23.55.00 Fuel-Fired Heaters
23.55.33.13 Oil-Fired Unit Heaters
23.55.13 Fuel-Fired Duct Heaters 23.55.13.13 Oil-Fired Duct Heaters 23.55.13.16 Gas-Fired Duct Heaters 23.55.23 Gas-Fired Radiant Heaters 23.55.23.13 Low-Intensity Gas-Fired Radiant Heaters 23.55.23.16 High-Intensity Gas-Fired Radiant Heaters 23.55.33 Fuel-Fired Unit Heaters 23.55.33.13 Oil-Fired Unit Heaters 23.55.33.16 Gas-Fired Unit Heaters