Carrier 48/50AJ specifications •Lo Cool Mode mechanical stages =

Table 48 — Cool/Heat Set Point Offsets Configuration

ITEM		EXPANSION		RANGE	UNITS	CCN POINT	DEFAULT
D.LV.T	COOL/HEAT SETPT. OFFSETS
L.H.ON	Dmd Level	Lo Heat On	-1 - 2		^F	DMDLHON	1.5
H.H.ON	Dmd Level(+) Hi Heat On		0.5	- 20.0	^F	DMDHHON	0.5
L.H.OF	Dmd Level(-) Lo Heat Off		0.5 - 2		^F	DMDLHOFF	1
L.C.ON	Dmd Level	Lo Cool On	-1 - 2		^F	DMDLCON	1.5
H.C.ON	Dmd Level(+) Hi Cool On		0.5 - 20.0		^F	DMDHCON	0.5
L.C.OF	Dmd Level(-) Lo Cool Off		0.5 - 2		^F	DMDLCOFF	1
C.T.LV	Cool Trend Demand Level		0.1	- 5	^F	CTRENDLV	0.1
H.T.LV	Heat Trend Demand Level		0.1	- 5	^F	HTRENDLV	0.1
C.T.TM	Cool Trend Time		30 - 600		sec	CTRENDTM	120
H.T.TM	Heat Trend Time		30 - 600		sec	HTRENDTM	120

Timeguards — In addition to the set points and offsets which determine the trip points for bringing on and bringing off cool modes there is a timeguard of 8 minutes which enforces a time delay between the transitioning from a low cool to a high cool mode. There is a timeguard of 5 minutes which enforces a time delay between the transitioning from a heat mode to a cool mode.

Supply Air Set Point Control — Once the control has deter- mined that a cooling mode is in effect, the cooling control point (Run Status→VIEW→CL.C.P) is calculated and is based upon either Setpoints→SA.HI or Setpoints→SA.LO, depending on whether a high or a low cooling mode is in effect, respectively. In addition, if supply air reset is config- ured, it will also be added to the cooling control point.

Refer to the SumZ Cooling Algorithm section for a discus- sion of how the A Series ComfortLink™ controls manage supply-air temperature and the staging of compressors for these control types.

Thermostat Cool Mode Selection (C.TYP = 3 and 4) — When a thermostat type is selected, the decision making pro- cess involved in determining the mode is straightforward. Upon energizing the Y1 input only, the unit HVAC mode will be LOW COOL. Upon the energizing of both Y1 and Y2 in- puts, the unit HVAC mode will be HIGH COOL. If just input G is energized the unit HVAC mode will be VENT and the supply fan will run.

Selecting the C.TYP = 3 (TSTAT – MULTI) control type will cause the control to do the following:

•The control will read the Configuration→UNIT→ SIZE configuration parameter to determine the number of cooling stages and the pattern for each stage.

•An HVAC mode equal to LOW COOL will cause the unit to select the Setpoints→SA.LO set point to control to. An HVAC mode equal to HIGH COOL will cause the unit to select the Setpoints→SA.HI set point to control to. Supply air reset (if configured) will be added to either the low or high cool set point.

•The control will utilize the SumZ cooling algorithm and control cooling to a supply air set point. See the section for the SumZ Cooling Algorithm section for information on controlling to a supply air set point and compressor staging.

Selecting the C.TYP = 4 (TSTAT – 2 STG) control type means that only two stages of cooling will be used. On unit sizes 020, 025 and 027 (with three compressors), an HVAC Mode of LOW COOL will energize one compressor in Cir- cuit A; an HVAC Mode of HIGH COOL will energize all three compressors. On unit sizes 030 and larger (with four compressors) an HVAC Mode of LOW COOL will energize both compressors in Circuit A; an HVAC Mode of HIGH COOL will energize all four compressors. Refer to the sec- tion on Economizer Integration with Mechanical Cooling for more information.

2-Stage Cooling Control Logic (C.TYP = 4 and 6) — The logic that stages mechanical cooling for the TSTAT and SPT 2-Stage cooling control types differs from that of the multi- stage control types. This section will explain how compressors

are staged and the timing involved for both the Low Cool and High Cool HVAC Modes.

There are either three or four compressors divided among two refrigeration circuits. Circuit A always contains two compressors (Outputs→COOL→A1 and A2). Circuit B has either one com- pressor (Outputs→COOL→B1) on size 020-027 units or two compressors (Outputs→COOL→B1 and B2) on size 030-060 units. For 2-stage cooling control, regardless of configuration, there is no minimum load valve (MLV) control. The decision as to which compressor should be turned on or off next is decided by the compressor’s availability and the preferred staging order.

Either A1 or A2 may start first as there is a built-in lead/lag logic on compressors A1 and A2 every time the unit stages to 0 compressors. Also, based on compressor availability, it should be noted that any compressor may come on. For example, on a 3 compressor unit, if no compressors are currently on, compressor A2 is currently under a minimum off compressor timeguard, and 2 compressors are to be turned on, then com- pressors A1 and B1 will be turned on immediately instead of A1 and A2.

Low Cool Versus High Cool Mechanical Staging — The num- ber of compressors to be requested during a cooling mode are divided into 2 groups by the control, HVAC mode = Lo Cool and HVAC mode = Hi Cool.

If the economizer is not able to provide free cooling (Run Status→ECON→ACTV = NO) then the following staging occurs:

•Lo Cool Mode mechanical stages = 2

•Hi Cool Mode mechanical stages = 3 (for 020 through 027 size units)

•Hi Cool Mode mechanical stages = 4 (for 030 through 060 size units)

If the economizer is able to provide free cooling (Run Sta- tus→ECON→ACTV = YES) then the following staging occurs:

1.If the economizer’s current position is less than Configu- ration→ECON→EC.MX – 5 and mechanical cooling has not yet started for the current cool mode session then: Lo Cool Mode mechanical stages = 0

Hi Cool Mode mechanical stages = 0

2.During the first 2.5 minutes of a low or high cool mode where the economizer position is greater than Configura- tion →ECON→EC.MX – 5% and mechanical cooling has not yet started:

Lo Cool Mode mechanical stages = 0 Hi Cool Mode mechanical stages = 0

3.If the economizer position is greater than Configuration →ECON→EC.MX – 5% for more than 2.5 minutes but less than 5.5 minutes and mechanical cooling has not yet started then:

Lo Cool Mode mechanical stages = 1 Hi Cool Mode mechanical stages = 1

4.If the economizer position is greater than Configuration →ECON→EC.MX – 5% for more than 5.5 minutes but less than 8 minutes and mechanical cooling has started

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