ASHRAE Journal - July 2024 - 36

TECHNICAL FEATURE
FIGURE 2 Probability distribution of house temperature for TDHP and MSHP
systems (06/04/2021 - 10/13/2021).
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Temperature (°C)
that provides a common evaporating or condensing
temperature for all of the delivered airflow. The
conditioned air is delivered throughout the house via
a zoned ducted air distribution system. In contrast,
a MSHP uses individual air handlers for each zone,
and the refrigerant is delivered to each air handler
via refrigerant lines. Each air handler has its own
expansion device, and, theoretically, can operate at its
own evaporating/condensing temperature, although
not necessarily by design.
Data Acquisition System
The study collected data on indoor temperature,
relative humidity and energy consumption, alongside
outdoor weather parameters. Thermocouples and
humidity sensors were deployed, taking measurements
every 15 seconds, with adherence to ASHRAE standards
for accuracy requirements as shown in Online Table 2.
T-type thermocouples, calibrated for accuracy, were
used for temperature readings, while sensors with
a 2% accuracy operated for relative humidity. A total
of 29 thermocouples and six humidity sensors were
placed throughout the house. Data acquisition system
included modules for thermocouples, voltage and
current readings managed by a real-time software. A
weather station recorded outdoor conditions at oneminute
intervals.
Results and Discussion
The assessment of cooling performance involved
maintaining both the TDHP and MSHP systems at a
setpoint temperature of 23.9°C (75°F) throughout
the summer. Additionally, the TDHP system
included a relative humidity control setpoint, set at
a dehumidification target of 50% relative humidity.
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TDHP
MSHP
TDHP
Mean: 23.8°C
StDev: 0.20°C
MSHP
Mean: 23.8°C
StDev: 0.32°C
The MSHP system lacked a specific humidity
control setpoint but featured a " dry mode " for
dehumidification. Both the " dry mode " and standard
cooling mode were tested for the MSHP system in this
study. However, the application of " dry mode " was
limited to one week due to its tendency to excessively
cool the space in favor of humidity reduction.
To ensure comparable temperature control, the
thermostats for both systems were positioned next
to each other. Data collection occurred six days a
week, with the seventh day reserved for transitioning
between the systems each week.
Temperature Performance
The indoor temperature is regulated by a system
during its operation when the compressor is active.
Throughout the day, the compressor goes into an
idle state during certain periods, depending on
internal heat gains and weather conditions, resulting
in the system being inactive. To assess the systems'
performance accurately, only data recorded during
operational periods-those actively controlling the
temperature-were considered. In this study, a system
was categorized as being in " not controlling " mode
when it remained idle for at least one hour.
The data during active temperature control periods
were used for system comparison, involving the
generation of a probability histogram for cumulative
data recorded over the cooling season that lasted more
than four months. In Figure 2, a probability histogram
for the average temperature distribution of the house
is presented, comparing both the TDHP system and
the MSHP system. The cooling season findings indicate
that the mean indoor temperature for both systems,
TDHP and MSHP, was 23.8°C (74.8°F), demonstrating
close alignment with the setpoint. The standard
deviation of the distribution was 0.20°C (0.36°F) for
the TDHP system and 0.32°C (0.58°F) for the MSHP
system. As a result, the TDHP system exhibited better
temperature control supported by a more concentrated
distribution closer to the setpoint with a lower
standard deviation.
To visualize temperature control, daily plots
illustrating the room average temperature for each
zone were generated. For a meaningful comparison
between both the TDHP and MSHP systems, two days
with comparable outdoor conditions were chosen.
Probability
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