Honeywell M8405 - Manual (Page 14)

SOLID STATE ECONOMIZER SYSTEM
63-2484—2 14
M7415 Damper Actuator
Single M7415 Damper Actuator accepts thermistor sensor
input from a C7046A or a C7150B Mixed Air Sensor mounted
in discharge or mixed air duct. See Fig. 27.
During the occupied period, on a call for cooling, when outdoor
air temperature or enthalpy conditions are low, the M7415
Actuator proportions to maintain between 50°F (10°C) and
56°F (13°C) at the thermistor sensor.
If the mixed or discharge temperature is above 56°F (13°C),
the M7415 Actuator opens to admit additional outdoor air until
the temperature returns to the 50°F (10°C) to 56°F (13°C)
range. If the mixed or discharge air temperature is below 50°F
(10°C), the actuator proportions closed, shutting the outdoor
air damper until the temperature returns to the 50°F (10°C) to
56°F (13°C) range. During the occupied period, the actuator
does not close past the minimum position.
If the fully open M7415 Actuator cannot satisfy the space
demand, mechanical cooling is sequenced on.
During the unoccupied period, the M7415 Actuator overrides
the minimum position setting and drives fully closed. On a loss
of power, the actuator spring-returns fully closed.
If terminals P and P1 are jumpered, the M7415 drives fully
open; however, if terminals P and P1 are left open, the M7415
drives fully closed. The M7415 minimum position adjustment
drives the motor open when the resistance across P and P1 is
minimal. Increasing the amount of resistance across these
terminals drives the actuator closed.
M8405 Damper Actuator
An spst low-voltage controller is used to control the M8405
Actuator as follows:
a. Fully open—when the controller circuit closes to pro-
vide 24 Vac to terminals D and T, the actuator is
energized and runs fully open.
b. Fully closed—when the controller circuit opens,
power is removed from terminals D and T, and the
actuator spring-returns to the fully closed position.
c. Mid-position—when the controller circuit closes to
provide 24 Vac to terminals T and X, the actuator is
energized to run to the adjustable mid-position (mini-
mum position).
Adjustable minimum position can be reached from either the
fully closed or fully open position. From fully closed, the
actuator drives open to the minimum position; from fully open,
the actuator spring-returns to minimum position.
W7459A,C Solid State Economizer Logic
Module
The purpose of an economizer is to use outdoor air for cooling
whenever possible, to reduce air conditioner compressor
operation. The W7459 Economizer System, when wired as
shown in Fig. 27, responds to a signal from the cooling
thermostat. This system uses a C7400 Enthalpy Sensor or a
C7650A Temperature Sensor. It responds to both dry bulb
temperature and humidity, allowing the use of outdoor air at
higher temperatures for free cooling when the humidity is low.
The economizer functions as a true first stage of cooling and
provides maximum fuel economy during the cooling cycle. The
economizer is automatically locked out during heating. It holds
the outdoor air damper at the minimum position setting. On a
call for cooling by the space thermostat, the system operates
as follows:
When the enthalpy or temperature of the outdoor air is below
the setpoint, the outdoor air damper is proportioned open (and
the return air damper is proportioned closed) to maintain
between 50°F and 56°F (10°C and 13°C) at the mixed/
discharge air sensor. During economizer operation, the
mechanical cooling is operated by stage 2 cooling on the
space thermostat.
When the enthalpy or temperature of the outdoor air is above
the setpoint, the outdoor air damper closes to its minimum
position. A call for cooling from the space thermostat turns on
the mechanical cooling.
During the unoccupied period, the M7415 Damper Actuator
spring-returns the outdoor air damper to the fully closed
position.
W7212A Economizer Logic Module
W7212A economizer logic module used with C7400 enthalpy
sensors, M7215 or MS7505 damper actuators and C7232 CO
2
sensors to proportion outdoor and return air dampers from
economizer and demand control ventilation in commercial
HVAC equipment.
W7212A Optional Applications
HEAT PUMP CHANGEOVER (W7213, W7214 ONLY)
In heat pump applications, the controller must have control of
the changeover valve. To provide the logic module with the
information necessary for proper information, there must be a
connection to the logic module O/B terminal. This terminal
alerts the logic module as to when the system operates in
cooling (the only time the economizer is used).
W7213 (CHANGEOVER TERMINAL B)
Connect the B terminal according to the following details:
— 24V power to B: System is in heating mode.
— No power to B: System is in cooling mode.
W7214 (CHANGEOVER TERMINAL O)
Connect the O terminal according to the following details:
— 24V power to O: System is in cooling mode.
— No power to O: System is in heating mode.
SETTINGS AND ADJUSTMENTS
Adjusting Minimum Damper Position
The minimum position potentiometer keeps the outdoor air
damper from completely closing during system operation to
allow ventilation.