CONTROL METHODS
HeatNet Control V3
Page 17
The
condensing boiler set
(Priority 1) has a
higher Priority to
stop
when one of these
conditions are met. Values are adjustable.
LAST
:
The condensing boilers (Priority 1) are always
stopped LAST.
OA T < 15F
:
The condensing boilers (Priority 1) are
stopped first when the OA temperature is less than Mixed
Boiler Outdoor Air Temperature.
RET > 140F
:
The condensing boilers (Priority 1) are
stopped first when the Return water temperature is greater
than the Mixed Boiler Return temperature. (This may not be
applicable in most configurations since the local return
temperature on the Master is used to provide a delta
temperature across the heat exchanger). A System Return
sensor will be required. However, the return temperature
sensor may have been moved on the Master to provide
system return temperature on existing installations and is
still supported).
Start/stop settings
Any combination of Start Conditions and Stop Conditions
can be used to optimize the mixing of condensing
(Priority 1) and non-condensing boilers (Priority 2) for best
performance/economy.
The default start setting always starts the condensing boilers
(Priority 1 example) first, except for the lead boiler setting.
The
lead boiler
will always start first if enabled, unless
there is a boiler already running (this includes a Member
boiler in LOCAL). The default stop condition setting always
stops the condensing boilers (Priority 1) last.
If prolonging the life of the heat exchanger(s) on non-
condensing boilers is very important, consider starting the
condensing boilers (Fusion-Series) when the return water
temperature is below 140F.
The return water temperature sensor would
need to be moved from the Master’s return
inlet to the system return. The
EXCHGR
DELTA
may need to be adjusted in
SETUP:
AUX FUNCTIONS: HEAT EXCHANGER
to prevent the Master from going to ½ input
when a high
DELTA T
is reached.
This method would lead to the non-condensing boilers
carrying the load when the system temperature stabilizes
above 140F, since non-condensing boilers will start first
with the Return water temperature is > 140F. The
condensing boilers can then be stopped first when the RET
water temperature is above the 140F. Remember, any
combination of the Start and Stop conditions may be applied
for best performance and economy in the system. Also, non-
condensing boilers may be set to go offline when a return
temperature is too low using the SETUP: AUX
FUNCTIONS: HET EXCHANGER: TEMP DISAB menu.
Base load boilers can also be mixed in the same way as
condensing and non-condensing boilers. The base load
boiler(s) can be prioritized in one set (example, Priority 2)
and non-base load boilers (Priority 1). The non-base load
boilers can then be set to fire first and once they are all
firing, the base load boiler would fire.
To minimize the cycling of a large base load boiler, consider
using the stop condition. Change it to the OA T < 15F
(Outside Air Temperature) condition. This setting may be
used to stop the Priority 1 boiler set when the OAT drops
below the OA T setpoint, thus leaving the large base loaded
boiler on and shutting off the condensing boilers first. This
is also true when using the OA T setting to start the
Priority 1 boiler set when the OA T is above the start
setpoint. To use temperatures as start and stop conditions,
the system design temperatures must be known.
Selecting Mixed Boilers
There are a few factors to consider when choosing which
type of boilers to use in a mixed system. These factors need
to be considered when boilers are added or shed. When
BTUs are introduced into the system by adding boilers, the
amount of introduced BTUs should be smooth (linear). If
these factors are not considered, discontinuity in BTUs may
occur when boilers are added and as a result, short cycling
will occur.
1.
Turndown
: This is the ratio of minimum fire rate to
maximum fire rate: Example: a 20% minimum
modulation = 5:1 turndown (100%mod / 20% mod). A
(1) million BTU boiler = 200,000 BTUs minimum in.
2.
MOD MAX CLAMP
: This value determines the
maximum modulation % at which the boilers will fire
to, until all available boilers are firing.
3.
Total System BTUs
.
4.
Desired Effective Turndown
. This is the lowest
firing rate of the system relative to the maximum firing
rate of the system. The larger the value, the lower the
BTUs that can be delivered to a light load.
5.
Piping
.
Mixed System Type 1:
High System Turndown
The following examples are of mixed boiler systems with
high effective system turndown and fault tolerance built in.
When boiler types are the same, the system turndown is
limited to the boiler’s min input and fault tolerance is
always present. When the system has mixed boiler types,
consideration needs to be taken on what types can be mixed
properly to achieve a high system turndown and provide
some fault tolerance.
