Controls Variable Air Volume (VAV) Only
temperature from setpoint, i.e., the further away from setpoint, the faster the fresh air damper will 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. For comparative enthalpy economizer control, outdoor air enthalpy must be below the enthalpy of the return air.
The outdoor air dampers may be set for a maximum of 25 percent outdoor
Supply Air
Temperature
Supply Air
Temperature
Setpoint
Heating
Reset
Ending (Cooling)
Beginning (Heating)
Reset Temperature
Cooling | Amount of | |
Temperature | ||
Reset | ||
Reset | ||
|
Outdoor Air
or
Zone
Beginning (Cooling) Temperature
Ending (Heating)
Reset Temperature
air, through the unit mounted Human Interface Panel or a signal from the
Figure 12. Supply Air Temperature Reset
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 fresh air damper is closed.
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 60 second
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 locked 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,
Electric Heating
The individual stages of electric heat will be sequenced on the zone demand. The number of available stages will depend on the unit size and heat capacity selected.
Hot Water or Steam
On units with hot water or steam heating, the supply air temperature can be controlled to a heating setpoint during the Occupied mode. The supply air temperature heating setpoint and deadband are user- defined at the Human Interface Panel. VAV Occupied heating on hot water and steam heat units is enabled by closing a
Supply Air Setpoint Reset
Supply air setpoint reset can be used to adjust the supply air temperature setpoint on the basis of a zone temperature or on outdoor air temperature. Supply air setpoint reset adjustment is available from the Human Interface Panel for supply air heating and supply air cooling control.
Outdoor air cooling reset is sometimes used in applications where the outdoor temperature has a large effect on building load. When the outside air temperature is low and the building cooling load is low, the supply air setpoint can be raised, thereby preventing subcooling of critical zones. This reset can lower usage of primary cooling, thus savings in mechanical cooling kw, but an increase in supply fan kw may occur.
Outdoor air heating reset is the inverse of cooling, with the same principles applied.
For both outdoor air cooling reset and heating reset, there are three
–beginning reset temperature
–ending reset temperature
–amount of temperature reset
Zone reset is applied to the zone(s) in a building that tend to over cool or
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