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IM 1234-1 • BACNET THERMOSTAT
Fan Speed Setup
The BACnet Thermostat can control either variable speed fans
or fans with up to three speeds. Fans speed control is set up
from the user interface by a user with Password 2.
One, two, and three speed fans
– The outputs are energized
as the cooling or heating loop varies from 0 to 100%. The fan
always starts as soon as a cooling or heating valve is active.
Table 15: Fan Speed
Output Terminal
Fan L
Fan M
Fan H
Fan speed
OFF
ON
OFF
ON
OFF
ON
One
5%
Active valve
N/A
N/A
N/A
N/A
Two
5%
Active valve
N/A
N/A
95%
80%
Three
5%
Active valve 65%
50%
95%
80%
Modulating fans
– The fan starts on a call for either heating
or cooling. The fan runs at the minimum fan speed until the
heating or cooling loop reaches 50%. From 50% to 100% of
the loop, the fan speed ramps from the minimum speed to
maximum speed. The minimum and maximum fan speeds are
set from the user interface during system setup.
Modulating Cooling and Heating
for Rooftop Units
Modulating cooling and heating is used in BACnet Thermostat
controlled systems with modulating valves and chilled or hot
water coils. A single modulating valve can be used with staged
cooling or heating.
Cooling
Modulating cooling does not start until the cooling loop reaches
100%. Once the loop reaches 100%, the analog cooling output
modulates over 10 volts DC as the cooling loop changes from
0 to 100%. When the cooling loop returns to 0%, cooling stops
until the loop again reaches 100%.
If the economizer is enabled, cooling will not start modulating
until the economizer damper is fully open.
Heating
Modulating heating does not start until the heating loop
reaches 100%. Once the loop reaches 100%, the analog
heating output modulates over 10 volts DC as the heating loop
changes from 0 to 100%. When the heating loop returns to 0%,
heating stops until the loop reaches 100% again.
Valve Action
The BACnet Thermostat supports both normal and reverse
valve action that can be set from the user interface.
•
Normal
– The valve is fully closed when the output signal
is 0 volts and fully open when the output is 10 volts DC.
•
Reverse
– The valve is fully closed when the output
signal is 10 volts and fully open when the output is 0 volts
DC.
Staged Heating and Cooling for
Rooftop and Heat Pump Units
Staged heating and cooling is used for application other than
chilled or hot water systems. Typically the BACnet Thermostat
controls gas heat, electric heat, or direct expansion (DX) cooling
with stage heating and cooling. Staged cooling or heating can be
mixed with a modulating valve for cooling or heating.
Staged Cooling
As the demand for cooling increases, the BACnet Thermostat
starts the first stage of cooling when the cooling PID loop rises
above 95% and the first stage of cooling has been turned off for
at least the time set by the value of MINIMUM OFF TIME. The
second stage of cooling is turned on when the cooling loop rises
above 99% and the first stage has been turned on for the period
set by STAGE DELAY and the second stage has been turned off
for at least as long as the value of MINIMUM OFF TIME.
As the demand for cooling is satisfied, the second stage is
turned off when the cooling PID loop drops below 50%. This
first stage is turned off when the cooling loop drops below 5%.
Staged Heating
As the demand for heating increases, the BACnet Thermostat
starts the first stage of heating when the heating PID loop rises
above 95%. The second stage of heating is turned on when
the heating loop rises above 99% and the first stage has been
turned on for the period set by STAGE DELAY.
As the demand for heating is satisfied, the second stage is
turned off when the heating PID loop drops below 50%. This
first stage is turned off when the heating loop drops below 5%.
