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Make-up Air Systems

Direct Indoor/Outdoor Make-up Air (MDB/MRB)

Designed to provide an economical and efficient means of supplying tempered make-up air, our direct-fired, make-up air units are available in eight airflow configurations. All units allow indoor or outdoor mounting and have a 100% thermal efficiency.


100% Makeup Air Systems

The make-up air is heated directly by the gas flame, eliminating the need for a heat exchanger and associated efficiency losses of indirect-fired equipment. Model MDB units provide maximum application flexibility with input ratings up to 7,425,000 Btu/hr and maximum airflow capability of 60,000 CFM.


Return Air Units Systems

Model MRB units provide maximum application flexibility with input ratings up to 5,500,000 Btu/hr, maximum airflow capability of 41,000 CFM and maximum return air capability of 75%. With direct-fired MRB units, a minimum 25% outside air must always be supplied to the building.



100% Makeup Air Systems

For buildings requiring 100% make-up air, direct-fired units provide the best fuel efficiency (100% thermal efficiency, 92% sensible). The make-up air is heated directly by the gas flame, eliminating the need for a heat exchanger and associated efficiency losses of indirect-fired equipment.

Return Air Units Systems

Direct-fired units can be used as combination make-up air and heating/ventilating units, requiring the capability of operating with recirculated air as well as make-up air.



  • 100% thermal efficiency (92% sensible) results in lower fuel bills
  • Natural or propane gas manifolds are factory-assembled to provide flexible fuel options and reduce field-installation costs
  • Factory-wired electrical panel with numbered terminals significantly simplifies field wiring
  • 100% factory flame-tested
  • Weatherproof roof with drip ledge provides protection from water being drawn into the unit from the roof
  • 18-gauge galvanized steel casing provides high corrosion protection for long life
  • Adjustable drives through 10 HP simplify air balancing
  • Four access doors provide maximum access for easy adjustments and service
  • 22:1 firing rate turndown for optimum temperature control
  • Units are third party certified by ETL