Two Sensor Or Differential Enthalpy - Honeywell Design and Application Technical Reference Manual

Two Sensor or Differential Enthalpy

A dual sensor enthalpy control is equipped
with the same outdoor air enthalpy sensor and
an additional second enthalpy sensor in the
return air. This is also referred to as differential
enthalpy. On a call for cooling or when the
mixed air temperature goes above the high
MAT range or setpoint, additional air with the
lower enthalpy, outdoor or return, is selected
to be brought into the conditioning section of
the air handler. For analog economizers the
setpoint on the logic module is turned to D
whenever differential enthalpy is used. This is
a very efficient method of controlling outdoor
air usage since the return and outside air
comparison is continuous and automatic year-
EXHAUST
AIR
OUTSIDE
AIR SENSOR
DUAL SENSOR OR DIFFERENTIAL ENTHALPY
DAMPER
TB8220
COMMERCIAL
PRO
Section 2 - Enthalpy Theory And Controllers
round. It eliminates operator error by
eliminating seasonal changeover which is
frequently overlooked. Though it may appear
wasteful to cool outdoor air at a higher dry
bulb temperature than return air, the savings
are verifiable through psychrometric
calculations. The amount of mechanical
cooling required to dehumidify air often
exceeds the amount required to lower the dry
bulb temperature. In buildings where there is a
substantial amount of cooking, laundry or
other moisture generating activity this type of
control sequence can result in substantial
savings in cooling costs.
RETURN AIR
SENSOR
MIXED OR
COILS FAN
SUPPLY AIR
SENSOR
29
RETURN AIR
FROM ROOM
SUPPLY
AIR TO
BUILDING
W7220 ECONOMIZER
CONTROLLER
WITH DEMAND
CONTROL
VENTILATION
M23904A
Honeywell Economizers 63-8594-02