used in the Boiler Room. This is a compact unit with on-board CO sensor, relays, and a buzzer alarm. A combustible sensor can be added to this model now or in the future should you need to monitor for natural gas.