ASHRAE Journal - February 2021 - 20

TECHNICAL FEATURE

Supply Air Temperature (°F)

a reduction in secondary ﬂow, and
FIGURE 4 MAU supply air temperature control improved by cascade control implementation.
the SHWST rose to match that of the
85
Cascade Control
PHWST. This improved downstream
83
Implemented
FTR sections' ability to heat the con81
dos. It was quickly realized, however,
79
77
that this closed-loop control was
75
insufﬁcient for TSA control of the MAU.
73
Supply Air Temperature
No amount of tuning could get away
71
Setpoint
from large temperature ﬂuctuations
69
around the TSA setpoint (Figure 4).
67
65
These TSA ﬂuctuations were traced
1/2/2020
1/2/2020
1/2/2020
1/2/2020
1/2/2020
1/2/2020
1/2/2020
back to ﬂuctuations in the PHWST. The
13:26
13:40
13:55
14:09
14:24
14:38
14:52
boiler plant consisted of two atmospheric boilers, each with two stages of heat. Bringing
loops in the system, a concept detailed in other HVAC
on an additional stage of heat at times would raise the
applications.10
PHWST (and SHWST) 30°F (16.7°C) or more in three
Conclusion
minutes. Disabling a stage of heat would at times drop
Cascade control has numerous applications in conthe supply temperature at about the same rate.
ventional and unconventional HVAC systems. However,
These undesirable ﬂuctuations in SHWST resulted in
unintended ﬂuctuations in the GHWST. The ﬂuctuations the term is rarely used, nor is the concept universally
understood. This article's intentions are to help people
in GHWST resulted in large deviations in TSA from setpoint. The system needed a way to identify and react to
understand the concept; and to allow readers to better
these disturbances before they so drastically affected the articulate these sequences in speciﬁcations and submitTSA. Cascade control was required.
tals, comprehend them quicker and realize when its
The MAU heating sequence was then adjusted a secapplication is appropriate. Foundational understanding
ond time. The hot water valve at the heat exchanger
will bring value to the project delivery process and ultinow modulates to keep the GHWST at setpoint, which
mately the performance of HVAC systems.
is reset to keep the TSA at its setpoint. The moment this
References
cascade control logic was implemented can be seen in
1. Montgomery, R., R. McDowall. 2011. Fundamentals of HVAC
Figure 4. Oscillations in supply air temperature around
Control Systems. Atlanta: ASHRAE
2. ASHRAE Guideline 36-2018, High-Performance Sequences of
its setpoint were immediately reduced to less than 1°F
Operation for HVAC Systems.
(0.6°C) in amplitude. Ongoing monitoring of the sys3. ANSI/ASHRAE/IES Standard 90.1-2019, Energy Standard for
tem has conﬁrmed this level of performance has been
Buildings Except Low-Rise Residential Buildings.
4. ANSI/ASHRAE Standard 62.1-2019, Ventilation for Acceptable
sustained.
Indoor Air Quality.
Not only did implementing cascade control improve
5. Taylor, S., J. Stein, G. Paliaga, H. Cheng. 2012. " Dual maximum
the ability of the system to maintain TSA at setpoint, it
VAV box control logic. " ASHRAE Journal (12):16 - 24.
6. Ryan, M., G. Henze. 2017. " Airside system-type prediction
was also determined that this unstable control loop
enabled by intelligent pressure independent control valves. " Journal
was a trigger that had ripple effects elsewhere in the
of Architectural Engineering 23(3).
7. Thuillard, M., F. Reider, F., G.P. Henze, G. P. 2014. " Energy
system. Previous hunting of the valve created periods
efﬁciency strategies for hydronic systems through intelligent
of low load on the heat exchanger followed by periods
actuators. " ASHRAE Transactions. 120(1).
of high load. Cascade control allows a steady load on
8. Henze, G.P., W. Henry W., M. Thuillard, M.
2013. " Improving Campus Chilled Water Systems
the heat exchanger. Implementing cascade control
with Intelligent Control Valves: A Field Study. "
resulted in smaller ﬂuctuations in the PHWST. Once
Proceedings of the 2013 ASCE Architectural Engineering
https://bit.ly/3aC5wVH
observed temperature oscillations in the PHWST of
Conference.
Rate this Article
9.
Energy
Design
Resources.
2009.
" CoolTools™
30°F (16.7°C) have been reduced to never more than
Chilled Water Plant Design Guide. " Energy Design Resources.
12°F (6.7°). This is prime example of how one unstable
10. Sellers, D. 2004. " Troubleshooting and tweaking air-handling
systems. " HPAC Engineering (12).
control loop can have ripple effects into other control
20

ASHRAE JOURNAL

ashrae.org

FEBRUARY 2021

https://www.bit.ly/3aC5wVH http://ashrae.org

ASHRAE Journal - February 2021

Table of Contents for the Digital Edition of ASHRAE Journal - February 2021