Controls Constant Volume (CV) Only
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,
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
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.
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