Gas- Or Oil-Fired Unit Heater; Unit Ventilators; General - Honeywell AUTOMATIC CONTROL Engineering Manual

CEILING
THERMOSTAT
COIL
DIFFUSER
Fig. 18. Down-Blow Unit Heater.

GAS- OR OIL-FIRED UNIT HEATER

The gas-fired unit heater (Fig. 19) is used when a central hot
water or steam system is not available. In a gas-fired unit heater,
a gas burner heats a heat exchanger while a fan forces air across
the exchanger to warm the space. The two-position control
system comprises a thermostat, solenoid gas valve, safety cutout
switch, pilot safety switch, and fan switch.
FAN SWITCH
(MAY BE COMBINATION FAN
AND LIMIT CONTROLLER)
FLUE
EXCHANGER
FAN
GAS-FIRED
UNIT HEATER
SOLENOID
GAS VALVE
GAS
SUPPLY
Fig. 19. Gas-Fired Unit Heater.
When space temperature falls below the thermostat setpoint,
the thermostat contacts close to energize the gas valve. If the
pilot safety switch indicates the pilot burner is lit, the gas valve
energizes. The burner warms the heat exchanger and the fan
switch turns on the fan. The burner operates until space
temperature warms to setpoint. If unacceptably high
temperatures occur in the heater (e.g., during fan failure), the
safety cutout switch closes the solenoid gas valve. After the
room thermostat shuts off the burner, the fan continues to run
until the heat exchanger cools.
FAN
MOTOR
VALVE
HOT WATER
OR STEAM
SUPPLY
DOWN-BLOW
UNIT HEATER
THERMOSTAT
C3028
SAFETY
CUTOUT
S S
SWITCH
HEAT
THERMOSTAT
BURNER
PILOT
S
SAFETY
SWITCH
C3024
INDIVIDUAL ROOM CONTROL APPLICATIONS
Some larger industrial gas-fired unit heaters have two stages:
two-position (low fire) and modulating (to high fire). These
units are controlled by room thermostats designed to sequence
the two-position and modulating stages on a decrease in space
temperature.
Oil-fired unit heaters operate similarly to gas-fired unit heaters
and include safety controls designed for oil burners.

UNIT VENTILATORS

GENERAL

A unit ventilator consists of dampers, a filter, a fan, a heating
and/or cooling coil, and the necessary controls (e.g., a valve
and damper actuator). Unit ventilators use outdoor air,
recirculated or return air from the space, or a mixture of both.
Unit ventilators are designed for many capacities and are used
in areas where occupancy density indicates a need for controlled
ventilation (e.g., classrooms, conference rooms). A unit
ventilator control system varies heating, ventilating, and cooling
(if available) while the fan runs continuously.
Figure 20 shows a "blow-through" unit ventilator. Dampers
at the bottom of the unit control the amounts of outdoor air and
return air brought into the unit. The air passes through the filter
section and enters the fan section, where the fan blows it across
the coil.
WALL
COIL
FAN
OUTDOOR
AIR
Fig. 20. Blow-Through Unit Ventilator.
415 ENGINEERING MANULA OF AUTOMATIC CONTROL
DISCHARGE
AIR
FILTER
OA, RA
DAMPERS
RETURN
AIR
C3037