ASHRAE Journal - August 2024 - 53

TECHNICAL FEATURE
FIGURE 8 Fan power for the three warm-up strategies.
Conclusions
7
6
5
4
3
2
1
12 a.m. 1 a.m.
Setback
Long Warm-Up
No Setback
2 a.m.
3 a.m.
Time of Day
temperature. As expected, the no setback approach
incurs signifi cantly higher heating energy use to
overcome the increased envelope losses.
Though only limited data are available, there was little
difference in heating energy between the long warm-up
and baseline setback approach. Though the long
warm-up approach did not measurably reduce heating
energy use in this testing, this is an important result
because this strategy achieved a signifi cant reduction
in peak heating load without incurring a measurable
increase in heating energy. The reduction in peak heating
load would likely be greater if evaluated against a
conventional step-change setpoint approach (Figure 1a),
whereas the baseline setback strategy here used some
degree of a ramped setpoint change (Figure 1b).
Also, the hot water plant controls were not altered
or tuned during this intervention to take advantage
of the increased opportunity for improved plant
performance, and the existing settings largely
prevented condensing operation. For buildings with
electric heat, reductions in peak heating load have the
potential to reduce operating costs through reduced
peak demand charges.
In addition to the impact on heating energy and
loads, alternative warm-up strategies present an
opportunity to reduce fan energy. Figure 8 shows the
total supply fan power for the different strategies
across the same comparable mornings shown in
Figure 5. The long warm-up approach reduced fan
power compared to the setback approach, with about
10% fan energy savings (though based on very limited
data). The no setback approach had the lowest fan
power but increased fan energy consumption because
of the longer runtime.
4 a.m.
5 a.m.
6 a.m.
This pilot testing was limited in duration and had
relatively few days of operation for the two alternative
warm-up strategies because the study was initiated
in spring and there were limited cool nights available
for testing. Though more extensive study is needed
to expand on this work and better understand its
implications across different climates and HVAC system
types, this limited fi eld testing confi rms the potential for
signifi cant reductions in peak heating loads.
The ramped warm-up strategy is the default approach
in the building automation system installed in this test
building-it is readily available for this platform and for
replication in others, but unfortunately in the authors'
experience is often left untuned or adjusted to leverage
its potential benefi ts. With basic tuning and a longer
duration, this strategy can provide better assurance
of achieving comfort by the start of occupied mode,
potentially without the energy penalty of the stepchange
approach and without the risk of not recovering
in time because of imperfect optimal start. Most
building controls systems use the standard step-change
approach, and this is what is described in Guideline
36-2021, but these could be modifi ed to use a ramped
strategy with relatively simple programming changes.
Improving HVAC heating system performance in
setback and warm-up modes is of great urgency with
increasing interest and mandates to reduce building
fossil fuel consumption-and the rapid pivot to building
electrifi cation in some areas of the U.S. When deployed
with effective heating plant control sequences (e.g.,
Guideline 36), opportunities may also exist for improved
heating effi ciency. More importantly, the reduction in
peak heating loads at plant and zone levels may help
overcome challenges electrifying the building stock.
The reduced plant peak loads may allow more
aggressive equipment sizing, reducing fi rst-costs
and equipment space requirements and minimizing
the impact on building electrical infrastructure. The
reduced zone coil heating loads have the potential to
allow existing hot water pipe distribution and heating
coils to be reused for heat pump retrofi ts with lower
supply water temperatures (avoiding prohibitively
expensive and invasive distribution and coil retrofi ts).
On the coldest days of the year in cooler climates where
envelope loads alone may require all of the reduced coil
capacities at lower supply water temperatures, applying
A U G U ST 2 0 2 4 ashrae.org ASHRAE JOURNAL
53
Supply Fan Power, kW
http://www.ashrae.org
ASHRAE Journal - August 2024

Table of Contents for the Digital Edition of ASHRAE Journal - August 2024
