ASHRAE Journal - October 2014 - 54

COLUMN ENGINEER'S NOTEBOOK
Steven T. Taylor

Return Fans in VAV Systems
BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE

Air-handling systems with outdoor air economizers must include a means to relieve
excess outdoor air supplied to the building to prevent the building from being overpressurized. Over-pressurization causes exterior doors to stand open if they open to
the outside (a potential security issue) or cause doors to be difficult to open if they
open to the inside (a potential safety and ADA issue) and may cause air to whistle
through exterior doors and elevator doors.
There are three common types of relief systems:
* Relief dampers (aka barometric relief);
* Relief fans (aka powered exhaust fans); and
* Return fans (aka return/relief fans).
For most applications, this list is also in order of first
cost (low to high) and energy efficiency (best to worst)
(see References 1 and 2 for detailed comparisons). While
typically the most expensive and least efficient option,
return fans are the system of choice for applications with
high return air pressure drops (greater than about 1 in.
w.c. [250 Pa]), such as fully ducted return systems. This
month's column focusses on two schemes for controlling
return fan speed for building pressure control: Airflow
Tracking and Direct Pressure Control. My next column
(in November) will outline how to design and control
economizer dampers in these systems, including how to
maintain minimum ventilation outdoor rates.
Before getting into the details, it is important to note
that building pressure control need not be precise. The
maximum building pressure to avoid the pressurization problems described above is typically about 0.05
to 0.10 in. w.c. (12 to 25 Pa).3 But lower pressures down
to neutral are generally acceptable (e.g., per Section
5.9.2 of ASHRAE Standard 62.14) and slightly negative
pressures may even be preferred in cold climates for
buildings with internal moisture sources to minimize
condensation in the envelope. So there is a wide range of
acceptable building pressure allowing some "slop" in the
controls.

Option 1: Airflow Tracking
Figure 1 shows a typical VAV system with a return fan
using Airflow Tracking (aka volumetric fan tracking)
controls. With Airflow Tracking, the return fan speed is
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controlled to maintain the return airflow rate at a setpoint equal to the supply airflow rate less a fixed differential airflow rate. This differential indirectly maintains
the building at a positive building pressure and is on the
order of 0.05 to 0.15 cfm/ft2 (0.3 to 0.8 L/s·m2) for typical
buildings, with the range a function of building air tightness and desired level of pressurization. The setpoint is
often determined empirically by field tests during the
commissioning phase; simply adjust the differential
airflow setpoint in the field until the desired building
pressure (e.g., 0.05 in. w.c. [12 Pa]) is achieved using a
handheld differential pressure (DP) sensor across an
entry door.
Return airflow is commonly measured using one of the
following:
a) Sum of Zone Airflow. If the return air system has
airflow control VAV boxes, as in some hospital applications, the return airflow rate may be determined by
simply summing zone airflow rates as measured by
VAV box velocity pressure sensors. VAV box airflow
measurements are fairly accurate,5,6 and, as noted
above, extreme accuracy is not required for building
pressure control, so the accumulated error from summing up measurements from several zones is generally
acceptable.
b) Speed Tracking. In a system with a high return air
pressure drop with fixed geometry, such as a manually balanced ducted return system (no return air VAV
boxes), fan speed will track fan airflow fairly well per
the fan laws. The only moving parts in the system are the
relief air and return air dampers but if they are properly
selected and controlled (to be discussed in my November
Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a member of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.
ASHRAE Journal - October 2014

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