Precautions And Conditions For Successful Operation; Blocked Airflow; Power Failure; Coil Freeze-Up - Honeywell AUTOMATIC CONTROL Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS
Three room temperature setpoints are shown, free cooling, a
heating deadband, and a mechanical cooling deadband. The free
cooling setpoint may be manual on the wall module, or software
(with software bounds). All unit ventilators are switched to the
free cooling economizer mode of operation globally anytime the
OA is suitable (see the Air Handling System Control Applications
section). When the OA is unsuitable for free cooling assistance,
and chilled water is available, the OA damper is returned to the
minimum ventilation.
PRECAUTIONS AND CONDITIONS FOR
SUCCESSFUL OPERATION
Unit ventilators require protection from blocked airflow,
power failure, and coil freeze-up.

Blocked Airflow

Proper airflow is essential to satisfying space temperature
and ventilation requirements. Objects located directly over the
discharge air vents can inhibit or block airflow. Cleaning or
replacing the filter as needed and cleaning dust and dirt from
unit ventilator coils improves airflow through the unit ventilator.

Power Failure

Precautions for power and air failure must be specified when
the automatic control system is designed. Pneumatic systems
require an electric-pneumatic switch to exhaust the valve and
damper actuator diaphragms on a power or air failure. The outdoor
air damper closes and the return air damper and coil valve open.
Electric, electronic, and digital control systems should be specified
with spring-return actuators that close the outdoor air damper and
open the return air damper on power failure.

Coil Freeze-Up

Causes of coil freeze-up include central system circulating
pump or boiler failure, outdoor air damper or control valve
malfunction, and uneven temperature distribution. A low-limit
controller can help prevent coil freeze-up by overriding other
control system components to close the outdoor air damper
and open the return air damper and coil valve. Effective freeze-
up protection depends on available heat in the system and flow
through the coils.
Coils can also freeze when low-temperature outdoor air leaks
through defective dampers. Frequent inspection of dampers
should be made to detect bent and broken damper linkages,
warped damper blades, and defective or missing blade seals
that can contribute to coil freeze-up.
ENGINEERING MANULA OF AUTOMATIC CONTROL
Low temperature switches to stop the fan and close the OA
damper should be provided where freezing OA conditions are
likely. With digital systems, during unoccupied periods, the low
temperature switch may start the fan and the recirculating pump.

FAN COIL UNITS

GENERAL

Fan coil units are similar to unit ventilators except that fan
coil units do not have dampers and typically do not have an
outdoor air intake. They may also be configured for installation
above a ceiling with ceiling or wall mounted discharge and
return air grills. Fan coil units provide heating and/or cooling
for single-zone areas such as apartments, offices, and individual
hotel or hospital rooms. Figure 30 shows a typical fan coil unit
comprising a finned-tube coil, a fan section, and a filter. The
fan circulates air from the space across the coil. The coil may
use steam or hot water from a central system or electric
resistance elements to satisfy heating requirements. Chilled
water or DX coils can be used for cooling. Units used for cooling
only or for both heating and cooling have a built-in condensate
drain pan to collect and drain condensate on the cooling cycle.
WALL
COIL
FAN
OUTDOOR
AIR
(OPTIONAL)
Fig. 30. Fan Coil Unit.
Fan coil units are classified as two-pipe heating, two-pipe
cooling, two-pipe heating/cooling, or four-pipe heating/cooling.
Control for a fan coil unit typically comprises a room or return
air thermostat for individual room control. The thermostat
regulates a valve while a fan moves air through the unit and
across the coil. The fan runs continuously, is scheduled, or is
cycled on and off by the thermostat. The fan is often three speed
with a local three-speed switch. Some applications control only
the fan operation and allow the conditioning medium to flow
continuously in the coil. Fan coil units can use pneumatic,
electric, electronic, or digital control.
422
DISCHARGE
AIR
FILTER
RETURN
AIR
C3038