Honeywell W7750A Allowable Heating and Cooling Equipment Configurations, Heat Pump Control

EXCEL 10 W7750A,B,C CONSTANT VOLUME AHU CONTROLLER

Allowable Heating and Cooling Equipment Configurations

Each W7750 device can control a variety of different types of mechanical cooling and heating equipment within roof top air handlers. See Fig. 13 through 17 for a conceptual overview of some typical configurations. For specific wiring details, see the Prepare Wiring Diagrams section.

STAGED HEATING/COOLING CONTROL

Staged equipment control is available for up to four stages of heating or four stages of cooling. On the W7750, the stages are activated through digital outputs (Triacs on the W7750B,C and dry-contact relays on the W7750A) one for each stage wired to 24 Vac contactors (see Fig. 27 and 30 in Step 4. Prepare Wiring Diagrams section for wiring details). Note that the number of physical Digital Outputs (DOs) on the controller limits the total number of stages that can be controlled. For example, the W7750A Model has six digital outputs, and because one is used for the supply fan, there are five DOs available for any combination of heating and cooling stages

positions the actuator based on the length, in seconds, of the pulse from the digital output. For PWM, the controller outputs a pulse whose length consists of two parts, a minimum and a maximum. The minimum pulse time represents the analog value of 0 percent and the maximum pulse length that represents an analog value of 100 percent. If the analog value is greater than 0 percent, an additional time is added to the minimum pulse time. The length of time added is directly proportional to the magnitude of the analog value. The PWM actuator will begin to use the analog value at the end of the pulse and will continue to use this value until a new pulse is received. Refer to Appendix B under PWM Control for an example. Series 60 actuators are generally less expensive than those for PWM, but the trade-off is that PWM requires only a single controller digital output while floating control uses two DOs. Refer to Appendix B under Series 60 Modulating Control for an example. Fig. 14 illustrates a system with modulating heating and cooling (see Fig. 29 and 31 in Step 4. Prepare Wiring Diagrams section.

(with a maximum of four stages of heating and four stages for cooling). The W7750B Model offers two additional DOs, for a total of eight. The W7750C offers five DOs and three Analog Outputs (AOs). Fig. 13 shows a typical application of two stages of heat and two stages of cooling.

	COOL	HEAT
	COIL	COIL
MIXED	FAN	DISCHARGE
		AIR
AIR
	-	+
FAN	COMPRESSORS

MIXED

AIR

FAN

STARTER

COOL

COIL

FAN

-

CHILLED

WATER

VALVE

HEAT

COIL

DISCHARGE

AIR

+

HOT

WATER

VALVE

STARTER

EXCEL 10 T7560A,B OR T7770

CVAHU

W7750A,B,C

Y1 Y2

EXCEL 10

CVAHU

W7750A,B,C

GAS COMBUSTION

CONTROLS

W1 W2

T7560A,B OR T7770

M17491

M17492

Fig. 14. Fan, modulating heating and modulating cooling.

NOTE: Pneumatically actuated valves can be controlled using a pneumatic transducer device. See Fig. 17. Also, transducer devices are available from third party vendors to convert PWM outputs to a voltage or current signal if desired.

HEAT PUMP CONTROL

The W7750 Controller handles heat pump applications similarly to staged heating/cooling control. Heat pump

Fig. 13. Fan with two stages of heating and two stages

of cooling.

MODULATING HEATING/COOLING CONTROL

The W7750 Controller provides modulating equipment control for heating and cooling equipment (and economizer dampers, see Fig. 16) using either Series 60 Floating Control or Pulse Width Modulated (PWM) control, (PWM control is available on the W7750B,C only). The Series 60 Modulating Control is provided through two Relay digital outputs on the W7750A or two Triac digital outputs on the W7750B,C (one to pulse the valve actuator open and one to pulse it closed). PWM control

applications are supported by providing outputs for up to four compressor stages, a change-over relay for the refrigerant reversing valve, and up to four stages of auxiliary heat. Note that the W7750A Model has six digital outputs, and therefore, with one DO used for the supply fan and one for the change- over relay, there are four outputs available for any combination of compressors and auxiliary heat stages. The W7750B Model offers two additional DOs for a total of eight, while the W7750C Model offers five DOs and 3 AOs. Fig. 15 illustrates a typical heat pump system with auxiliary heat.

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