CONTROL METHODS
HeatNet Control V3
Page 20
Boiler System Response 1
(2) CK 1500s, (2)
CK 2500’s
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 50%, 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) CK 1500 (one of the CK 1500s was taken
offline) were used with (2) CK 2500’s and the Mod-Max is
set to 50%, the CK 1500 would fire to 750 MBTUs and wait
for the CK2500 (
Boiler System Response 2
graph). Now, the
minimum input rate would be 500M (CK 2500) + the 300M
(CK 1500) (already running, but dropped to low fire, but it
needs to go to 18.75%, when the CK 2500 starts), the total
being 800M. The turndown limits the boiler to running at a
minimum of 20%. With a 50% MOD-MAX clamp, there
would be 50 MBTUS more than needed that would be
added to the system when the CK 2500 fired.
The PID algorithm would then compensate for the
discontinuity (bump) in BTUs and the CK2500 would short
cycle. To compensate for this, the Mod Max percent would
need to be increased by 3%, but should be increased by at
least 5% to 55% to allow tolerance. 10% is a better
tolerance choice if room is available. This allows the load to
fluctuate without causing short cycles.
1500 * .55 = 825 MBTUS.
This new Mod-Max value will allow the sum of the low fire
BTUs of both boilers to fire at; 300 + 500 = 800 MBTUs
with room of 25 MBTUs, and prevent the short cycle
condition.
Boiler System Response 2
(1) CK 1500, (3)
CK 2500, 50% Mod-Max
While a CK 1500 running with a CK 2500 is an acceptable
solution, it may not be an optimal choice unless (2) CK
1500s are used in the Priority 1 set and one is allowed to be
taken offline (for Redundancy).
A system employing this redundancy where (1) is allowed
to be taken offline is listed in the
MIXED BOILER SYSTEM
chart. This system uses (2) CK 1500s and (2) CK 2500s.
Two of the CK 1500s are treated as one when adding the
min inputs of the Priority 1 set.
In summary, the system should be tuned using the boiler
selection charts and the MOD-MAX value. Since selecting
the Priority 1 boiler is integral to the fault tolerance of the
system, it is important to note any discontinuities in BTUs if
a Priority 1 boiler fails when multiple Priority 1 boilers are
used.
Mixed System Type 2:
Condensing/Non-Condensing
In the following examples, condensing boilers will be used
with non-condensing mass boilers. The reason for creating a
mixed system is primarily to control the system cost.
Note: In a mixed condensing/non-condensing system,
boilers with differing sizes, as outlined in the Mixed System
Type 1: High System Turndown section may also be used.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0
1,500,000
2,750,000
4,000,000
5,000,000
6,000,000
I
n
p
u
t
,
%
System Load, Btu/Hr
Blr1 (750 MBTU)
Blr1+Blr2+Blr3 (2750 MBTU)
Blr1+Blr2+Blr3+Blr4 (4000 MBTU)
Blr1+Blr2 (1500 MBTU)
8,000,000
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0
2,000,000
3,250,000
5,000,000
7,000,000
9,000,000
I
n
p
u
t
,
%
System Load, Btu/Hr
Blr 1+2 (2000 MBTU)
Blr 1+2+3 (9000 MBTU)
Blr 1+2+3 (3250 MBTU)
Blr 1 (750 MBTU)
750,000
