30 November 1998
of this, the economizer cycle discussed herein is based on a comparison of the temperature of the
outside and return air. Furthermore the decision to enable an economizer cycle to operate is partially
based on the temperature of the return air to determine whether the space requires heating or cooling.
This decision process relies on a deadband between heating and cooling. Federal regulations (10 CFR
435) require the use of economizer cycles and deadbands between heating and cooling in most situations
for all federally owned buildings.
b. The mixed air temperature and economizer control loops are shown in figure 3-6. The actuators
on the dampers operate like the actuator on a control valve. The outside air damper and relief air damper
are normally closed and operate in parallel with each other. The return air damper is normally open and
works opposite to the outside air and relief air dampers. The mixed air temperature control loop is linked
to the economizer control logic.
Figure 3-6. Mixed air temperature and economizer control loops.
c. Outside air will not be used when the control system is in the unoccupied mode or in the
ventilation-delay mode. A normally open (NO) relay contact in the circuit to IP keeps the outside air
damper closed under these conditions, and also when the supply fan is off. An open relay contact in the
circuit between TC and high signal selector TY keeps the dampers open to the manual setting of
minimum position switch MPS, when the system is in the minimum outside air mode, and the outside air
damper is allowed to open by the absence of other constraints. When both of these relay contacts are
closed, the control system is then operating in both the occupied and economizer modes. Controller TC
maintains the mixed air temperature by controlling the IP to modulate the dampers beyond minimum
position. The signal from MPS or the signal from TC operates through high signal selector TY to operate
the IP, which sends a pneumatic signal to positive-positioner PP to control the damper actuators. The
output of IP to the damper actuator positioners can be read on PIs at the panel and at the damper
location. Mixed air temperature sensing element and transmitter TT sends a signal to TC, which changes
its output to operate the dampers between minimum outside air position and full outside air.
d. The temperature sensing elements and transmitters TT in both the outside air intake and the
return air duct send temperature signals to economizer controller EC.
e. The economizer controller EC requires a setpoint for each of two contacts that determine whether
the coil of the relay that puts the system in the economizer mode is energized or de-energized. The
setpoints and switching differentials for each of the contacts are adjustable in EC. One of the contacts,
configured as a PV contact, responds to the temperature sensing element and transmitter TT in the
return air duct and prevents the economizer mode from operating when the HVAC system is heating the
space that it serves. The return air temperature setpoint of the contact will be selected at a temperature
that is below the expected cooling season return air temperature but higher than the expected heating
season space temperature. Two-deck multizone and dual-duct multizone systems present a special
situation and will be discussed separately. The other contact, configured as a deviation (DEV) contact,
responds to the difference in the signals of outside air temperature and return air temperature. The
setpoint of the DEV contact requires a calculation by the designer. The designer will indicate the return
air temperatures at which the PV contacts open and close and the temperature differences between the
outside air temperature and the return air temperature at which the DEV contacts open and close.