VAV Guidelines - Taylor Engineering

F IGURE 103:
A IRSIDE E CONOMIZER
C ONFIGURATION WITH
B AROMETRIC R ELIEF
FROM ASHRAE
G UIDELINE 16-2003
X178X OUTSIDE AIR/RETURN AIR/EXHAUST AIR CONTROL
Where barometric relief is not an option, relief fans (Figure 104) are the best bet. Relief fans
always use less energy than return fans and can incorporate barometric relief as the first stage
of building pressure control (see sequence below). In addition to the energy benefits, relief
fans are relatively compact, reducing impact on space planning and architectural design. The
two largest limitations are acoustics and static pressure. Acoustical control can usually be
achieved by placing the relief fans out of the line of site from the return shaft. Systems with
high return pressures (e.g., ducted returns) will generally require return fans.
The following is an example control sequence for a system with two relief fans and an
automated damper at each:
� Relief system shall only be enabled when the associated supply fan is proven on and the
minimum outdoor air damper is open.
� Building static pressure shall be time averaged with a sliding five-minute window (to
reduce damper and fan control fluctuations). The averaged value shall be that displayed
and used for control 30 .
30
A single building static pressure sensor is usually sufficient, or one per wing or tower for large, irregularly shaped
buildings. The high side should be in an interior space on the second floor (first floor is too variable due to lobby
doors). Do not tap into a single tube in multiple locations in order to get an average signal. The pressure
differences between the various taps creates a flow in the tube and a false reading.