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The constant volume 100% direct-fired make-up air system heaters provide a constant rate of supply air to the space. The gas flame in the unit heats the fresh make-up air directly, eliminating energy losses associated with other types of heating. For most applications, introducing slightly more make-up air into the space than what is exhausted produces the best results. A slightly positive building pressure assures the best performance. Some applications, such as the commercial kitchens, require more air exhausted than introduced to control over pressurization of the kitchen.
The two-speed, direct-fired make-up air system functions the same as constant volume, however provides two different air-delivery rates by using either an 1800/900 RPM motor or 1800/1200 RPM motor.
Two-speed units offer flexibility to buildings with two fixed exhaust loads or where an increase of airflow is desired. Units for buildings with two fixed-exhaust loads should have CFM ratios that match the available two-speed motor RPM ratios. The low speed is interlocked with the first exhaust fan, while high speed operates when both exhaust fans are running, allowing one unit to handle two different exhaust loads. Low speed is used for general ventilation and heating. However, if the co-detector senses high limits of carbon monoxide, the two-speed unit goes to high speed and the second exhaust fan turns on to ventilate the space. In regards to booths designed for painting and curing of paint, high speed is used in paint mode and low speed is used in curing mode to dry the painted surface.