ASHRAE Journal - September 2020 - 18

TECHNICAL FEATURE

TEMPERATURE

ANNUAL HOURS AT
TEMPERATURE

OPERATING LOAD
(BTU/H)

PLANT
FIRING RATE

EFFICIENCY

-10°F to -5°F

0.3

5,500,000

96%

89.6%

-5°F to -0°F

0.8

5,133,333

90%

89.6%

0°F to 5°F

7.1

4,766,667

84%

89.6%

5°F to 10°F

23.6

4,400,000

77%

90.1%

10°F to 15°F

51

4,033,333

71%

90.7%

15°F to 20°F

90.2

3,666,667

64%

91.2%

20°F to 25°F

201.6

3,300,000

58%

91.7%

25°F to 30°F

355.2

2,933,333

51%

92.2%

30°F to 35°F

600.9

2,566,667

45%

92.7%

35°F to 40°F

815.4

2,200,000

39%

93.2%

40°F to 45°F

764

1,833,333

32%

93.6%

45°F to 50°F

921.3

1,466,667

26%

94.6%

Boiler Controller

50°F to 55°F

887.5

1,100,000

19%

95.2%

55°F to 60°F

846.6

733,333

13%

95.5%

60°F to 65°F

765.2

366,667

6%

96.0%

Building management systems (BMS) are common
options to control many building elements, specifically
HVAC equipment. Integration of boiler plant components into the building automation system (BAS)/BMS
allows communication over multiple BAS protocols,
including BACnet and Modbus. Boilers and equipment
with multiple integrated BAS protocols and transmission options gives the controls contractor the ability to
choose the one that provides the best equipment control
and monitoring without the additional complications
associated with gateways.

reduce equipment life and increase downtime.

Different Return Temperatures and Dual Returns
Many facilities combine different heating systems
to provide the optimum solution. Often, the systems
require dissimilar supply temperatures, as well as different return temperatures. Depending on the system, it
is important to factor the low-temperature return loops.
Some loop temperatures can be reduced further by feeding the return of a high-temperature loop into the supply of a low-temperature loop. In these cases, the lower
return water temperatures significantly improve boiler
plant efficiency.
These benefits can be optimized by designing plants
with dual return boilers. Normally, the lower inlet return
is used for the lower temperature return to significantly
improve overall plant efficiency by as much as 7%.

Building Height and Boiler Pressure Rating
When designing boilers for high-rise buildings, it is
important to consider the whole system's operating
pressure. Using equipment or components below the
required pressure may complicate system design and
add significant costs by requiring additional pumps,
valves, heat exchangers or pressure reducing stations.
A better approach is to design a plant using boilers and
all the required components with pressure ratings of
18

FIGURE 4 Approximately 10% of the time the plant shall be running below 10%
load. *Data is based on New York City climate zone using ASHRAE bin data.
1,000
900
800
700
600
500
400
300
200
100
0
6 13 19 26 32 39 45 51 58 64 71 77 84 90 96
Plant Firing Rate (%)

Annual Hours at Temperature

TABLE 1 ASHRAE weather data vs. Btu load: New York, N.Y.

ASHRAE JOURNAL

ashrae.org

SEPTEM BER 2020

150 psi to 160 psi-ideal for high-rise buildings.

Outdoor Reset
Boiler controls can also be used for outdoor reset and
to establish setback schedules for ongoing operational
benefits. Reset helps more efficiently meet heating
loads while maintaining a comfortable environment. An
outdoor air temperature sensor determines the boiler
supply temperature. When the outdoor air temperature
drops, the boiler supplies higher temperature water to
meet high load requirements. As the warmer weather
arrives, less energy is needed for space heating and
lower temperature water is supplied to the heating coils.
Return water temperatures are also lowered. The result
is higher boiler efficiency.

Parallel Modulation
It requires less energy for a group of modulating boilers, each firing in parallel at "part load," to heat a building than for a single boiler operating at "full fire" to carry
the entire workload. In low-mass and mid-mass boilers,


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ASHRAE Journal - September 2020

Table of Contents for the Digital Edition of ASHRAE Journal - September 2020

Contents
ASHRAE Journal - September 2020 - Intro
ASHRAE Journal - September 2020 - Cover1
ASHRAE Journal - September 2020 - Cover2
ASHRAE Journal - September 2020 - 1
ASHRAE Journal - September 2020 - Contents
ASHRAE Journal - September 2020 - 3
ASHRAE Journal - September 2020 - 4
ASHRAE Journal - September 2020 - 5
ASHRAE Journal - September 2020 - 6
ASHRAE Journal - September 2020 - 7
ASHRAE Journal - September 2020 - 8
ASHRAE Journal - September 2020 - 9
ASHRAE Journal - September 2020 - 10
ASHRAE Journal - September 2020 - 11
ASHRAE Journal - September 2020 - 12
ASHRAE Journal - September 2020 - 13
ASHRAE Journal - September 2020 - 14
ASHRAE Journal - September 2020 - 15
ASHRAE Journal - September 2020 - 16
ASHRAE Journal - September 2020 - 17
ASHRAE Journal - September 2020 - 18
ASHRAE Journal - September 2020 - 19
ASHRAE Journal - September 2020 - 20
ASHRAE Journal - September 2020 - 21
ASHRAE Journal - September 2020 - 22
ASHRAE Journal - September 2020 - 23
ASHRAE Journal - September 2020 - 24
ASHRAE Journal - September 2020 - 25
ASHRAE Journal - September 2020 - 26
ASHRAE Journal - September 2020 - 27
ASHRAE Journal - September 2020 - 28
ASHRAE Journal - September 2020 - 29
ASHRAE Journal - September 2020 - 30
ASHRAE Journal - September 2020 - 31
ASHRAE Journal - September 2020 - 32
ASHRAE Journal - September 2020 - 33
ASHRAE Journal - September 2020 - 34
ASHRAE Journal - September 2020 - 35
ASHRAE Journal - September 2020 - 36
ASHRAE Journal - September 2020 - 37
ASHRAE Journal - September 2020 - 38
ASHRAE Journal - September 2020 - 39
ASHRAE Journal - September 2020 - 40
ASHRAE Journal - September 2020 - 41
ASHRAE Journal - September 2020 - 42
ASHRAE Journal - September 2020 - 43
ASHRAE Journal - September 2020 - 44
ASHRAE Journal - September 2020 - 45
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ASHRAE Journal - September 2020 - 48
ASHRAE Journal - September 2020 - 49
ASHRAE Journal - September 2020 - 50
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ASHRAE Journal - September 2020 - 65
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ASHRAE Journal - September 2020 - 69
ASHRAE Journal - September 2020 - 70
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ASHRAE Journal - September 2020 - 72
ASHRAE Journal - September 2020 - 73
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ASHRAE Journal - September 2020 - 76
ASHRAE Journal - September 2020 - 77
ASHRAE Journal - September 2020 - 78
ASHRAE Journal - September 2020 - 79
ASHRAE Journal - September 2020 - 80
ASHRAE Journal - September 2020 - Cover3
ASHRAE Journal - September 2020 - Cover4
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