750-265 30
Stack limit (Safety limit and Rate limit)
The stack limit is a safety limit. However, the stack rate limit portion which attempts to reduce the firing rate to avoid reaching the
stack limit is not.
STACK RATE LIMIT
If the stack limit is enabled, as the temperature approaches
the stack limit temperature, the Stepped Modulation rate limit
function (see “Stepped modulation rate limit” on page 30) is
active.
Outlet high limit (Safety limit and Rate Limit)
The outlet high limit is a safety limit. However, neither the CH
pump control nor the outlet rate limit portion that attempts to
reduce the firing rate to avoid reaching the outlet high limit are
safety functions.
OUTLET HIGH LIMIT CH PUMP CONTROL
Whenever the outlet high limit has been reached the CH
pump will be turned on. It will remain on until the outlet
temperature is lower than the outlet high limit setpoint minus
5°F.
OUTLET RATE LIMIT
Whenever the outlet sensor is not used as the modulation
sensor, the outlet rate limit function is active. (This will occur
when modulating via the DHW sensor, the Header sensor, or
as a LL slave.) In these cases, as the outlet temperature
approaches the outlet high limit setpoint, the Stepped
Modulation rate limit function (see “Stepped modulation rate
limit” on page 30) is active.
Stepped modulation rate limit
The Delta-T, Stack, and Outlet limit functions all use the same
stepped modulation limiting, which reduces the maximum
allowed modulation rate in five steps as the monitored
temperature approaches the limit.
The limiting performs as follows:
A range is determined by calculating:
range = Maximum modulation rate
- Minimum modulation rate
NOTE: The DHW maximum modulation rate is used when
firing for DHW, and for other sources the CH maxi-
mum modulation rate is used.
A step size is determined by dividing this range by 5:
stepsize= range/5
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
Table 13. Limits and Rate Override: Stack Limit Parameters.
Parameter Comment
Stack limit enable Enable, Disable
This parameter enables or disables the entire stack temperature limit function.
Stack limit setpoint Degrees or None
If the stack temperature reaches or exceeds the safety limit temperature given by this
parameter then the response defined below will occur.
Stack limit response Lockout, Recycle & delay
If the stack temperature exceeds the safety limit, then a response will occur. If the selected
response is a lockout, then the burner control locks out.
However, if the selected response is Recycle & Delay, the burner control recycles and holds
while waiting for a delay (see below) to expire, and after the delay it tries again (assuming
that demand is still present).
Stack limit delay MM:SS
This parameter provides the delay time for the Stack limit.
Table 14. Limits and Rate Override: Outlet High Limit Parameters.
Parameter Comment
Outlet high limit setpoint degrees or None
If the outlet temperature reaches the value given by this parameter then a response will occur
Outlet high limit response Lockout, Recycle & hold
This parameter selects the response. If lockout is selected, the burner control locks out. If
Recycle & hold is selected, the burner control recycles and waits for the outlet temperature to
fall. It will remain in this holding condition until the outlet temperature is lower than the outlet
high limit setpoint minus 5°F.
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