U.S. Boiler Company Expands Citadel Boiler and Water Heater Line

Now available in seven capacities, the Citadel can be configured to use natural gas or propane.
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Jan. 14, 2026
U.S. Boiler Company
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U.S. Boiler Company has expanded and improved its Citadel line of condensing boilers and volume water heaters. The Citadel is now available in seven capacities from 399 to 1500 MBH, all with 10:1 turndown. Available in natural gas and propane configurations, the Citadel is AHRI certified and rated up to 97% thermal efficiency, or 98% for water heaters.

The boilers can be stacked two high without the need for accessory racking systems, while zero side and top clearance optimize heating capacity per square foot of mechanical room space. The Citadel is also now compatible with U.S. Boiler Company’s USB-Connect mobile app, which streamlines installation, service, and troubleshooting. Modbus communication comes standard. All models are suitable for indoor/outdoor installations.

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