SETUP & OPERATON HeatNet Control V3
Page 16
With the traditional Non-Mixed boiler system, the effective
turndown increases by the turndown ratio for every boiler
added. The min fire rate is equal to the minimum BTUs that
can be delivered to the system.
Number of boilers * Turndown Ratio = Effective System
Turndown: 5 * 4:1 = 20:1.
Figure 5 Mixed Boiler System
When selecting the Priority 1 boiler(s) for a high effective
system turndown, the BTU Min Input is selected first. (See:
Futera III/Fusion Boiler Btu Chart). Next, the MOD-MAX
value of this Priority 1 boiler needs to be greater than: Mod
MAX % =
(Priority 1 ‘s Min Input + Priority 2 ‘s Min Input)
Max Input of the Priority 1 boiler
The reason for this is to keep the continuity of BTUs linear
without a BTU bump (discontinuity) when boilers are added
or shed. This is illustrated in the Boiler System Response 2
graph.
If redundancy is not required, the min inputs of the
Priority 1 boilers may be summed to lower the Mod Max %
value so smaller Priority 1 boilers can be used. The sum of
the min inputs would then need to be divided by the sum of
the Max Input of the Priority 1 boilers. The effect of this
would create a higher turndown. See: EXCEPTION NOTES:
Mod MAX % =
(((Priority 1 Min) * (#Priority 1’s)) + Priority 2 Min)
Max Input of Priority 1 boiler * (#Priority 1’s)
Example: (2) CB/CW500, (2) MB/MW1250
Redundancy: (125 + 312) / 500 = 88%
No Redundancy: (125 * 2) + 312) / (500 * 2) = 56%
EXCEPTION NOTES:
1. Mixing more than two different size/type boilers
becomes more complex than the scope of this manual
and is not recommended.
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
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