T7200D,E, T7300D,E,F AND Q7300 SERIES 2000 PROGRAMMABLE COMMERCIAL THERMOSTATS AND SUBBASES
63-4355—2 22
O and B Terminals for Heating or Cooling Changeover
The Q7300C controls heat pump changeover. The Q7300D,H
is only used in a heat pump application when the heat pump
controls the reversing valve changeover. See Table 13 for the
operation of the O and B terminals on the Q7300C, H2003.
O and B Terminals on a Conventional Subbase
Conventional subbase O and B terminals can be used to
drive dampers and valves. The O terminal is energized on a
call for cooling and the B terminal is energized on a call for
heating.
Table 13. Operation of Q7300 O and B terminals.
a
When the system is calling for heat or is de-energized, but
the last function was heat.
b
When the system is calling for cool or is de-energized, but
the last function was cool.
c
When the last function was heating, the operation is the
same as Auto (Heat). If the last function was cooling, the
operation is the same as Auto (Cool).
Fan Operation
When the fan is set to On, the fan is energized continuously in
the Occupied period. The Auto setting energizes the fan with
the heating or cooling system. If the subbase is Q7300D, the
fan cycles with Y1 and W1 during Unoccupied period.
Emergency Heat
When the system is set for Em Heat (on select T7300
Thermostats), the auxiliary heat is stage one and the
compressor stages are locked off. The fan cycles with the
auxiliary heat.
Communicating Subbase
Select T7300F Thermostats with the Q7300H Communicating
Subbase are designed for conventional or heat pump
applications in a LonWorks® network. The Q7300H Subbase
allows remote access and the sharing of clock, schedule,
setpoints, bypass and other system parameters in a
LonWorks® network. See fig. 32 for a T7300F/Q7300H wiring
layout in a LonWorks® network.
GENERAL OPERATION INFORMATION
Cycle Rates
The thermostat control algorithm maintains the temperature
by cycling stages of heating or cooling to meet setpoint.
System Selection O Terminal B Terminal
Heat Open Closed
Cool Closed Open
Auto (Heat)
a
Open Closed
Auto (Cool)
b
Closed Open
Emergency Heat Open Closed
Off
cc
Subbase Auxiliary Relay
The Q7300 Auxiliary Relay output acts as a time-of-day
switch to be used with an economizer minimum position
control. The normally open (A1) auxiliary relay contacts are
closed during the Occupied period and open during the
Unoccupied period. The normally closed (A3) auxiliary relay
contacts are open during the Occupied period and closed
during the Unoccupied period. The contacts are in the normal
state during recovery.
Interfacing with
Electromechanical Economizers
(Subbases with Auxiliary Relay)
Mechanical cooling is often used when outside
temperatures are in the 50°F (10°C) to 60°F (16°C) range
and humidity is below 50 percent. In central and northern
climates, hundreds of hours fall into this temperature
category. By permitting 80 to 100 percent outside air into the
system, mechanical cooling may not be needed at all,
particularly during Spring and Fall.
An economizer is used to take advantage of the outside air.
The typical economizer consists of an outside air damper,
motor, outdoor air changeover control and a minimum
position potentiometer. The motor controls the dampers.
Suitability of the outside air for cooling is determined by the
outdoor air changeover control. The potentiometer is used for
adjusting the minimum position of the economizer dampers,
which provide a minimum amount of fresh air for ventilation.
The economizer reduces compressor run time, thereby saving
energy and extending the compressor life. The drawback to
using the economizer is that during the Unoccupied period, if
there is no call for cooling or outdoor air is not suitable for free
cooling, the economizer is controlled to minimum position. This
position allows some percentage of outdoor air to enter the
building, regardless of air suitability. The situation can cause
the heating or cooling to run more often than if suitable outdoor
air is permitted to enter the building.
The thermostat can take advantage of an economizer by
connecting the auxiliary relay contacts (A1 and A2) of the
subbase to control the economizer minimum position
potentiometer. The contacts close during the Occupied
period, allowing the economizer to operate normally. The
contacts are open during the Unoccupied period, disabling
the minimum position feature of the economizer. The open
contacts cause the economizer to drive dampers fully closed
instead of staying at minimum open position. This reduces the
possibility of unsuitable outdoor air from entering the building,
which lowers the internal load on the HVAC system and saves
additional energy.
Other Uses for the Auxiliary Relay
Examples of other uses of the auxiliary relay are hot water
heaters, lighting or baseboard heat. The additional loads are
connected to the auxiliary relay contacts on the subbase. The
contacts are rated for 1.6A at 30 Vac, but can be adapted to
higher current applications with the use of an external relay
and transformer. See Fig. 18.
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