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14 RT-PRC031-EN
CV Units Only
Occupied Zone
Temperature Control
Cooling/Economizer
During Occupied cooling mode, the
economizer (if provided) and
mechanical cooling are used to
control zone temperature. The zone
temperature cooling setpoint is user-
defined at the Human Interface Panel
or from the BAS/Network.
If the conditions of outside air is
appropriate to use “free cooling”, the
economizer will be used first to
attempt to satisfy the cooling zone
temperature setpoint; then if
required the hydronic valve will be
modulated to maintain supply air
temperature setpoint.
On units with economizer, a call for
cooling will modulate the fresh air
dampers open. The rate of
economizer modulation is based on
deviation of the zone temperature
from setpoint, i.e., the further away
from setpoint, the faster the fresh air
damper will open.
First stage of cooling will be allowed
to start after the economizer reaches
full open.
Note: The economizer is only
allowed to function freely if
one of the following
conditions is met: For dry
bulb economizer control, the
ambient temperature must
be below the dry bulb
temperature control setting.
For reference enthalpy
economizer control, outdoor
air enthalpy must be below
the enthalpy control setting.
At outdoor air conditions
above the enthalpy control
setting, mechanical cooling
only is used and the outdoor
air dampers remain at
minimum position. For
comparative enthalpy
economizer control, outdoor
air enthalpy must be below
the enthalpy of the return air.
If the unit does not include an
economizer, primary cooling only is
used to satisfy cooling requirements.
The outdoor air dampers may be set
for a maximum of 25 percent
outdoor air, through the unit
mounted Human Interface Panel or a
signal from the BAS/network, if the
air handler is equipped with 0 to 25
percent motorized fresh air dampers.
A temperature sensor, located on the
entering air side of the chilled water
coil, will send a signal to the
hydronic valve to drive it full open
when a potential freeze condition is
detected. The supply fan is then
turned off and the outside air
damper is closed.
Heating
Gas Heating: Two-Stage
Upon a call for heating, the HEAT
module closes the first stage heating
contacts beginning the firing
sequence. First, the heat exchanger
combustion blower begins
operation. Upon positive proving of
combustion airflow, a 60 second pre-
purge cycle is executed. Then the
ignition sequence takes place.
If ignition is not proven, the safety
control locks out and must be
manually reset. As long as there is a
call for heat, the safety control can
be reset, which starts another purge
cycle and try for ignition.
As additional heat is required, the
HEAT module will close the second
stage heating contacts and
depending on heat module size, will
open either the second stage of the
gas valve, or a second stage gas
valve.
During heating operation, an
electronic flame safety control
provides continuous flame
supervision. If combustion should
become unstable for any reason,
heating will automatically shut
down. On the low heat and medium
heat for all units, after a one minute
delay, plus another 60 second pre-
purge cycle the ignition cycle begins.
On all other heat sizes the heating
section will be shutdown and locked
out until manually reset at the
ignition module and unit mounted
Human Inferface Panel after the first
shutdown due to flame instability.
As the heating requirement is
satisfied, the HEAT module will open
the second stage heating relay, de-
energizing the second stage of heat.
When the requirement is fully
satisfied, the first stage contacts are
opened, de-energizing the first stage
of heat.
Gas Heating: Modulating Gas
Upon a call for heating, the HEAT
module closes the heating contacts,
beginning the firing sequence. First,
the heat exchanger combustion
blower begins operation. Upon
positive proving of combustion
airflow, a pre-purge cycle is
executed. Then the ignition
sequence takes place.
If ignition is not proven, the safety
control locks out and must be
manually reset. As long as there is a
call for heat, the safety control can
be reset, which starts another purge
cycle and try for ignition.
Once ignited, as additional heat is
required, the combustion air damper
opens, increasing the firing rate.
During heating operation, an
electronic flame safety control
provides continuous flame
supervision. If combustion should
become unstable for any reason,
heating will automatically shut down
and be blocked out until reset at the
unit mounted Human Interface
panel.
As the heating requirement is
satisfied, the HEAT module will
modulate the combustion air
damper closed, and the firing rate
will lower to maintain the desired
outlet temperature. When the
requirement is fully satisfied, the
heating contacts are opened, de-
energizing the heat. The specific
sequence of operation of the gas
heat will depend on the size of the
heat exchanger.
Controls Constant Volume (CV) Only