18 • Fan Control
026-1727 Rev 3
7.4.3.2.
Modulating Heating
In addition to the primary and secondary heat
stages, the DAC supports one modulating heating
output. This modulating output must be classified as
either a primary or secondary heat source.
7.4.3.2.1.
Primary Mod Heat
If the modulating heat output is classified as a
primary heating source, it will be activated before any
other primary heat stages when the application calls
for heating. Once activated, the modulating heat
output will modulate to maintain the supply air
temperature at the active supply air heating setpoint.
A PID loop will be used to determine the capacity
output. If the modulating heat output reaches 100%
and remains there for the Heat On-Delay period, the
first primary heating stage will be activated.
Additional primary heating stages will be brought on
according to the Heat On-Delay as long as the
modulating heating output remains at 100%. No
additional stages of heating will be brought on while
the modulating heat output is less than 100%. When
all primary heating stages have been activated,
secondary heat stages will be allowed to stage on
similar to the primary heating stages.
Secondary and primary heat stages will be
sequentially turned off as demand for heating is
reduced. The highest secondary stage will turn off
after the modulating heat output has been at minimum
capacity for the Heat Off-Delay. Additional heat
stages will be turned off in the same way.
7.4.3.2.2.
Secondary Mod Heat
If the modulating heat output is classified as a
secondary heating source, it will be activated only
after all primary heat stages have staged on, but
before any defined secondary heat stages are
activated. Once activated, the modulating heat output
will modulate to maintain the supply air temperature
at the active supply air heating setpoint. A PID loop
will be used to determine the capacity output. If the
modulating heat output reaches 100% and stays there
for the Heat On-Delay period, the first secondary
heating stage will be activated. Additional secondary
heating stages will be brought on according to the
Heat On-Delay as long as the modulating heating
output remains at 100%. No additional stages of
heating will be activated while the modulating heat
output is less than 100%.
Secondary heat stages will be sequentially turned
off as demand for heating is reduced. The highest
secondary stage will turn off after the modulating heat
output has been at minimum capacity for the Heat
Off-Delay period. Additional heat stages will be
turned off in the same way. Once all secondary heat
stages are off, the modulating heat output will remain
on until it goes to minimum capacity and stays there
for at least the Heat Off-Delay period. After this time,
the modulating heat output will turn off, leaving only
the primary heat stages still active. The primary heat
stages will then stage off as previously described.
7.4.3.3.
Heating OAT Lockout
Heating will only be allowed when the outside air
temperature is below the heat lockout temperature
setpoint. If the outside temperature rises above this
setpoint, heating will be deactivated. Heating will
only be re-enabled when the outside temperature falls
2°F below the lockout setpoint.
7.5. Fan Control
The DAC supports control of either a single speed
fan or a variable speed (VS) indoor fan. The method
used to control the fan depends on the application
mode setting (CAV or VAV – see
Figure 7-2
- Heat Staging Primary Graph
Figure 7-3
- Heat Staging Secondary Graph
