| Term 
 | Definition 
 
        | Air provided to replace exhaust air |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | A change involving extension, addition or change to arrangement, type or purpose of installation. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | heat required to raise 1 pound of water 1 degree Fahrenheit |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | the minimum distance through air measured between a heated surface and a combustible material. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | A location not accessible without damaging the structure or a finished surface.  Spaces accessible by readily removable panels or doors are not concealed. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | appliance that derives combustion air directly from the outside and discharges all flue gasses to the outside. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | Low point in the system to collect condensate and allow for removal. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | Natural, liquefied petroleum, or manufactured gas or a mixture of these. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | Burners, burner igniters, or electrical switches capable of igniting flammable vapors. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | Propane or propylene, butanes or butylenes or mixtures of these that are gas at normal pressure but liquid under moderate pressure. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | A gas tight joint obtained by mechanically tightening to metal surfaces, including flanged, flared, threaded or compression joint. |  | 
        |  | 
        
        | Term 
 | Definition 
 
        | A pipe or conduit for conveying combustion products and air to the atmosphere. |  | 
        |  |