CONTROL METHODS
HeatNet Control REV 1.38-1
Page 22
Heating Method 2
STAGE Control T1-T2
1.
Closing only one of these contacts tells the boiler to
operate at MINIMUM FIRE.
2.
Closing the other contact will fire the boiler at
MAXIMUM output (the same rate as closing the AA
input).
Figure 16
Stage Control Input
The maximum output of the boiler is based on
the MAX VFD setting in the calibration mode
and not the nameplate rating.
The AA, HEAT DEMAND (LOCAL) input, the H-Net, the 4-
20ma input will all override the stage control inputs
Heating Method 3
4-20ma Control
Placing a current source between the + and – 4-20ma inputs
will allow remote control of the boilers firing rate. An
adjustable starting milliamp current signal here will start and
then fire the boiler at the minimum fire rate. See
:
ADVANCED SETUP:4-20mA INPUT
.
See section
OPTIONAL FEATURES Using the 4-20ma input
for extensive detail.
A 20mA signal will fire the boiler at the maximum firing rate,
a 4 mA signal at the minimum rate. This means that a 20%
(4mA) input signal is required to start the boiler, but if the
boiler is classified as having example:4:1 turn down ratio, the
boiler can only be fired as low as 25% of output. Any signal
between 20% and 25 %, will fire the boiler at the minimum
fire rate of 25%. If the MINIMUM setting of the boiler is set
above the example: 4:1 turndown of 25% (such as 33%), a
control signal change between 25% and 33% will not change
the boilers firing rate. Once the control signal rises above the
MINIMUM fire rate, the control signal will then affect control
of the boilers fire rate.
The AA terminal, the HEAT DEMAND, and the H-Net
NETWORK are the only inputs that will override the 4-20ma
input.
Heating Method 4
AA Input
HIGH FIRE input Control: The AA input will fire the boiler
at HIGH fire (maximum output of the boiler). No other inputs
can override this input.
Figure 17
AA – High Fire Input
Heating Method 5
MODBUS communications
The fifth method uses an RS485 digital communications cable
with the MODBUS protocol to control the boiler using the H-
Net network. The Boiler or Boiler network will run as in
Method 1, but instead of the HEAT DEMAND input, a
software form of the HEAT DEMAND input is used (40001:
Boiler/System Enable/Disable). See: MODBUS
COMMUNICATIONS section. The MODBUS protocol
allows writing and reading registers using MODBUS
commands.
Figure 18
MODBUS Connections
The System Setpoint Timer also needs to be loaded
periodically to allow the H-Net system to fallback to Method
1 in the event communications is lost from the Building
Management System (BMS). This feature can be disabled in
the
ADVANCED SETUP:COMMUNICATIONS
menu.
Method 2: LOW FIRE, HIGH FIRE
Close contacts here
T1
T2
