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Enthalpy Switch
The state of the enthalpy switch can be viewed under Inputs
GEN.I
ENTH. Enthalpy switches are installed as standard on
all P Series rooftops. When the switch reads high, free cooling will
be disallowed.
The enthalpy switch opens (reads high) when the outdoor enthalpy
is above 24 Btu/lb or dry bulb temperature is above 70°F and will
close when the outdoor enthalpy is below 23 Btu/lb or the dry bulb
temperature is below 69.5°F.
NOTE: The enthalpy switch has both a low and a high output. To
use this switch as designed, the control must be connected to the
low output. Additionally, there is a switch logic setting for the en-
thalpy switch under Configuration
ting must be configured to closed (CLSE) to work properly when
connected to the low output of the enthalpy switch.
There are two jumpers under the cover of the enthalpy switch.
One jumper determines the mode of the enthalpy switch/receiver.
The other is not used. For the enthalpy switch, the factory setting
is M1 and should not need to be changed. See Fig. 9 for a diagram
showing the settings on the enthalpy switch.
The enthalpy switch may also be field converted to a differential
enthalpy switch by field installing an enthalpy sensor (33CSENT-
SEN or HH57ZC001). The enthalpy switch/receiver remains in-
stalled in its factory location to sense outdoor air enthalpy. The ad-
ditional enthalpy sensor (33CSENTSEN) is mounted in the return
airstream to measure return air enthalpy. The enthalpy control
jumper must be changed from M1 to M2 for differential enthalpy
control. For the 2-wire return air enthalpy sensor, connect power
to the Vin input and signal to the 4 to 20 mA loop input. See Fig. 9
for diagram showing the settings and inputs on the enthalpy
switch.
There is another way to accomplish differential enthalpy control
when both an outdoor and return air relative humidity sensor are
present. See Economizer Changeover Select section below for fur-
ther information.
Fig. 9 — Enthalpy Switch Jumper Positions
ECONOMIZER CHANGEOVER SELECT (E.SEL)
The control is capable of performing any one of the following
changeover types in addition to both the dry bulb lockout and the
standard external input:
E.SEL = 0
none
E.SEL = 1
Differential Dry Bulb Changeover
E.SEL = 2
Outdoor Enthalpy Changeover
E.SEL = 3
Differential Enthalpy Changeover
Differential Dry Bulb Changeover
As both return air and outside air temperature sensors are installed
as standard on these units, the user may select this option, E.SEL
= 1, to perform a qualification of return and outside air in the en-
abling/disabling of free cooling. If this option is selected and out-
side-air temperature is greater than return-air temperature, free
cooling will not be allowed.
Outdoor Enthalpy Changeover
This option should be used in climates with higher humidity con-
ditions. The P Series control can use an enthalpy switch or enthal-
py sensor, or the standard installed outdoor dry bulb sensor and an
accessory relative humidity sensor to calculate enthalpy of the air.
Setting Configuration
ECON
user configure Configuration
Enthalpy Changeover Select, and install an outdoor relative
SW.LG
ENT.L. This set-
E.SEL = 2 requires that the
ECON
OA.E.C, the Outdoor
humidity sensor. A control expansion module (CEM) is required.
Once the sensor and board are installed, enable Configura-
tion
ECON
ORH.S, the outdoor relative humidity sensor con-
figuration option. This will automatically enable the CEM board,
if it is not enabled already.
If the user selects one of the Honeywell curves, A, B, C, or D, then
OA.E.C options 1-4 should be selected. See Fig. 10 for a diagram
of these curves on a psychrometric chart.
OA.E.C = 1 Honeywell A Curve
OA.E.C = 2 Honeywell B Curve
OA.E.C = 3 Honeywell C Curve
OA.E.C = 4 Honeywell D Curve
OA.E.C = 5 custom enthalpy curve
If the user selects OA.E.C = 5, a direct compare of outdoor enthal-
py versus an enthalpy set point is done. This outdoor enthalpy set
point limit is configurable, and is called Configuration
ECON
OA.EN.
Depending on what Configuration
ured for, if the outdoor enthalpy exceeds the Honeywell curves or
the outdoor enthalpy compare value (Configuration
OA.EN), then free cooling will not be allowed.
NOTE: If the user wishes to disable the standard enthalpy control
from running concurrently, a field-supplied jumper must be in-
stalled between TB201 terminals 5 and 6.
CONTROL CURVE
A
B
C
D
Fig. 10 — Psychrometric Chart for Enthalpy Control
Differential Enthalpy Changeover
This option compares the outdoor-air enthalpy to the return-air en-
thalpy and chooses the option with the lowest enthalpy. This op-
tion should be used in climates with high humidity conditions.
This option uses both humidity sensors and dry bulb sensors to
calculate the enthalpy of the outdoor and return air. An accessory
outdoor-air humidity sensor (ORH.S) and return-air humidity sen-
sor (RRH.S) are used. The outdoor-air relative humidity sensor
80
ECON
OA.E.C is config-
ECON
CONTROL POINT
(approx Deg) AT 50% RH
73
68
63
58
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