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

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

GAS- OR OIL-FIRED UNIT HEATER

The gas-fired unit hea ter (F ig. 19) is used when a centr al hot
water or steam system is not a vailable. In a g as-fired unit hea ter,
a gas bur ner hea ts a hea t exchang er while a fan forces air acr oss
the e xchang er to w ar m the space . The two-position contr ol
system compr ises a ther mosta t, solenoid gas valve, safety cutout
switch, pilot saf ety switch, and f an s witch.
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 temper atur e falls below the ther mosta t setpoint,
the ther mosta t contacts c lose to ener gize the g as valve. If the
pilot saf ety switch indica tes the pilot b ur ner is lit, the g as valve
ener gizes. The bur ner w ar ms the hea t exchang er and the f an
switch tur ns on the f an. The bur ner oper ates until space
temper atur e war ms to setpoint. If unacce pta bly high
temper atur es occur in the hea ter (e .g., dur ing fan failur e), the
safety cutout s witch closes the solenoid g as valve. After the
room ther mosta t shuts of f the b ur ner, the fan contin ues to r un
until the hea t exchang er cools.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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 lar ger industr ial gas-fired unit hea ter s have two stages:
two-position (lo w fire) and modula ting (to high f ire). These
units are controlled by room thermostats designed to sequence
the tw o-position and modula ting sta ges on a decr ease in space
temperature.
Oil-fired unit hea ter s oper ate similar ly to gas-fired unit hea ter s
and inc lude saf ety contr ols designed f or oil b ur ner s.

UNIT VENTILATORS

GENERAL

A unit v entila tor consists of damper s, a filter, a fan, a hea ting
and/or cooling coil, and the necessar y contr ols (e.g., a valve
and damper actua tor). Unit v entila tor s use outdoor air ,
recirculated or return air from the space, or a mixture of both.
Unit ventila tor s ar e designed f or man y capacities and ar e used
in ar eas wher e occupanc y density indica tes a need f or contr olled
ventila tion (e .g., classr ooms, confer ence r ooms). A unit
ventila tor contr ol system var ies heating, ventila ting, and cooling
(if available) while the f an r uns contin uously.
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.
411
DISCHARGE
AIR
FILTER
OA, RA
DAMPERS
RETURN
AIR
C3037