OPTIONAL FEATURES
HeatNet Control REV 1.38-1
Page 20
Heating Control Methods
An overview of the (5) methods for controlling the Smith
series boiler are presented here. They are outlined in more
detail at the end of this section. See
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Heating Method 1
The first method is to use the Smith Series boiler in its stand-
alone modulating method. This method uses a PID algorithm
to maintain a setpoint and is enabled using the HEAT
DEMAND input. Closing a relay contact or switch across the
HEAT DEMAND input will cause the Master boiler to
control all member boilers using H-Net.
A member boiler may also be controlled by the HEAT
DEMAND input (LOCAL mode). The member boiler will
then ignore commands from the Master and maintain its
LOCAL SETPOINT at the supply sensor.
Heating Method 2
The second method is to view the Smith boiler as two
separate boilers or as a HIGH/LOW boiler using T1 & T2.
Heating Method 3
The third method is to allow a remote 4-20 ma or 0-10 VDC
signal to control the firing rate (modulation) of the boiler
using the 4-20ma input, along with the 4-20ma REMOTE
ENABLE input.
Heating Method 4
The fourth method turns the boiler ON and OFF @ 100%
modulation using the AA terminal.
Heating Method 5
The fifth method uses an RS485 digital communications cable
with the MODBUS protocol. The boiler is controlled by
writing and reading registers using MODBUS commands. A
bridge module may also be used to convert BACnet or
LonWorks protocols to MODBUS.
Short cycling may occur when a firing rate is
sent to a member boiler that would cause the
supply temperature to rise high enough to trip
the operating limit (low flow rate). After the
supply temperature falls, the boiler would
restart and the process may continue. A
member boiler would use its supply (outlet)
sensor to protect itself from short cycling by
limiting the firing rate coming from the Master.
This occurs in the event that the member’s
supply temperature increases above the
(OPERATE LIMIT- OPERATE LIMIT
BAND).
Operating Limit
When the master boiler or an external control input is used to
control a member boiler (i.e. AA, T1-T2, 4-20ma, H-Net), a
software operating limit on the member boiler will be used to
limit the maximum output of the member boiler. This
operating limit can be adjusted in the
SETUP:SETPOINTS:OPERATING LIMIT
.
There is also an associated operating limit band that must be
set in conjunction with the operating limit to help prevent this
LIMIT from being reached. Its purpose, is to limit the output
of the boiler as it approaches the operating limit. If the band is
set to 10 degrees, then for every degree that it approaches the
operating limit, the maximum output will be lessened by 10%.
With a band of 20 degrees, for every degree that it approaches
the band, the maximum output will be lessened by 5%. You
can think of this operating limit as a smart aquastat which
prevents the High Limit from tripping. This method
minimizes boiler short cycling when using external inputs.
The minimum setting is 1 degree and effectively turns the
limit band OFF. The default setting is 20F.
Input Priorities
The Smith-Series
control inputs are
prioritized
so that
multiple levels of external control can be employed at the
same time. This means that if we are firing the boiler with a
low Priority input and a higher Priority input is called for, the
boiler will now fire at the higher Priority input. When the high
Priority input is removed, the boiler will revert back to the
lower Priority input that is still being called.
Priority 1
The AA terminal has absolute control, and if used, will
always fire the boiler at 100% output, regardless of any other
input. The 4-20mA input may be raised to this Priority using
ADVANCED SETUP: 4-20mA INPUT:PRIORITY
.
Priority 2
The HEAT DEMAND input is the next, and provides the
means to operate the boiler in LOCAL MODE when an
external control is not present, has failed, or needs to be
