York HFC-410A User Manual

FORM 100.50-EG5 (108)

JOHNSON CONTROLS

the Return Air Temperature  Setpoint, Morning Warm-
Up ends when occupancy occurs (BAS, Intelli-Start, or 
contact closure), or when the Maximum Morning Warm-
Up Time has expired.

The unit can be equipped with one of three types of 
optional economizers: dry-bulb, single-enthalpy, or com-
parative-enthalpy. When the unit controller determines 
that Outside Air is suitable for economizing, the unit 
controller will control the outside air damper(s) open 
to provide economizer cooling. If economizer cooling 
alone is insuffi cient for the cooling load, the unit control-
ler shall stage up compressors, one at a time, to meet 
demand.

The control logic for the three types of economizers is 
as follows:

The dry-bulb economizer is the default economizer 
control scheme. With the dry-bulb economizer, the unit 
controller monitors the Outside Air temperature only 
and compares it to a reference temperature setting. 
Outside Air is deemed suitable for economizing when 
the Outside Air temperature is determined to be less 
than the reference temperature setting. This method of 
economizing is effective, but is prone to some change-
over ineffi ciencies due to the fact that this method is 
based on sensible temperatures only and does not take 
Outside Air moisture content into consideration.

With the optional, single-enthalpy economizer, the unit 
controller monitors the Outside Air enthalpy in addition 
to the Outside Air temperature and compares it to a 
reference enthalpy setting and a reference temperature 
setting. Outside Air is deemed suitable for economizing 
when the Outside Air enthalpy is determined to be less 
than the reference enthalpy setting and the Outside Air 
temperature is less than the reference temperature set-
ting. This method of economizing allows the reference 
temperature setting to be set higher than the dry-bulb  
Economizer and is a more effi cient packaged rooftop 
economizer.

With the optional, dual-enthalpy economizer, the unit 
controller monitors and compares the Outside Air and 
Return Air enthalpies, in addition to comparing the 
Outside Air temperature to the reference temperature 
setting. Outside Air is deemed suitable for economizing 
when the Outside Air enthalpy is determined to be less 
than the Return Air enthalpy and the Outside Air tem-

perature is less than the reference temperature setting. 
This method of economizing is the most accurate and 
provides the highest degree of energy effi ciency for a 
packaged rooftop economizer.

When the unit goes into the Occupied mode of operation, 
the unit controller shall open the Outside Air Damper to 
a fi xed minimum position. The damper shall remain at 
this position as long as the unit is in the occupied mode, 
and the economizer is not suitable for cooling.

With Variable Air Volume units, there are two Minimum 
OA Damper Positions: one when the unit is at full speed 
and the second when the unit is at approximately half 
speed. These two points allow the control to linearly 
reset the position of the OA damper in response to fan 
speed.

When the unit goes into the Occupied mode of operation, 
the unit controller shall monitor the speed of the supply 
fan and open the Outside Air damper to a calculated 
minimum position based on the fan speed. This minimum 
position shall vary as the speed of the fan changes. The 
damper shall remain at this calculated position as long 
as the unit is in the occupied mode, and the economizer 
is not suitable for cooling.

The optional barometric exhaust system consists of a 
lightweight barometric relief damper installed on the end 
of the unit in the Return Air section. As more outside air 
is introduced into the controlled zone due to Economizer 
and Ventilation control sequences, the pressure inside 
the building rises. As building static pressure increases 
to overcome any exhaust duct static pressure, air will be 
allowed to escape through the barometric relief damper. 
Because this type of exhaust is not powered, the amount 
of air exhausted will be limited to the static pressure that 
will need to be overcome.

This optional variable-volume, powered-exhaust system 
consists of a fi xed-speed fan confi gured with a propor-
tionally controlled discharge damper. The Rooftop Unit 
controller monitors the pressure inside the building and 
controls the Exhaust Damper and the Exhaust Fan. 
If the Building Pressure rises, the Exhaust Damper is