Install Instructions

Page 43
NEOTHERM Residential Boilers
HIGH EFFICIENCY RESIDENTIAL BOILERS
®
3. Set the DHW priority time. If the DHW function has the
priority, the priority will continue during the priority time.
If your system has just one boiler –
How to get there: From the “Home” screen, press “I” to
go to “Info/ Install.” Choose “Advanced Setup,” then go
to “DHW Conguration.” Select “DHW Priority Time.”
If your system has more than one boiler, and is
controlled using the Lead/ Lag function –
How to get there: On the unit used as the Lead Lag
Master, from the “Home” screen, press “I” to go to “Info/
Install.” Choose “Advanced Setup,” then go to “DHW
Conguration.” Select “DHW Priority Time.”
Figure 32. Lead / Lag Operation, 2 boilers.
Low demand -
The first boiler in
sequence fires at
less than 65%
First
boiler
Second
boiler
Demand increases -
Once the first boiler
reaches 65%,
the second boiler
switches on, and
both modulate
together between 20%
and 65% (adjustable up to 20% and 85%).
Nearing max. demand -
Once both reach 65%,
then they are both
allowed to go over
65%
8.N About Lead Lag Operation
If an installation includes two or more boilers (up to 8
residential units), they may be set up for “Lead Lag”
operation. One boiler will be set up as the “Master”, and
the others will operate as “Followers.” Figure 38 shows
an installation with four boilers. (A Lead Lag system can
include up to eight boilers.) The boiler controllers are
connected in a “daisy chain” using a Modbus connection.
For the full wiring diagram for lead lag, See Figure
17 on page 28.
A single System sensor is used to monitor the demand
for heat. The input from this sensor is used by the Master
controller to control the modulation rates of all of the
operating boilers.
Lead Lag Modulation Cycle -
Note - We will explain the modulation cycle here, in case
you need to understand how the Lead Lag system actually
operates. If you are installing the unit(s) and want to skip
this section, just remember that, as the heating demand
increases, the Lead Lag system puts more boilers on-line.
As the heating demand is reduced, the Lead Lag system
shuts o󰀨 some of the boilers.
The signal from the System sensor is sent to the unit
operating as the Lead Lag Master. A Run sequence is
initiated when the system temperature falls to the Lead
Lag Central Heat setpoint. (Actually this is the LL CH
setpoint less the On Hysteresis value. The default setting
for On Hysteresis is 5°F, but this is adjustable.)
Notice that, when a boiler is operating as part of a Lead
Lag system, it does not use the normal CH setpoint – it
uses the special LL CH setpoint on the Lead Lag Master.
The unit acting as the Lead Lag Master decides
which boiler is assigned to start rst. This assignment
is rotated across all of the available boilers so that
any one boiler does not run signicantly longer than
the others. The Lead Lag controller tracks the run
times for all of the boilers, and uses this to calculate
the starting order for the boilers. This means that
each time the system starts up, a di󰀨erent boiler
may start rst.
• When the Run sequence is initiated, the boiler with
the least amount of runtime will re.
• If the heating demand increases so that the
ring rate of that rst boiler rises to your boiler
congurations Base Load Value (see Table 18), the
next boiler in the sequence will start up and begin
ring at 20 to 35% fan speed (depending on your
conguration). After this, the entire conguration
of boilers will modulate up or down together, in