SETUP & OPERATON HeatNet Control V3
Page 17
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 Non-Condensing Boilers
Figure 8 Typical efficiency of condensing boilers
(GAMA BTS2000 method)
In the Mixed Boiler System table line 2 example, (2)
MB/MW 500s are set as Priority 1 and MB/MW 1250s set
as Priority 2. With a MOD MAX of 60%, each 500 can run
to 300M (600M total) before a 1250 is called ON (Add
Delay timer). Once both 500s are running and the 1250 is
called on, all (3) boilers will drop to a total of the 600M
BTUs: The sum of the 500, 500, and 1250 would equal
about 27% modulation: (.27 * 500M) + (.27 * 500M) + (.27
* 1.25MM) or: 135M +135M + 337M = 607M and operate
at higher combustion efficiencies (noncondensing boilers
have minimal effect individually, but can have an effect if
many are used).
If CB/CW Fusion boilers are substituted for the MB/MW
Futera III boilers, the efficiency is greatly increased due to
the condensing mode of these boilers. When using CB/CW
Fusion boilers, during the first 2850 MBTH of load, the
combustion efficiency is maximized by running the CB/CW
Fusion boilers from low to middle input rates. See: Typical
Efficiency of Condensing Boiler graph.
Figure 9 Boiler System Response 1
(2) MB/MW 500s, (3) MB/MW 1250s
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%.
Now, if (1) MB/MW 500 (one of the MB/MW 500s was
brought offline) were used with (3) MB/MW 1250s and the
Mod-Max is set to 60%, the MB/MW 500 would fire to 300
MBTUs and wait for the MB/MW 1250 (Boiler System
Response 2 graph). Now, the minimum input rate would be
312M (MB/MW 1250) + the 125M (MB/MW 500) (already
running, but dropped to low fire when the MB/MW 1250
fired), the total being 437M. With a 60% MOD-MAX
clamp, there would be 137 MBTUS more than needed and
added to the system when the MB/MW 1250 fired.
The PID algorithm would then compensate for the
discontinuity (bump) in BTUs and the MB/MW 1250 could
shut off (short cycle).
This discontinuity is observed in the graph below, (Boiler
System Response 2) where the jump from the MB/MW 500
@60% to the firing of the MB/MW 1250 is apparent.
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