ASHRAE Journal - September 2020 - 15

TECHNICAL FEATURE

Temperature Priority

FIGURE 1 Thermal efficiency of a condensing boiler at different firing rates at 5% O2.

Efﬁciency (%)

systems with multiple functionality, such as
indirect hot water (DHW), reheat, etc. For combination plants where there will be multiple
temperature setpoints, three design options are
available:

Temperature priority designs (Figure 2) allow
the temperature to be raised to satisfy the needs
of the higher of the two temperatures. There are
two notable disadvantages-one side will sacriﬁce its temperature to satisfy the other (thereby
reducing comfort) and overall plant efﬁciency
will decrease. Even if additional valves and
controls are used to improve temperature control, overall plant efﬁciency is lowered because
the whole system supply and return temperatures are
increased.

100
99
98
97
96
95
94
93
92
91
90
89
88
87
86
85

Boiler Firing Rate
100%
5%

5% O2

15% O2

9% O2

5% O2

90
100
110 120
130
140
Return Water Temperature with 20 Degree Rise (°F)

150

160

170

FIGURE 2 Combination plant application with single setpoint temperature control.

Boiler Pump

System Pump

System Supply

Two Independent Plants
Another option is to have two independent plants,
each with its own boilers, pumps, controls and piping.
This approach requires a larger plant size and brings
additional learning curve requirements. Furthermore,
if N + 1 redundancy is desired, an additional boiler is
necessary for both plants, increasing major equipment
costs.

Single Plant with Smart Controller
The most efﬁcient approach is one plant with a
smart controller that can meet temperature and load
requirements without the large oversizing associated
with two independent plants and dedicated controls.
It also signiﬁcantly improves the overall plant efﬁciency without sacriﬁcing comfort and temperature
control.
What makes all this possible is the swing boiler concept, which allows certain boilers to be assigned for
space heating while others are for DHW. In normal operation, each boiler is equipped with its own integrated
controller. The integrated controller of the master/
manager boiler manages all the slave/client boilers and
swing valves. This controller is mounted on the front of
the boiler and manages all the boilers' electronic inputs
and outputs, allowing boilers with swing valves to be
easily switched from space heating to DHW or vice versa,
depending on load requirements.

Four-Port
Buffer
Tank

Boiler

Indirect Water Heater
System Return

This design adds a layer of security, as well. For
example, if the DHW boiler cannot meet its load, the
controller will direct the swing valves to divert its
designated boilers from space heating to DHW. As per
ASHRAE, it is assumed that both space heating and
DHW peak loads may not happen concurrently.3 This
offers a major beneﬁt in reducing the number of boilers required to meet plant peak load, shrinking the
footprint.
Another beneﬁt is that the boiler system condenses
more because the high and low temperature supply
are separated. Models with dual returns can leverage this by allowing boilers to condense signiﬁcantly
more when the lower temperature return water is
fed into a special inlet designed to reduce ﬂue gas
temperatures further. When combined with a lowmass boiler and its faster adaptation to plant load
changes, the footprint of the plant becomes even
more compact.
SEPTEM BER 2020

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ASHRAE Journal - September 2020

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