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27UH-PRC002-EN
Controls
Pilot Control
Standing pilot is standard on indoor duct
furnaces, propeller fan unit heaters and
centrifugal fan unit heaters. High
efficiency units (centrifugal, propeller and
duct furnaces) ship as standard with
intermittent pilot ignition. The separated
combustion propeller and centrifugal fan
unit heaters also ship as standard with
intermittent pilot ignition.
Intermittent pilot ignition contains a
solid-state ignition control system that
ignites the pilot by spark for each cycle of
operation. When the pilot flame is
proven, the main burner valve opens to
allow gas flow to the burners. Both the
pilot and burners are extinguished
during the off cycle.
Intermittent pilot ignition is ideal for
limited access installations where
manual lighting of the pilot may be
difficult. Nuisance pilot outages on units
mounted in areas subject to occasional
severe drafts can also be eliminated.
Energy savings will be realized using this
system as the pilot is extinguished
during the off cycle. Intermittent pilot
ignition should be considered on units
that have long shutoff periods.
(Required on shipments to California.)
Fan Control
The supply fan motor is activated directly
through the fan time delay relay on
indoor units provided with single-phase
motors up to one hp. Contactors or
starters are required on all other units
with single-phase motors 1½-hp and
above, and with all three-phase motors.
On indoor units, contactors and starters,
where required, are provided. (See table
below.)
Gas Controls
Single-Stage Control
Indoor gas heating units are provided
with an automatic single-stage gas valve
as standard. This valve is an on/off type
control, typically activated by a low
voltage single-stage thermostat.
Two-Stage Control
Indoor units with optional two-stage
control are provided with a two-stage
gas valve capable of firing at 100 percent
and 50 percent of rated input. Ignition is
at low fire (50 percent of the unit’s rated
input) and the unit is typically controlled
by a low voltage two-stage thermostat.
Hydraulic Modulating Control
Units with optional hydraulic modulating
control are provided with a modulating
gas valve capable of firing from 50
percent to 100 percent of rated input.
Ignition is at low fire (50 percent of
input). The hydraulic modulating valve is
controlled by a sensing bulb located in
the discharge airstream which
modulates the gas input from 50 percent
to 100 percent of rated input. The unit is
also provided with an automatic electric
valve in series with the hydraulic
modulating valve, which typically cycles
the unit in response to a low voltage
single-stage thermostat. Units that do
not utilize a thermostat will operate as a
standing pilot unit. The ignitor will light
the pilot. The pilot will remain on until
power is disconnected from the units.
Voltage ½ hp ¾hp 1hp 1½hp 2hp 3hp 5hp 7½hp 10hp
115/60/1 1 1 1 2 2 N/A N/A N/A N/A
230/60/1 1 1 1 2 2 N/A N/A N/A N/A
208/60/3 2 2 2 2 2 2 2 4 4
230/60/3 2 2 2 2 2 2 2 4 4
460/60/3 3 3 3 3 3 3 3 5 5
Notes:
1. Contactors not required or provided on units with single phase motors up to one hp.
2. Provided with contactor with line voltage holding coil on all 1
1
/2 hp and above single-phase motors and on all 208
230V three-phase motors up to five hp.
3. Provided with contactor with low (24V) holding coil on all 460V three-phase motors up to five hp.
4. Provided with size 2 starter on all units with 7
1
/2 and 10 hp 208/230V three-phase motors.
5. Provided with size 1 starter on all units with 7
1
/2 and 10 hp 460V three-phase motors.
Hydraulic Modulating Control with
Bypass
An additional electric valve is provided in
parallel with the hydraulic modulating
valve. The electric valve bypasses the gas
flow around the hydraulic modulating
valve, allowing full fire and overriding
the modulating valve’s sensing bulb
control. The electric valve is activated by
a low voltage single-stage thermostat.
(Thermostat not included)
(N/A on 30-75 MBh units)
Electronic Modulating Control
This optional control is available for use
with natural gas units only. Units with
electronic modulating control are
provided with an electronic modulating
valve capable of firing from 100 percent
to 50 percent of rated input. Ignition is at
full fire (100 percent of unit’s rated input).
The electronic modulating valve is
controlled by a room thermostat or a
duct thermostat with remote setpoint
adjustment which modulates the gas
input from 100 percent to 50 percent of
rated input. An optional override room
thermostat is available for use with the
duct thermostat. The override room
thermostat allows full fire and overrides
the duct thermostat when the room
temperature falls below the override
room thermostat’s setpoint. (N/A on 30-
75 MBh units)
Electronic Modulating — 4-20 mA/0-10
VDC Input
Provides modulated heat output. Ignition
is at full fire (100 percent input), and
modulates the gas input from 100
percent to 40 percent rated input.
The modulating gas valve shall operate
in response to a 4-20 mA or a 0-10 VDC
input from an external DDC control.