Honeywell W7750A Staged Cooling Control, ONE Stage, TWO Stages, Three Stages, Four Stages

EXCEL 10 W7750A,B,C CONSTANT VOLUME AHU CONTROLLER

STAGED COOLING CONTROL

The Excel 10 W7750 Controller supports up to four stages of D/X cooling. As space temperature rises above the current Cooling Setpoint, the controllers mode of operation is switched to the COOL mode. When in the COOL mode, all heating outputs are driven closed or off (with the exception that occurs during IAQ Override Operation, see above), and the staged cooling outputs are enabled for use. When in the COOL mode, the PID cooling control algorithm compares the current space temperature to the EffectiveCoolSetPt, and

calculates a PID error signal. This error signal causes the cooling stage outputs to be cycled as required to drive the space temperature back to the setpoint. Fig. 50 illustrates the relationship between PID error and staged output activity. As the error signal increases and the space temperature is getting farther away from setpoint, or is remaining above setpoint as time elapses, additional stages of cooling are energized until, if PID error reaches 100 percent, all configured stages are on.

PID

ERROR

NO. STAGES

0%

25%

33%

50%

66%

75%

100%

> 100%

CONFIGURED

ALL STAGES

ONE STAGE

CYCLING

LOCKED ON

TWO STAGES

STAGE 1

STAGE 1 LOCKED ON

ALL STAGES

CYCLING

STAGE 2 CYCLING

LOCKED ON

THREE STAGES

STAGE 1

STAGE 1 LOCKED ON

STAGE 1,2 LOCKED ON

ALL STAGES

CYCLING

STAGE 2 CYCLING

STAGE 3 CYCLING

LOCKED ON

STAGE 1

STAGE 1,2

STAGE 1,2,3

FOUR STAGES

STAGE 1

LOCKED ON

LOCKED ON

LOCKED ON

ALL STAGES

CYCLING

STAGE 2

STAGE 3

STAGE 4

LOCKED ON

CYCLING

CYCLING

CYCLING

M10112

Fig. 50. Staged output control versus PID Error.

If economizer dampers are configured, and the outdoor air is suitable for free cooling, the economizer operates as the first stage of cooling. For example, if a controller was configured with two stages of mechanical cooling and an economizer, the application should be viewed in Fig. 50 as a three-stage system.

Setpoints for the PID gains allow for unit-by-unit adjustment of the control loop, if required; however, any change from the default values should be minimal.

The PID control algorithm used to control staged cooling is anticipator-driven, and is similar to the algorithm used in the T7300 commercial thermostat. All staging events are subject to a minimum interstage time delay, which is based on the cycles per hour user setting (CoolCycHr). The minimum interstage time delay ranges from 90 seconds (at 12 cycles per hour) to 8.5 minutes (at two cycles per hour), see Table

17.The user has the option to disable the minimum run timer (DisMinClTimer for cooling). If the minimum run timer is disabled, the interstage time delay is fixed at 20 seconds. The cycling rate is separately selectable for heating and cooling between 2 and 12 cycles per hour (cph).

Table 17. Interstage Minimum Times

Cycles/Hour Selection	Minimum On/Off time (Min.)
2	8.5
3	5.5
4	4.0
5	3.5
6	3.0
7	2.5
8	2.0
9	2.0
10	2.0
11	1.5
12	1.5

STAGED HEATING CONTROL

The Excel 10 W7750B,C Controller supports up to four stages of heating. As space temperature falls below the current Cooling Setpoint, the controller mode of operation is switched to the HEAT mode. When in the HEAT mode, all cooling outputs are driven closed or off, and the staged heating outputs are enabled for use. When in the HEAT mode, the PID

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