Mixed System Type 2: Condensing / Non-Condensing - HydroTherm HeatNet KN -6 User Information

SETUP & OPERATION
Mixed System Type 2:
Condensing / Non-Condensing
This mixed system may also have mixed boilers with
differing sizes as outlined in the Mixed System Type 1:
High System Turndown section. In the following examples
condensing high mass boilers will be used with non-
condensing low mass boilers. The reason for creating a
mixed system is primarily to control the system cost.
For the examples, the RBI FIII series water heaters will be
used. These boilers are non-Condensing, fully modulating,
low mass, and HeatNet compatible. Also, consider the
SMITH series 28 with the HeatNet option in the Priority 2
set.
Figure 13 Mixed Boilers: Example: Condensing/Non-Condensing
Figure 14 FIII Boiler Btu Chart (MBH)
MB/MW 500 750
Max Input 500 750
Min Input
166 250
3:1
Mod Max
400 600
80%
Mod Max
350 525
70%
Mod Max
300 450
60%
Mod Max
250 375
50%
1000 1250 1500 1750
1M 1.25M 1.5M 1.75M
333 416 500 583
800 1 1.2M 1.4M
700 875 1.1M 1.2M
600 750 900 1.1M
500 625 750 875M
Figure 12 Mixed Boiler System
System Effective MOD
MMBTU Turndown MAX
3.45 28:01:00
4.2 35:01:00
2.3 29:01:00
Using the boiler charts and the examples used in: Mixed
System Type 1: High System Turndown, a mixed hybrid
boiler system can be designed. The Priority 1 boilers should
be setup so as to keep the non-condensing boilers from
seeing return water temperatures of less than 140F to ensure
a long heat exchanger life.
Normally, the Priority 1 boilers will fire first. Once all the
Priority 1 boilers are firing, the next boiler to fire (after the
ADD BOILER timer expires) would be the Priority 2 (non-
condensing). If the return water temperature has not come
up to ~140F, the non-condensing boilers could fire in a
condensing mode. The ADD BOILER delay timer would
have to be set to a long enough period to ensure this does
HeatNet Control V3 2.x
Priority 1 Priority 2
KN 5:1 MB/MW 3:1
FIII 750,
62% KN6, KN6
750, 750
FIII 1000,
76% KN6, KN6
1000, 1000
FIII 500,
62% KN4, KN4
500, 500
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