Dual bypass direct fired heating system with pressure control

A heating system for use in conditioning air in a building by supplying air to a duct system of the building, the heating system comprising:

• an enclosure having an outside air opening for drawing outside air into the enclosure and a return air opening for drawing return air from within the building into the enclosure;
• a burner box positioned in the enclosure adjacent the outside air opening such that air flowing into the enclosure through the outside air opening must pass through the burner box, the burner box comprising a direct fired burner for heating at least a portion of air flowing through the outside air opening and flowing over the direct fired burner, the direct fired burner being configured for optimized operation at a predetermined target pressure difference across the burner, the burner box comprising:
• a main body defining an open upstream end and an open downstream end, the open upstream end being divided into a first bypass opening and a second bypass opening spaced from the first bypass opening by a burner opening, the main body further including:
• a mixing chamber adjacent the open downstream end;
• a first bypass passage extending from the first bypass opening to the mixing chamber;
• a second bypass passage extending from the second bypass opening to the mixing chamber;
• and
• a burner passage extending from the burner opening to the mixing chamber, the direct fired burner being positioned in the burner passage, such that the burner air flow is drawn into the main body from the outside environment through the burner opening and directed through the burner passage over the direct fired burner for heating,
• wherein air drawn into the main body from the outside environment through the first and second bypass passages is shunted around the burner passage and converges with the heated air from the burner passage in the mixing chamber downstream from the burner passage before exiting the main body through the open downstream end of the main body, the return air opening of the enclosure being positioned outside the main body, the open downstream end of the main body lying within an orthogonal projection of the return air opening projecting orthogonally from the return air opening through the enclosure such that mixed air exiting the mixing chamber through the open downstream end of the main body of the burner box is mixed with the return air entering the enclosure.