ASHRAE Journal - February 2021 - 19

TECHNICAL FEATURE

control also allows one to recognize
FIGURE 3 High-rise condominium's hot water system serving MAU heat exchanger and terminal loads.
opportunities for its implementa9
17
tion in unconventional applications.
11
8
Cascade control is appropriate when
10
18
limits need to be placed on the values
19
the manipulated variable can obtain
7
3
5
6
or when the manipulated variable is
15 16
1
2
subject to disturbances. The previ14
13
4
ous examples all highlight instances
12
when limits need to be placed on
1 - Boiler 1
11 - Heat Exchanger Control Valve
the manipulated variable. The following example details how cascade
2 - Boiler 2
12 - From Condos
control can be used to more effec3 - Common Pipe
13 - Makeup Air Unit (MAU)
tively address system disturbances
4 - Secondary Hot Water Return Temperature (SHWRT)
14 - Face/Bypass Dampers
to improve the controllability of a
5 - Heat Exchanger
15 - Heating Coil
system.
6 - Outdoor Air
16 - Cooling Coil
This author's ﬁrm was asked to
investigate inadequate heating at
7 - Primary Pumps
17 - To Condo FCUs and FTR
a high-rise condominium com8 - Primary Hot Water Supply Temperature (PHWST)
18 - Glycol Hot Water Supply Temperature (GHWST)
plex. During the previous winter,
9 - Secondary Hot Water Supply Temperature (SHWST)
19 - Supply Air Temperature (TSA)
Minnesota's polar vortex of January
10 - Secondary Pumps
2019 resulted in many condos unable
to maintain space temperatures above
55°F (12.8°C). The hot water system had a constant speed to the system's inability to provide heat to the spaces
primary/variable speed secondary pumping conﬁguraserved. This scenario has been astutely referred to as the
tion (Figure 3). The secondary pumps provided heat to
" death spiral " in literature discussing low ∆T syndrome
downstream ﬁnned tube radiation (FTR) and fan coil
in chilled water systems.9
The primary reason for excessive secondary ﬂow in the
units (FCU) located in the residences, as well as to a hot
water-to-glycol hot water heat exchanger. The glycol hot high-rise condominium complex was due to the MAU's
heating sequence of operation. During heating operawater solution was then used to heat incoming ventilation, the hot water valve serving the heat exchanger was
tion air at the makeup air unit (MAU).
commanded full open. This resulted in substantially
The root causes of insufﬁcient heating were many,
higher glycol hot water supply temperatures (GHWST)
but the system's biggest shortcoming was secondary
than needed. This did not result in overheating of the
ﬂow exceeding primary ﬂow. This occurs when secondsupply air, however, as the MAU's face and bypass dampary ﬂow does not track on load, i.e., the secondary ﬂow
ers were modulated to effectively maintain the supply
remains high even during periods of low heating load.
air temperature (TSA) at setpoint.
When secondary ﬂow exceeds primary ﬂow, water travTo bring the secondary ﬂow rate below the primary
els opposite of the intended direction in the common
ﬂow rate, the heating sequence for the MAU was
pipe, which results in the secondary hot water supply
adjusted as follows: during heating operation, the face
temperature (SHWST) falling below that of the primary
damper fully opens, the bypass damper fully closes, and
hot water supply temperature (PHWST). Degrading
the heat exchanger's control valve modulates to mainSHWSTs reduce downstream heating mechanisms'
tain the TSA at setpoint.
ability to provide heat, which further increases secondThis strategy minimized the amount of hot water
ary ﬂow, which sends more secondary ﬂow backward
ﬂow needed at the heat exchanger to meet the heating
through the common pipe, which in turn further
requirements of the MAU.
degrades the SHWST.
Implementing this strategy immediately resulted in
The boilers, which control the PHWST, are oblivious
FEBRUARY 2021

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ASHRAE Journal - February 2021

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