ASHRAE Journal - August 2024 - 50

TECHNICAL FEATURE
FIGURE 3 Demonstration site.
Field Testing
setpoint from the unoccupied to the occupied level
(Figure 1b). Starting warm-up earlier and gradually
ramping the heating setpoint allows for a slower, paced
recovery with a reduced peak heating load (Figure 2b).
With this approach, each individual zone will enter
heating mode at a different time depending on when
its temperature intersects with the ramped setpoint,
thereby further spreading out the heating demand (as
opposed to having every zone simultaneously enter
heating mode). This alternative approach may reduce
fan energy by allowing VAV systems to operate at lower
airflows and reduce heating energy use by allowing
for operation at lower water temperatures (with the
associated improvement in equipment efficiency
and reduction in distribution losses) and potentially
avoiding cycling on lag equipment.
In the extreme edge case, maintaining a constant
heating setpoint at all times (Figure 1c) completely avoids
morning recovery load (Figure 2c). Peak heating load is
greatly reduced with just a smaller peak corresponding
to envelope losses and perhaps conditioning outdoor air
during the coldest time of day. However, this strategy
results in increased envelope losses from the space to
the outdoors during unoccupied periods since higher
space temperatures are maintained.
For buildings considered for electrification retrofits,
a major barrier is delivering sufficient heating capacity
with the lower supply water temperatures that heat
pumps can produce, typically around 130°F (54°C)
compared to 180°F (82°C) for gas-fired boilers.
Replacing distribution piping and heating coils to
accommodate lower supply temperatures may be
prohibitively expensive and disruptive. However, these
modified morning warm-up control strategies offer the
potential to avoid these costly replacements by reducing
peak heating coil loads and recovering more slowly
instead at lower supply water temperatures.
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ASHRAE JOURNAL ashrae.org A U G UST 2024
Three alternative morning warm-up strategies were
tested at an academic building (Figure 3) in northern
California (ASHRAE Climate Zone 3B) in the spring
of 2023. Constructed in 2020, the building houses
classrooms, laboratories, faculty offices and a student
center in separate wings across the single story,
55,000 ft2 (5100 m2) facility. Each of the four wings is
conditioned by a separate VAV reheat system with airside
economizers, with hot water generated by a plant with
two condensing boilers. The laboratory HVAC system
operates 24/7 with 100% outdoor air and was excluded
from the intervention, though its energy use is included
in the monitoring data. The three strategies tested were:
1. Setback. This is the setback and warm-up control
strategy that was programmed in the existing control
system. Though this approach included a setpoint
ramp per Figure 1b, the ramp was not tuned, and the
warm-up mode was limited to 1.5 hours prior to the
start of occupied mode, so the recovery was effectively
compressed into a relatively short time period.
2. Long Warm-Up. This approach used the same
FIGURE 4 Zone heating setpoints for the three different warm-up strategies
in solid lines. Dashed lines show actual temperatures for a given day for each
approach. Occupancy begins at 6 a.m.
72
70
68
66
64
62
60
58
12 a.m.
3 a.m.
6 a.m.
Time of Day
FIGURE 5 Peak heating loads as a function of average outdoor air temperature.
60
50
40
30
20
10
35
40
Setback
Long Warm-Up
No Setback
18 Btu/h·ft2
Temp Setpoint
No Setback
Setback
Long Warm-Up
9 a.m.
12 p.m.
45
50
55
Average Outdoor Air Temperature (12 a.m. - 6 a.m.), °F
60
65
LOS MEDANOS COLLEGE
Peak Hot Water Load (12 a.m. - 6 a.m.), Btu/h·ft2
Temperature, °F
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ASHRAE Journal - August 2024

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