ASHRAE Journal - August 2024 - 52

TECHNICAL FEATURE
FIGURE 6 Heating load profiles for the three warm-up strategies.
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25
20
15
10
5
12 a.m.
1 a.m.
Setback
Long Warm-Up
No Setback
baseline setback approach. This roughly represents the
maximum reduction in peak heating load that can be
achieved based on modified warm-up control strategies.
The long warm-up approach also significantly reduced
2 a.m.
3 a.m.
Time of Day
ramped control logic, but the warm-up period was
extended to three hours, and the optimal start tuning
was adjusted to use the full period.
3. No Setback. As an edge case, the unoccupied
heating setpoint was set equal to the occupied heating
setpoint to effectively eliminate setback operation and
the morning recovery load.
Figure 4 shows the heating setpoints (solid lines) and
actual zone temperatures (dashed lines) for the three
control strategies evaluated. Data are from days with
similar average morning temperatures of about 48°F
(8.9°C) as indicated in Figure 5. The setpoint ramps for
the setback and long warm-up approaches vary slightly
each day based on ambient conditions, and the resulting
heating loads also vary based on how much each zone
needs to recover. Note that the full setpoint stepchange
that is the dominant standard practice was not
evaluated because the testing was performed with only
setpoint and parameter adjustments for simplicity, and
the control system was already configured for " ramped "
warm-up per Figure 1b.
As expected, the long warm-up and no setback
approaches both reduced peak heating load compared
to the baseline setback approach. Figure 5 shows the peak
heating load for the whole building plotted against the
average outdoor air temperature (OAT), with each data
point evaluated over the period from midnight to 6 a.m.
each day. The peak heating load correlates well with the
average OAT for the setback approach (shown in gray).
Though there were only a few days where the no
setback approach was tested, the results consistently
show (in green) a dramatic reduction in peak heating
load of about 18 Btu/h·ft2 (57 W/m2) compared to the
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ASHRAE JOURNAL ashrae.org A U G UST 2024
4 a.m.
5 a.m.
6 a.m.
peak heating load, but more so on colder mornings. The
diminishing reduction for warmer mornings is because
the optimal start delays the onset of warm-up mode
on those days. Note that the absolute heating loads are
relatively high in all cases here because they include
the ventilation and recovery loads for the laboratory
system, which operates continuously but does use night
setbacks. Again, the laboratory system controls were
unchanged throughout the study.
Though the scatter plot in Figure 5 shows clear
reductions in peak heating loads, it is easier to visualize
and understand the impact of different warm-up
strategies by observing heating load profiles (Figure 6).
So that the recovery loads are comparable across the
different strategies, load profiles shown are for weekdays
when average morning outdoor air temperatures were
about 48°F (8.9°C) (blue box, Figure 5).
The setback approach incurs the highest peak loads,
with peaks just prior to the start of occupied mode.
Though only two days of data were available, the heating
loads for the long warm-up mode were consistent,
starting earlier and with a significantly reduced peak.
The heating load for the no setback approach was nearly
flat with the recovery load effectively eliminated (except
for the laboratory wing) and achieved even further
reductions in the heating peak.
The heating energy consumption was also evaluated
for the different warm-up control strategies. Figure 7
shows the total overnight (9 p.m. to 6 a.m.) heating
energy use as a function of the average outdoor air
FIGURE 7 Heating energy use for the three warm-up strategies.
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Average Outdoor Air Temperature (9 p.m. - 6 a.m.), °F
No Setback
Setback
Long Warm-Up
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65
Hot Water Load, Btu/h·ft2
Heating Energy (9 p.m. - 6 a.m.), Btu/ft2
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ASHRAE Journal - August 2024

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