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SETTING TRIM LIMITS
O2 trim limits (expressed in degrees) are derived from trim limit ratio, trim limit at low fire (p3) and
the current firing rate of the burner. Care should be exercised when selecting these values. The trim
limit applies to both clockwise and counter-clockwise directions and is a deviation from the commis-
sioned position. The expression to determine trim limits is as follows:
Allowable Trim Limit =
Where:
R
TD
= user specified trim limit ratio (R
TD
>1)
M
P
= present firing rate of burner (0 to 100%)
T
0
= user specified trim limit at low fire (0% firing rate)
A large trim limit ratio will yield a large value of trim limit at the high fire position. Care must be
exercised when selecting these values.
A trim limit ratio of 1 will yield a constant trim limit from p03 to pHigh.
It is of absolute importance to realize that trim limit applies to both clockwise and counter-clockwise
direction from the commissioned or untrimmed servo position. The trim limit values must be
selected as to not reach the mechanical stop at either end of the trimmed servo travel. See section
relating to operating with O2 trim.
There is typically a 10:1 ratio between air and fuel in any combustion system and therefore if select-
ing fuel as the trimming servo, trim limits should be selected so as to allow only sufficient movement
to achieve the desired target.
Menu item
Value
Description
O2 LO ALM@LO (X)
0.5%
LO refers to O2 level alarm limits below the O2 curve. HI
refers to O2 level alarm limits above the O2 curve The
values entered are the deviation from the current target
value. The default value is 0.5% O2 deviation and the
range of values is 0.1% to 5.0% in 0.1% increment. Enter
values for each curve for low fire and high fire. The
PPC4000 interpolates the alarm value between these
points. See section titled GENERAL RULES FOR ALARM
NOTIFICATION.
O2 LO ALM@HI (X)
O2 HI ALM@LO (X)
O2 HI ALM@HI (X)
FLUE TEMP ALM
UNUSED
Action that takes place if programmed flue temp levels are
exceeded. UNUSED is no action taken. WARNING is
alarm energized, burner remains on line.
WARNING
FLUE TEMP LO (X)
40F
Range of value is 40F (4C) to 800F (426C). The values
represent the low limit and high limit to define the
acceptance zone. The PPC4000 interpolates the alarm
value between these points.
FLUE TEMP HI (X)
40F
FLUE LO TIME
0 min
Enables a Low Fire Hold time when the flue temperature,
as reported by the oxygen probe, is below the user setting
specified in FLUE TEMP LO. Set to 0 to disable.
1
100
1
0
P
TD
M
R
T
