When a rooftop unit includes a ventilation air damper option, the system must also include an effective means for allowing excess air to escape from the building, or undesirable over-pressurization can occur.

In most applications it is desirable to maintain the internal building pressure at a level slightly above the outdoor ambient pressure. This slightly positive pressure prevents untreated outdoor air from infiltrating the building through leaks in the building enclosure. As outdoor air is brought into the building through the rooftop unit for ventilation or economizer cooling purposes an equivalent amount of air must also exit the building to maintain a constant building pressure. Some of this air will exit the building through gaps in the building enclosure; however the majority must be removed through mechanical relief devices.

On rooftop units there are three main different building relief devices that can be used to help control building pressure: a Barometric relief, Power Exhaust fans or Return Fans.

• Barometric relief devices provide a low-resistance path for excess air to exit the building. Barometric relief devices rely on the positive pressure in the building to force gravity operated relief dampers open to allow excess air to exit the building. Barometric relief should only be utilized in applications with little or no return air duct (plenum return). On systems with ducted returns the pressure loss associated with the return air path will cause a lower pressure at the barometric relief damper, resulting in less air being exhausted from the building. Differences in the construction and operation of manual, 2-position dampers and economizer assemblies, may require additional dedicated barometric relief devices.
• Power Exhaust Fans pull air from the return section of the rooftop unit and discharge this air outside the building. Power Exhaust fans may be direct-drive or belt-drive and use constant speed or variable speed controls. Fan staging or speed control may be based on economizer damper position or respond directly to building pressure. Typically, a Power Exhaust option also requires an economizer.
• Return Fans pull air from the return duct system and discharge it into the rooftop unit’s return air plenum. Based on the position of the unit’s return air damper, which is downstream of the return air fan, this airflow may either be recirculated to the supply side of the unit or exit the building through the unit’s barometric relief damper. Return fans are applicable when the return duct system has a high pressure loss (they should never be used with plenum return systems). Because the return fan is located in the return air path it must operate whenever the rooftop unit’s indoor supply fan is activated. Proper control of return fan system is typically complex, requiring a more sophisticated control sequence.

Carrier Building Pressure Control Features and Benefits include:

• UL certified for single point power from Carrier rooftop unit
• Barometric relief or power exhaust options for every size of rooftop unit
• Factory integrated controls
• Factory installed option or field installed accessory†
• Available on units with vertical or horizontal return
• Propeller or centrifugal fan designs†

† Availability is unit and model dependent

Used On:

• Packaged rooftop units using Puron® Refrigerant (R-410A)
• Packaged rooftop units using R-22