AERCO C-More Controls Manual June 2010 - Manual (Page 116)

C-More Controller for Benchmark, Innovation & KC
USER MANUAL
Page 116 of 162 AERCO International, Inc. • 100 Oritani Dr. • Blauvelt, New York 10913 04/22/14
GF-112 Phone: 800-526-0288 OMM-0032_0E
Temperature Control under Low Flow Conditions
When the boilers in the BST system raise the water temperature under Low Flow conditions,
there is a delay (as long as 2-3 minutes observed in the field) until the heated water travels the
length of the outlet piping until it reaches the Header Sensor. During this delay, the PID
Controller assumes that the temperature is still too cold and further raises Fire Rate to meet the
temperature demand. By the time the heated water reaches the Header Temperature Sensor,
the Fire Rate is too high and must be reduced. Reducing the Fire Rate will lower the water
temperature, but by the time the cooler water reaches the Header Sensor, the Fire Rate is now
too low. This is a classic definition of a PID Loop ‘HUNTING” or cycling up and down in order to
maintain temperature.
The One Boiler Mode virtually eliminates the distance the water needs to travel until it reaches
the controlling Temperature Sensor, thereby eliminating the “HUNTING” or cycling up and
down.
The One Boiler Mode has three options as follows:
• Off
The One Boiler Mode is disabled.
• On-Outlet Temp (Select On-Outlet Temp if Isolation Valves are installed)
The BST substitutes the ignited units’ Outlet Temperature instead of the Header
temperature. This mode is used when Isolation valves are installed on the boilers.
• On-Avg Temp (Select On-Avg Temp if isolation valves are not installed)
The BST substitutes the average of the ignited units’ Outlet Temperature and the
Header temperature. This mode is used when no Isolation valves are installed on the
boilers.
With Isolation Valves Installed
In One Boiler Mode with isolation valves installed, the master will use the ignited boiler’s Outlet
Temperature sensor for PID control. The distant Header Temperature Sensor is monitored, but
not used for control in this mode of operation.
Without Isolation Valves Installed
In One Boiler Mode with no isolation valves installed, the master will take the average value of
the Header Temperature and the ignited boiler’s Outlet Temperature for PID control.
1 Blr Threshold (Span = 10% - 35%, Default = 25%)
When One Boiler Mode is selected, The BST will try to determine if a Low-Flow condition exists.
A Low-Flow condition is detected by comparing a “Long Fire Rate Average” of all ignited boilers
with the “1 Blr Threshold” item value. If the “Long Fire Rate Average” is less than the “1 Blr
Threshold” for a period of fifteen minutes, the BST will cycle down one boiler. This process will
be repeated until one boiler remains ignited, at which time the One Boiler Mode is activated to
prevent excessive cycling.
The 1 Blr Threshold can be set from 10% to 35%, the default value is 25%.
Setpoint Setback (Selections = Enable – Disable, Default Disable)
The Setpoint Setback item Enables or Disables the Setpoint Setback feature.
This Setpoint Setback feature, when enabled, allows the user to substitute the BST System
Setpoint during pre-programmed time intervals.