30 November 1998
The bias is set in the controller in L/s (cfm) units. Selecting the bias value requires consideration of the
exfiltration flow (Flowexf) required to pressurize the space and the space exhaust flow (Flowexh) (such as
that from toilet areas) as follows:
Bias = Flowexf + Flowexh
Flowexf = Space exfiltration flow, L/s (cfm)
Flowexh = Space exhaust flow, L/s (cfm)
Determination of the exhaust flow is straightforward. It is the total flow due to exhaust fans located in the
space served by the air handling unit. Determiming the exfiltration flow can be slightly more involved and
is related to building pressuization. Detailed guidance in determining this value is presented in
PROSPECT course 340 HVAC Control Systems Design , although a reasonable approximation can be
obtained by assuming that the Flowexf is 10 to 15 percent of the design airflow quantity. Use 10 percent
for a tight building and 15 percent for a loosely constructed building.
The bias quantity should be coordinated with the minimum outside air flow quantity as follows:
MIN OA = Flowventilation
if Flowventilation > Bias
MIN OA = Bias
if Flowventilation < Bias
= Fresh air ventilation quantity (per ASHRAE Standard 62), l/s (cfm)
= Flowexf + Flowexh , l/s (cfm)
= Building or space exfiltration flow, l/s (cfm)
= Building or space exhaust flow (ie. Toilets, etc), l/s (cfm)
For DDC applications, the DDC panel takes the place of flow controller FC. The ratio and bias logic
remains the same as for control via single-loop controllers, but is performed in software.
Figure 3-12. Return fan volume control loop.
9. HUMIDIFIER CONTROL LOOP. The humidifier control loop is shown in figure 3-13. Humidifier
control valve VLV is normally closed. It is inhibited from opening by the contact of a relay that is open
unless the fan is on, the system is in the occupied mode, and the ventilation delay period has expired.
When these conditions of operation are met, space relative-humidity sensing element and transmitter
RHT signals relative-humidity controller RHC to operate IP to control humidifier valve VLV. A high-limit
relative-humidity controller RHC receives a signal from a duct relative-humidity sensing element and
transmitter RHT downstream of the humidifier. Both controllers are reverse-acting. Low-signal selector
RHY allows the space relative-humidity controller to operate the valve if the high-limit relative-humidity
setpoint is not exceeded. The high-limit relative-humidity controller must be a proportional only controller.
For DDC applications, the DDC panel takes the place of the two relative humidity controllers RHC and the