Selecting and Controlling Economizer Dampers for VAV Systems with Return Fans

For over 20 years the generic method of maintaining the prescribed amount of outdoor ventilation air has been through the use of a supply-return fan tracking system. The latest draft of ASHRAE Standard 62-1989R requires “controls and devices to maintain outdoor air flow within 10% of required levels.” Because fan tracking systems measure large volumes of air to control the relatively small volume of outdoor ventilation air, they generally will not meet the 10% requirement. For example, if the outdoor air flow is 15% of the design supply flow and the air volume sensors in the supply and return ducts have an accuracy of 5%, then the worst case error is 5% of 100% (supply flow) plus 5% of 85% (return flow) or (5 + 4.25)/15= 62%. Maintaining an outdoor air flow with this uncertainty will not meet the revised standard.

The method of control shown in diagram 1 meets the 10% requirement of standard 62 and in addition controls the building pressure much closer than a fan tracking system.

When the system in diagram 1 is utilizing outdoor air for cooling (economizer cycle), the building differential pressure sensor (P-1) will control the low leakage opposed blade relief damper (D-1). As the building pressure rises above the set point of (P-1), the relief damper (D-1) will modulate open. This damper may be the same size as the duct to the relief louver. The high pressure port of sensor (P-1) should be referenced to the main building entrance lobby or similar critical location. The low pressure port should be referenced to the outdoors on the same side of the building as the entrance. A positive lobby pressure of 0.05″ W.C. is generally satisfactory. This will create a maximum exit velocity through an open entrance door of about 750 feet per minute (8.5 miles per hour). The maximum range of sensor (P-1) should be -0.1″ to +0.1″ W.C.. Unless the building has zones that are definitely pressure isolated, only one differential pressure sensor should be used. If more than one air handler is in the building the sensor normally controls all relief dampers in parallel on all air handlers.

The speed of return fan (F-1) is controlled by the return-relief plenum (PL-1) differential pressure sensor (P-2), to maintain a plenum pressure high enough to discharge the design relief air volume when the damper is wide open. The pressure in the relief plenum generally ranges from +0.1 to +0.3″ W.C. As the relief damper opens and the plenum pressure drops, the return fan speed will increase to maintain the set-point of sensor (P-2). The low pressure port on sensor (P-2) should be referenced to outdoor pressure on the same side of the building as the relief louver so as to compensate for wind effect. A sensor with a range of +0.0 to + 0.5″ W.C. is generally used.

Determining the set point of pressure sensor (P-2) can be calculated by summing the drops through the relief louver and damper (D-1) from the manufacturer’s cataloged data plus an allowance for the relief plenum and fittings.

The opposed blade low leakage return damper (D-2) is sized to have approximately the same drop as the total drop through the relief system. The use of opposed blade relief and return dampers (D-1 and D-2) will insure a more linear control of flow than will parallel blade dampers. Because of the high differential pressure between the relief and mixed air plenums (PL-1) and (PL-2), the velocity through the wide open return damper (D-2) generally ranges from 1500 to 3000 F.P.M. Return damper (D-2) and the low leakage outdoor air damper (D-3) are controlled by the supply air thermostat (T-1). As the supply air temperature rises, the return air damper modulates closed and the outdoor air damper modulates open.

The size of the low leakage parallel blade outdoor air damper (D-3) is not critical and can be louver size. Since the return air volume is fixed by the building pressure and the supply air volume is fixed by the supply duct static pressure sensor (P-3), the difference must be made up of outdoor air. The only function of the outdoor air damper (D-3) is to close when the HVAC system is off and to act as a check valve to restrict the flow of outdoor air caused by high winds into the outdoor-return air plenum; otherwise, the outdoor air damper (D-3) could be omitted. In this application, bigger is better, in fact reducing the size of the outdoor air damper introduces control loop interaction that tends to destabilize the control system.

The outdoor ventilation air is handled by the small constant volume injection fan (F-3). A pitot tube velocity sensor (V-1) in the high velocity discharge of this fan, maintains a constant flow of air by controlling the fan speed to meet the ventilation requirements of ASHRAE Standard 62. If the need for more or less ventilation air changes, the velocity can be reset providing the fan has been liberally sized. Parallel blade low leakage damper (D-4) is closed when fan (F-3) is off.

Injection fan (F-3) should be selected to handle the required ventilation air at the pressure losses in the fan intake and discharge duct system plus the pressure in the outdoor-return air plenum (PL-2). This pressure can vary from a negative pressure when the system is on the winter economizer cycle to a positive pressure when the system is on the summer refrigeration cycle.

The maximum negative pressure in the outdoor-return plenum (PL-2) is developed on the winter economizer cycle and is equal to the drop through the outdoor intake, damper, and ductwork when the supply fan (F-2) is handling the maximum design flow of 100% outdoor air.

The highest positive pressure in plenum (PL-2) is developed on the summer refrigeration cycle by the return fan when handling minimum design C.F.M. The positive pressure in the outdoor-return plenum (PL-2) will be the pressure in the return-relief plenum (PL-1) less the drop through the return damper (D-2). This drop will be the lowest when the return fan is handling the lowest flow. For example if the minimum return flow is 1/2 of maximum and since drop varies as flow squared; then the drop through the wide open return damper (D-2) will be (1/2) squared or 1/4 the drop at maximum flow. If the relief-return plenum (PL-1) pressure is set for +0.2″ W.C., the drop through the wide open return damper (D-2) at minimum flow will be 1/4 X 0.2″ W.C. or 0.05″ W.C. and the outside-return plenum (PL-2) pressure will be +0.2″ W.C. minus 0.05″ W.C. or + 0.15″ W.C..

In this example, injection fan (F-3) should be sized to handle the connected duct system plus 0.15″ W.C., the maximum positive pressure that will occur in the outdoor-return plenum (PL-2).

When the system is not on the economizer cycle, relief damper (D-1) and outdoor air damper (D-3) are closed and return air damper (D-2) is wide open.

Systems that utilize fan tracking controls have many control loops that interact and destabilize the system.

The control loops for the system in diagram1 are de-coupled. A change in flow in either the supply or return fan will have little or no effect on the flow from the other fan. The outdoor-return plenum, at or near atmospheric pressure, de-couples the two systems.

The use of the outdoor air injection fan provides a dependable, verifiable supply of ventilation air and it provides the flexibility to change the amount of outdoor air if the building usage changes or if the amount required for pressurization exceeds the amount required for ventilation.

Gil Avery, P.E.