SETUP & OPERATION
HeatNet Control REV 1.38-1
Page 15
Figure 8
Typical efficiency of non -condensing boilers
Figure 8
Typical efficiency of condensing boilers
(GAMA BTS2000 method)
In the first row of
Figure 5 – The Mixed Boiler System
table
KN6s are set as Priority 1 and 28A-W-5’s set as Priority 2.
With a MOD MAX of 54%, each KN6 can run to 324M
(648M total) before a 28A-W-5 is called ON (Add Delay
timer set long enough). Once both KN6s are running and the
28A-W-5 is called on and running, all (3) boilers will drop
to a total of the ~625M BTUs: The sum of the KN6, KN6,
and 28A-W-5. About 22% KN modulation: (.22* 600M) +
(.22* 600M) + (.33* 1.156MM) or: ~132M +~132M +
~385M = ~650M and operate at higher combustion
efficiencies. 22% is roughly between the top two lines on
the Typical Efficiency of Condensing Boilers chart for the
KN’s. The 33% is the minimum firing rate for the 28A-W-
5.
‘~’ is defined as approximate
Figure 9
Boiler System Response 1
(2) KN6’s, (2) 28A-W-5’s
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
In
p
u
t,
%
System Load, Btu/Hr
Blr 1+2 (1200 MBTU)
Blr 1+2+3 +4 (3512 MBTU)
Blr 1+2+3 (2356 MBTU)
Figure 9 - Boiler System Response 1
chart illustrates how
each boiler (in the example) is brought on and fires to 54%,
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 1270 MBTH of load, the combustion
efficiency is maximized by running the boilers from low to
middle input rates.
Now if a (2) boiler system using (1) KN6 with (1) 28A-W-5
and MOD-MAX set to 54, the KN6 would fire to 324
MBTUs and wait for the 28A-W-5 (Figure 10 -
Boiler
System Response 2
graph). Once the 28A-W-5 fired, the
modulation would drop down and the input rate would jump
to 505 MBTUs, 385M (28A-W-5 @ 33%) + the 120M
(KN6 @ 20%). The system load was looking for 324
BTUs, but there would be 181 MBTUs more than needed.
The PID algorithm would then compensate for the large
discontinuity (over fire bump) in BTUs and the 28A-W-5
would shut off (short cycle). This discontinuity is observed
in the graph below, (Figure 10 -
Boiler System Response 2
graph) where the jump from the KN6 @54% to the firing of
the 28A-W-5 is apparent.
