351579-YTS-B-0608
Johnson Controls Unitary Products
23
Pressure Switch
- The pressure switch closes on a
drop below a specific pressure. On a call for heat, the
ICB checks the pressure switch for open state before it
energizes the draft motor. If it is closed, then the ICB will
lock out the furnace and flash a code (Table 7). If open,
then the ICB will energize the draft motor and verify that
the switch closes before initiating the purging and igni-
tion sequence. If at any time during furnace operation
the pressure switch opens, then the ICB will de-energize
the gas valve and monitor the pressure switch. If the
pressure switch closes and the call for heat still exists,
then the ICB will retry the purging and ignition sequence.
Rollout Switch
- This temperature sensitive switch is
mounted on the burner assembly top shield. In the event
of the flame spilling out into the burner assembly area
and the rollout switch opens, the ICB will close the gas
valve and flash a code (Table 7). The ICB cannot be
reset automatically as the roll out switch is of the manual
reset type.
Flame Sense Circuit
- The flame sensor is mounted on
the left hand side of the burner assembly and is posi-
tioned such that the flame from the first burner from the
left side surrounds the tip of the sensor. On a call for
heat the ICB checks for the flame sense circuit to be
open. If open, then the ICB initiates the purging and igni-
tion sequence. Once the flame is present, the ICB moni-
tors the flame sense circuit for flame stability. If the flame
is unstable or lost completely, then the ICB will immedi-
ately close the gas valve. The ICB will retry the purging
and ignition sequence. If the flame is unstable or lost
more than 5 times during the same call for heat, then the
ICB will lock out that module and flash the appropriate
code.
Flash Codes
- The ICB will initiate a flash code associ-
ated with errors within the ignition and gas control sys-
tem. See Table 7 for the list of codes for the ICB.
12.
Gas Heat Two-Stage Module
: The arrangement and oper-
ation for two stage modules is similar to the single stage
modules except that the gas valve and the ignition control
are replaced with a two stage gas valve and a two stage
ignition control. For faster heat exchanger warm up, the
ICB will automatically run each module on high fire for 30
seconds given a low fire only call for heat. Because the use
of two stage modules is restricted to modulating gas heat,
there is a Modulating Furnace Control (MFC) with a dedi-
cated discharge air temperature sensor added between the
UCB and the two stage ignition control for each installed
module.
13.
Gas Heat Modulating Module
: The modulating module is
significantly different from the single and two stage mod-
ules. It consists of one module broken into two systems.
Each system has its own complement of gas valve, ignition
control, draft motor and safety devices. The burner orifice
manifold is internally sealed such that the left three burners
are supplied with fuel from one two stage gas valve while
the right two burners are supplied with fuel by a second two
stage gas valve. This modulating module is always present
on a modulating furnace with additional two stage modules
installed as required by the application.
Modulating Furnace Control
- The Modulating Furnace
Control (MFC) is designed to accept a W1 and W2 call
for heat and then control up to 8 stages of gas heat for
up to 17 steps of gas heat input rates. It is configurable
for air tempering in free cooling mode without any input
from the UCB when in constant volume air flow that is at
or above the minimum air flow for heating. It uses config-
urable values and sensor input to determine the amount
of heat that is required to meet the heating load up to the
maximum heat output that is installed. Below is the
sequence of steps for the three different systems avail-
able. The LF and HF designations represent the low fire
and high fire gas valve signal for each system, See
Table 8. Modulating Gas Furnace control board failure
codes are found in Table 9.
Table 7: Ignition Control Board Flash Codes
Flash Code
Description
Heart Beat
Normal Operation
1 Flash
Not Applicable
2 Flashes
Pressure / Centrifugal Switch Open with Inducer On
3 Flashes
Pressure / Centrifugal Switch Closed with Inducer
Off
4 Flashes
Lock Out From Too Many Failed Ignition Tries
5 Flashes
Lock Out From Too Many Flame Losses
6 Flashes
High Temperature Switch Open
7 Flashes
Rollout Switch Open
8 Flashes
Flame Present With Gas Off
9 Flashes
Exceeded Max Limit Trips In One Call For Heat (5)
10 Flashes
Gas Valve Stuck Off or On
