Unit Start Up
92 RT-SVX36K-EN
limit cutout (4S26) to the Honeywell ignition control board
(4U18).The ignition control board (4U18) starts a pre-
purge timing cycle. At the end of the pre-purge cycle, the
ignition transformer (4T7) and the pilot solenoid valve
(4L9) are energized.This starts a 10 second trial for pilot
ignition.When the pilot flame is established and sensed by
the flame sensing rod (4U19), stage 1 of the main gas valve
(4L7) and the 60 seconds sequencing time delay relay
(4DL6) is energized.
The system will operate in the low heat mode until there
is an additional call for heat is established by closing the
K3 contacts on the Heat Module.
On DischargeTemperature Control systems, the
sequencing time delay relay (4DL6) will energize the
combustion blower motor relay (4K33) which switches the
combustion blower motor to high speed and energizes the
2nd stage solenoid on the gas valve (4L7) after
approximately 60 seconds.
If the flame rod (4U19) does not detect a pilot flame within
the 10 second trial for ignition period, the control will
lockout. If a flame failure occurs during operation, the gas
valve (4L7), the sequencing time delay relay (4DL6), and
the combustion blower relay (4K33) is de-energized.The
system will purge and attempt to relight the pilot. If a flame
is not detected after this attempt, the Honeywell ignition
control (4U18) will lock out.The combustion blower motor
will continue to operate as long as a heating demand
exists and the system switch (4S24) is “On”.
Once the heating demand has been satisfied, the
combustion blower and the Honeywell ignition control
board (4U18) is de-energized.
Propane Gas
Units that operate on propane gas after 1990 have two (2)
additional controls that affect the combustion blower
motor operation and the sequence of the gas valve
operation.
With the post purge time delay relay (4DL4), the additional
service switch (4S24), and the additional 115 volt control
relay (4K31) installed, the sequence of operation is as
follows:
Power is applied to the Honeywell ignition control board
(4U18) through the high limit switch (4S26). The
Honeywell ignition control board (4U18) will sequence
through its pre-purge timing and pilot ignition sequence to
The post purge time delay relay (4DL4) delays the starting
of the combustion blower motor by approximately 60
seconds. Once the timing has elapsed, the combustion
blower motor will start, closing the combustion air switch
(4S25).
Energize the control relay (4K31) and the sequence time
delay relay (4DL6). 24 volts is applied from the new service
switch (4S24) through the normally open control relay
(4K31) contacts to energize the 1st stage solenoid on the
gas valve (4L7).
On an additional call for heat, the K3 contacts on the Heat
Module will close to energize the combustion blower relay
(4K33) which switches the combustion blower motor to
high speed and closes its normally open contacts allowing
24 volts to energize the 2nd stage on the gas valve (4L7).
Once the heating demand has been satisfied, the
Honeywell ignition control board (4U18) and the post
purge time delay relay (4DL4) is de-energized.The
combustion blower motor will continue to operate for
approximately 15 seconds to purge the heat exchanger on
the “Off” cycle.
Modulating Gas Sequence of Operation—
Full and Limited Modulating Gas Furnace
The control system for the rooftop units are wired to
ensure that the heating and cooling do not occur
simultaneously. Refer to the modulating heat wiring
diagram that shipped with the unit while reviewing the
following sequence of operation. As you review the
sequence of operations, keep the following in mind:
• The furnace will not light unless the manual gas valves
are open and the control circuit switch 4S24 is closed.
• The control systems are wired to ensure that heating
and cooling cannot occur simultaneously.
• The unit supply fans must run continuously so air flow
switch 4S38 will stay closed.
• Modulating Gas heat is available during both occupied
and unoccupied operation.
Whenever there is a call for heat, 1U50-K1 energizes and
combustion blower motor 4B11 begins to operate at High
speed on the 850 and 1000 MBH heaters.The blower will
operate on low speed for the 500 MBH. A relay 4K119 in
parallel with the main gas valve actuator control output,
insures the actuator will be open prior to proof of flame.
This will force the combustion air actuator 4U82 to the
open position, causing the auxiliary switch on 4U82 to
close.This insures complete purging of the combustion
chamber during the 60 second purge cycle.
Ignition control IC board 4U18 will not energize, however,
unless the supply air flow switch 4S38, combustion air
flow switch 4S25, high limit cutout 4S26, the auxiliary
switch on combustion air actuator 4U82 and the proof of
closure switch on gas valve 4L22 are closed.These are all
part of the safety interlock system.
With all these conditions satisfied, the IC board energizes
and initiates an internal 60 second pre-purge time delay.
When the pre-purge period expires, 4U18 energizes both
the ignition transformer 4T7 and solenoid 4L9 on the
intermittent pilot valve. At that point, 4U18 gi ves electrode
4E1 approximately 10 seconds to establish a pilot flame.
(The presence of this flame is proven by flame rod 4U19.)
If 4U18 does not detect a pilot flame at the end of this
period, it will shut down and lock out the ignition /
combustion circuit.
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