CleaverBrooks CFC-500 Operation, Service And Parts Manual page 171

Thus there are 5 steps in the modulation limiting:
step 0: unlimited (max is 100%)
step 1: max is 80% of range
step 2: max is 60% of range
step 3: max is 40% of range
step 4: max is 20% of range
step 5: limited to minimum modulation rate
If the monitored temperature is not within 12°F of the limit, then
no rate limiting occurs. The stepped rate limit behaves as
illustrated below:
Assuming that rate limiting has not been in effect, when the
monitored temperature crosses a threshold that is 10°F away
from the limit, then the maximum allowed firing rate is reduced
by one stepsize (to 80%) and thereafter it is reduced by one
stepsize every two °F until it is reduced to the minimum
modulation rate when the 2°F threshold is crossed. Assuming
that rate limiting has been in-effect then the thresholds for
returning to a less restrictive step are shifted by 2°F to provide
hysteresis. I.e. to go from step 4 to step 5 the threshold occurs
at 2°F, but to go the other way, from step 5 to step 4, the
threshold is 4°F.
STEPPED MODULATION RATE LIMITING
MAXIMUM
MODULATION
0
RATE (NO LIMIT)
1
2
3
4
5
MINIMUM
MODULATION
RATE
...>12 12
AT THIS POINT A RESPONSE OCCURS DUE TO REACHING A
SAFETY LIMIT.
Fig. 19. Stepped modulation rate limiting.
47
CB FALCON
10 8 6 4 2 0
DEGREES F FROM THRESHOLD
750-265