HydroTherm HeatNet KN -6 User Information page 16

SETUP & OPERATION
EXCEPTION NOTES:
1. Mixing more than two different size/type boilers
becomes more complex than the scope of this manual.
2. If using more than one Priority 1 boiler and the
calculated value is <
Priority 1Min * 2
Priority 1 Max Input
Use this result PLUS note 3 value as the
ModMax%.
3. Always add a few % (3-5%) to the calculated MOD
MAX % value to allow a guard band (tolerance).
4. If boilers are of different sizes, try to use larger
Priority 2 boilers.
If the calculated Mod MAX % value is greater
than 99%, the combination cannot be used
since short cycling will occur.
Once the Priority 1 and Priority 2 boilers are selected, they
can be multiplied in each Priority set to achieve the desired
system design BTUs. If the # of boilers becomes a large
number, a Priority 1 boiler with a higher Min Input may
need to be selected.
While considering the MOD-MAX value, the lower the
MOD-MAX the greater the combustion efficiency since it
effectively limits the input rate. The Typical Efficiency of
Non-Condensing Boilers chart can help illustrate how the
MOD-MAX value can affect the efficiency by limiting the
input until all boilers have fired. Non-condensing boiler
efficiency is relatively flat compared with condensing as
illustrated in the Typical Efficiency of Condensing Boiler
graph.
Figure 7 Typical efficiency of condensing boilers
(GAMA BTS2000 method)
In the Mixed Boiler System table (line 1), KN6s are set as
Priority 1 and KN10's set as Priority 2. With a MOD MAX
of 60%, each KN6 can run to 360M (720M total) before a
KN10 is called ON (Add Delay timer set long enough).
Once both KN6s are running and the KN10 is then called on
and running, all (3) boilers will drop to a total of the 720M
BTUs: The sum of the KN6, KN6, and KN10. About 33%
modulation: (.33* 600M) + (.33* 600M) + (.33* 1MM) or:
198M +198M + 330M = 726M and operate at higher
combustion efficiencies. 33% is roughly between the top
two lines on the Typical Efficiency of Condensing Boilers
chart.
Figure 8 Boiler System Response 1
(2) KN2s, (3) KN6s
When running non condensing boilers at low
input rates, the risk of condensing should be
considered.
The Boiler System Response 1 chart illustrates how each
boiler (in the example) is brought on and fires to 60%, drops
to a lower fire rate and then adds the next boiler (vertical
dashed lines). Once all boilers are firing, the modulation is
released allowing all boilers to fire to 100%. So, for the first
2500 MBTH of load, the combustion efficiency is
maximized by running the boilers from low to middle input
rates.
Now if a (2) boiler system
brought offline) using (1) KN6 with (1) KN20 and MOD-
MAX set to 60%, the KN6 would fire to 360 MBTUs and
wait for the KN20 (Boiler System Response 2 graph). Once
the KN20 fired, the input rate would jump to 520 MBTUs,
400M (KN20 @ 20%) + the 120M (KN6 @ 20%). There
would be 160 MBTUS more than needed.
The PID algorithm would then compensate for the large
discontinuity (over fire bump) in BTUs and the KN20
would shut off (short cycle). This discontinuity is observed
in the graph below, (Boiler System Response 2) where the
jump from the KN6 @60% to the firing of the KN20 is
apparent.
HeatNet Control V3 2.x
one of the
( KN6s & two KN20s
Page 16