Carrier 48/50AJ Inputs→GEN.I→E.SW, E.Sel =, Psychrometric Chart for, Enthalpy Control

modulate to the same control point (Sum Z) that is used to control capacity staging. The advantage is lower compressor cycling coupled with tighter temperature control. Setting this option to No will cause the economizer, if it is able to provide free cooling, to open to the Economizer Max. Position (EC.MX) during mechanical cooling.

ECONOMIZER OPERATION — There are four potential elements which are considered concurrently which determine whether the economizer is able to provide free cooling:

1.Dry bulb changeover (outside-air temperature qualification)

2.Economizer switch (discrete control input monitoring)

3.Economizer changeover select (E.SEL economizer changeover select configuration option)

4.Outdoor dewpoint limit check (requires an installed out- door relative humidity sensor installed)

Dry Bulb Changeover (OAT.L) — Outside-air temperature may be viewed under Temperatures→AIR.T→OAT. The con- trol constantly compares its outside-air temperature reading against the high temperature OAT lockout (OAT.L). If the tem- perature reads above OAT.L, the economizer will not be al- lowed to perform free cooling.

Economizer Switch (EC.SW) — The function of this switch is determined by Configuration →ECON→EC.SW. The state of the corresponding economizer input can be viewed under

Inputs→GEN.I→E.SW.

When set to EC.SW = 0, the switch is disabled. When set to EC.SW = 1, the economizer switch functions to enable/disable the economizer. When set to EC.SW = 2, the switch functions as an IAQ override switch. This functions just like the discrete IAQ input Inputs→AIR.Q→IAQ.I when Configuration →IAQ→AQ.CF→IQ.I.C=2 (IAQ Discrete Override). See the Indoor Air Quality Control section for more information.

When Configuration→ECON→EC.SW=1 and Inputs→ GEN.I→E.SW = No, free cooling will not be allowed.

Economizer Control Type (E.TYP) — This configuration should not be changed.

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 external switch enable 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 enabling and disabling of free cooling. If this option is selected and outside-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 conditions. The A Series control can use an enthalpy switch or enthalpy sensor, or the standard installed outdoor dry bulb sensor and an accessory relative humidity sensor to calculate the enthalpy of the air.

Setting Configuration→ECON→E.SEL = 2 requires that the user configure Configuration→ECON→OA.E.C, the Outdoor Enthalpy Changeover Select, and install an outdoor relative humidity sensor. Once the sensor is installed, enable Configuration→ECON→ORH.S, the outdoor relative humid- ity sensor configuration option.

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.

CONTROL CURVE	CONTROL POINT
	(approx Deg) AT 50% RH
A	73
B	68
C	63
D	58

Fig. 10 — Psychrometric Chart for

Enthalpy Control

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 comparison of out- door enthalpy versus an enthalpy set point is done. This out- door enthalpy set point limit is configurable, and is called Configuration→ECON→OA.EN.

Depending on what Configuration→ECON→OA.E.C is configured for, if the outdoor enthalpy exceeds the Honeywell curves or the outdoor enthalpy compare value (Configuration →ECON→OA.EN), then free cooling will not be allowed.

Differential Enthalpy Changeover — This option compares the outdoor-air enthalpy to the return air enthalpy and chooses the option with the lowest enthalpy. This option 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 sensor (RRH.S) are used. The outdoor air relative humidity sensor config (ORH.S) and return air humidity sensor config (Configuration→UNIT→SENS→RRH.S) must be enabled.

Outdoor Dewpoint Limit Check — If an outdoor relative humidity sensor is installed, then the control is able to calculate the outdoor air dewpoint temperature and will compare this temperature against the outside air dewpoint temperature limit configuration (Configuration→ECON→O.DEW). If the outdoor air dewpoint temperature is greater than O.DEW, then free cooling will not be allowed. Figure 11 shows a horizontal limit line in the custom curve of the psychrometric chart. This is the outdoor air dewpoint limit boundary.

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