59
Limit Switch Temperature Monitoring (LIM.M) — Variable
air volume applications in the low heat or tempering mode can
experience low airflow and as a result it is possible for nuisance
trips of the gas heat limit switch, thereby shutting off all gas
stages. In order to achieve consistent heating in a tempering
mode, a thermistor (Temperat ures
→
AIR.T
→
S.G.LS) is placed
next to the limit switch and monitored for overheating. In order
to control a tempering application where the limit switch
temperature has risen above either the upper or lower configu-
ration parameters (SW.L.T, SW.H.T), the staged gas control
will respond to clamp or drop all gas stages.
If the Limit Switch Monitoring configuration parameter
(LIM.M) is set to YES, all the modes will be monitored. If set
to NO, then only LAT Cutoff mode and Capacity Clamp mode
for RISE will be monitored.
If S.G.LS rises above SW.L.T or if (LAT – LAT last time
through the capacity calculation) is greater than (RISE)
degrees F per second, the control will not allow the capacity
routine to add stages and will turn on the Capacity Clamp
mode.
If S.G.LS rises above SW.H.T the control will run the capac-
ity routine immediately and drop all heat stages and will turn
on the Limiting mode.
If S.G.LS falls below SW.L.T the control will turn off both
Capacity Clamp mode and Limiting mode with one exception.
If (LAT – LAT last time through the capacity calculation) is
greater than “RISE” degrees F per second, the control will stay
in the Capacity Clamp mode.
If control is in the Limiting mode and then S.G.LS falls
below SW.L.T, and LAT is not rising quickly, the control will
run the capacity calculation routine immediately and allow a
full stage to come back on if desired this first time through
upon recovery. This will effectively override the “max capacity
stage” clamp.
In addition to the above checks, it is also possible at low cfm
for the supply-air temperature to rise and fall radically between
capacity calculations, thereby impacting the limit switch tem-
perature. In the case where supply-air temperature (LAT) rises
above the control point (HT.C.P) + the cutoff point (LAT.L) the
control will run the capacity calculation routine immediately
and drop a stage of heat. Thereafter, every time the capacity
calculation routine runs, provided the control is still in the LAT
cutoff mode condition, a stage will drop each time through.
Falling back below the cutoff point will turn off the LAT cutoff
mode.
INTEGRATED GAS CONTROL BOARD LOGIC — All gas
heat units are equipped with one or more integrated gas control
(IGC) boards. This board provides control for the ignition sys-
tem for the gas heat sections. On size 030-050 low heat units
there will be one IGC board. On size 030-050 high heat units
and 055-105 low heat units there are two IGC boards. On size
055-105 high heat units there are three IGC boards. When a
call for gas heat is initiated, power is sent to W on the IGC
boards. For standard 2-stage heat, all boards are wired in paral-
lel. For staged gas heat, each board is controlled separately.
When energized, an LED on the IGC board will be turned on.
See Table 51 for LED explanations. Each board will ensure
that the rollout switch and limit switch are closed. The induced-
draft motor is then energized. When the speed of the motor is
proven with the Hall Effect sensor on the motor, the ignition
activation period begins. The burners ignite within 5 seconds.
If the burners do not light, there is a 22-second delay before an-
other 5-second attempt is made. If the burners still do not light,
this sequence is repeated for 15 minutes. After 15 minutes have
elapsed and the burners have not ignited then heating is locked
out. The control will reset when the request for W (heat) is tem-
porarily removed. When ignition occurs, the IGC board will
continue to monitor the condition of the rollout switch, limit
switches, Hall Effect sensor, and the flame sensor. Forty-five
seconds after ignition has occurred, the IGC will request that
the indoor fan be turned on. The IGC fan output (IFO) is
connected to the indoor fan input on the MBB which will
indicate to the controls that the indoor fan should be turned on
(if not already on). If for some reason the overtemperature limit
switch trips prior to the start of the indoor fan blower, on the
next attempt the 45-second delay will be shortened by 5 sec-
onds. Gas will not be interrupted to the burners and heating will
continue. Once modified, the fan delay will not change back to
45 seconds unless power is reset to the control. The IGC boards
only control the first stage of gas heat on each gas valve. The
second stages are controlled directly from the MBB board. The
IGC board has a minimum on-time of 1 minute. In modes such
as Service Test where long minimum on times are not
enforced, the 1-minute timer on the IGC will still be followed
and the gas will remain on for a minimum of 1 minute.
Table 47 — Staged Gas Heat
Table 48 — Staged Gas Heat Control Steps (HT.ST = 0)
NUMBER OF
STAGES
HT.ST
CONFIGURATION
UNIT SIZE
48Z HEAT SIZE
20 030-050 Low
51030-050 High
055-105 Low
93 055-105 High
STAGE
RELAY OUTPUT
CAPACITY
%
Heat 1 Heat 2 Heat 3 Heat 4 Heat 5 Heat 6
MBB-RLY8 MBB-RLY7 SCB-RLY1 SCB-RLY2 SCB-RLY3 SCB-RLY4
IGC1 MGV1 IGC2 MGV2 IGC3 MGV3
0OFF OFF OFF OFF OFF OFF 0
1ON OFF OFF OFF OFF OFF 75
2ON ON OFF OFF OFF OFF 100