INTRODUCTION
HeatNet Control V3 3.x
Page 6
Figure 1
Heat Band
The control algorithm is based upon a Heat Band (Figure 1),
at the center of which is the setpoint. While below the Heat
Band, boilers are staged on and modulated up until the Heat
Band is entered. Once in the Heat Band, modulation is used
to maintain setpoint. Boilers are shut down only when the
top of the Heat Band is breached. Timers are also used to
prevent short cycling.
While staging the boilers on, a modulation clamp is used to
hold the boilers at a lower fire rate until the last boiler is
fired. Once the last boiler fires, the modulation clamp is
removed, and all boilers are allowed to fire above this
clamped percentage up to 100%. This “boiler efficiency”
clamp is defaulted to 70% and thus limits all the boiler’s
individual outputs to 70% until the last boiler fires. All
running boilers modulate up and down together, always at
the same modulation rate. As a general rule, this percentage
should be no lower than twice the minimum turndown to
minimize short cycling.
When additional boilers are needed to achieve setpoint in
the system, the MASTER boiler employs an ADAPTIVE
MODULATION algorithm to prevent over firing of the
system. The MASTER communicates over the H-Net to
view the exact status of each MEMBER boiler. When a new
boiler is added, the MASTER boiler adjusts the system
modulation rate lower to compensate for the BTUs that will
be introduced by the newly added boiler.
This adjustment occurs when the newly added MEMBER
boiler enters its ON CALL state (default setting). This can
be changed to PILOT when the new boiler is called using
the menu:
ADAPTIVE MOD: DROP DOWN.
Once the Main Valve
(on the newly added boiler) is opened, and the
DELAY
RELEASE
timer equals zero, the PID algorithm is allowed
to control the system modulation.
Setting
the DELAY
RELEASE timer will allow some “soak” time of the newly
added boiler before releasing modulation control to the PID.
The ADAPTIVE MOD menus are disabled
on a MEMBER boiler but are still visible.
MEMBER
If a “SYSTEM HEADER” or “DHW” (DHW MASTER
Only) sensor is
not
connected to J10, a boiler always
defaults to the role of MEMBER and its “HEATNET
ADDRESS” is set to “2”.
The MEMBER boiler can operate as part of a multi-boiler
system or as a stand-alone unit.
In a multi-boiler system, the MEMBER typically receives
its command signals from a designated MASTER-boiler. It
is also capable of receiving inputs from an external control
system. The boiler responds to these signals, to start/stop the
burner, and/or to modulate the firing rate. The outlet water
temperature is also monitored. If the outlet temperature
approaches the operating limit temperature setpoint
(adjustable), the boiler’s firing rate is limited, and its
modulation value is reduced to minimize short cycling. If
the operating limit is exceeded, or if an interlock trips, the
boiler is shut down. When connected with a network cable,
in a MASTER/MEMBER role, the MEMBERs' status is
interrogated by the MASTER boiler.