CONTROL METHODS
HeatNet Control V3
Page 22
The
Mixed Boiler System
table show some examples of
mixed systems using different sizes along with Fusion
condensing (Priority 1) and Futera III noncondensing
(Priority 2) boilers.
Using the boiler charts and the examples used in:
Mixed
System Type 1: High System Turndown,
a mixed boiler
system can be designed. The Priority 1 boilers should be
setup so as to keep the non-condensing boilers from seeing
return water temperatures of less than 140F to ensure a long
heat exchanger life.
Normally, the Priority 1 boilers (Condensing) will be set to
fire first. Once all the Priority 1 boilers are firing, the next
boiler to fire (after the ADD BOILER timer expires) would
be the Priority 2 (non-condensing). If the return water
temperature has not come up to ~140F, the non-condensing
boilers could fire in a condensing mode. The ADD BOILER
delay timer would have to be set to a long enough period to
ensure this does not happen. Even then, the load may be too
great.
When running with a remote BMS setpoint, care must be
taken that an Outside Air reset setpoint (or other setpoint)
sent by the BMS is not set too low. If the BMS system is
controlling the setpoint close to the condensing temperature,
the return water temperature may never rise sufficiently to
keep boilers out of a condensing mode. HeatNet online is a
good way to monitor this scenario if suspected.
The following note will explain an alternative way (not
depending on the ADD BOILER DELAY) to keep non-
condensing boilers from firing in a condensing mode.
NOTE:
If the firmware version for a HeatNet V2 board is at least
3.47(or a version 3 board), the non-condensing boiler
may hold itself off from being added to the HeatNet
Master’s available to fire list. This would effectively keep
the non-condensing boiler from firing in a condensing
mode, but as a result, may not satisfy the system setpoint.
In order to use this feature, the version 2 board would
need to monitor the system or local return temperature.
This can be done locally by setting SETUP: AUX
FUNCTIONS: HEAT EXCHANGER: TEMP DISAB:
RETURN if the there is no pump/valve limiting flow
continuously through the boiler. If there is a pump/valve
limiting the flow through the boiler, the SETUP: AUX
FUNCTIONS: HEAT EXCHANGER: TEMP DISAB: SYS
RET needs to be set. Then the Master boiler needs to set
SETUP: AUX FUNTIONS: HEAT EXCHANGER: SEND
RETURN: to which of its return temperatures it would
send to all boilers. These include the Local Return
temperature or the System Return temperature.
The Member’s menu “SETUP: AUX FUNCTIONS: HEAT
EXCHANGER: TEMP DISAB:” if set to RETURN or SYS
RET, will force the boiler to become unavailable to
HeatNet when the SETUP: AUX FUNCTIONS: HEAT
EXCHANGER: TEMP< 140F. This value is adjustable to
135F if a forced air fan is used. When the SYS RET or
RETURN temperature is <140F the boiler responds to a
HeatNet Master’s request as” unavailable”. As soon as
the return temperature reaches 140F, the boiler will
respond to the Master’s request that it is available to fire.
If the Master boiler is a version 2 board, the Master will
always transmit its Local Return temperature to all
boilers. If the Master is set to Priority 1 and all other
non-condensing boilers are set to Priority 2, the Master
should always remain on if there is a call for heat. This
requires that the Priority 1 boiler be set up to start first
and stop last. Using this method should always send a
valid return temperature to the Member boilers. This
method can also be used with a version 3 board, but a
system return sensor is preferred.
When this condition is in effect, the STATUS * screen will
indicate “blr offline”. While the boiler is in this “not
available” state, it can still be fired locally, and failsafe is
still available.
SETUP: AUX FUNCTIONS: HEAT
EXCHANGER: SEND RETURN:
OFF
The Master sends its return
temperature to all boilers
RETURN
The Master sends its return
temperature to all boilers
SYS RET
The Master sends the system
return temperature to all
boilers
SETUP: AUX FUNCTIONS: HEAT EXCHANGER:
LOW TEMP:
OFF
No check is made to the return
temperature – boiler remains
online
RETURN
Uses the boilers own return
sensor (No pump /valve
present)
SYS RETURN
Uses the System Return temp
received from the Master Boiler
(its Local or System Return).
SETUP: AUX FUNCTIONS: HEAT EXCHANGER:
TEMP < 140F
This is the adjustable threshold temperature below
which the boiler will take itself offline.
