boiler [1]
Device in which water is heated before being distributed to the radiators.
Aquastat 
Device regulating the water temperature.
pressure relief valve 
Mechanism for releasing pressure by letting off excess water or steam.
box 
Rigid metal casing for the boiler.
chimney 
Channel through which combustion gases are evacuated.
burner 
Device for mixing a flammable substance with air, and fanning and igniting this mixture in a boiler’s fire pot.