ASHRAE Journal - September 2014 - 18
TECHNICAL FEATURE
1.2
GSHP
76
VRF
74
Zone Temperature (°F)
75
0.8
W/ft2
1
0.6
0.4
0.2
72
ZONE 207
ZONE 209
ZONE 224D
ZONE 215A
ZONE 215C
71
70
0
12a
3a
6a
9a
12p
3p
6p
9p
12a
FIGURE 4 GSHP and VRF power use on April 3, 2013.
when the ambient air temperature peaked at about
100°F (38°C). The DOAS cooled the outdoor air to 55°F
(13°C), and its power use increased linearly as the
ambient air became warmer.
The different power use in mild weather appears to
result from the control strategies of the two systems and
the interactions with the DOAS. Throughout the building
the thermostats have BAS-specified base setpoints that
the occupants can adjust ±3°F (1.67°C) to suit individual
comfort levels. When the weather is mild, the fresh air
supplied by the DOAS is adequate to maintain most of
the zones on the second floor within the heating and
cooling setpoints for the GSHP system (typically 68°F
and 74°F [20°C and 23°C], respectively). As a result, few
heat pumps compressors operated then, with most heat
pumps running in ventilation mode. However, during the
same time periods, a much higher proportion of FCUs in
the VRF system were on with some of the units operating
in cooling mode while others ran in heating mode.
Each zone in the VRF system has a single setpoint,
which according to the manufacturer, is valid for the
current operation mode. We do not have complete
information about how the control strategy works,
but our interpretation is that an FCU can run in
one mode, maintaining a temperature within about
±1°F (0.55°C) until such time as the temperature
moves a certain amount away from the setpoint in
the opposite direction from the system's operation
in the current mode. For example, if the system
is in heating mode and the zone rises about 4.5°F
(2.5°C) above the setpoint temperature, the FCU will
change to cooling mode, and bring the zone temperature back to within ±1°F (0.55°C) of setpoint.
18
73
ASHRAE JOURNAL
ashrae.org
SEPTEM BER 2014
69
7a
10a
1p
ZONE 204
ZONE 224C
ZONE 206
ZONE 215B
ZONE 224B
4p
7p
FIGURE 5 GSHP zone temperatures on April 3, 2013.
The open office floor plan presents challenges for control schemes based on a
single set point for each zone.
We welcome input from VRF experts on this point.
As to whether or not this control strategy is consistent with Standard 90.1 requirements of at least a
5°F (3°C) deadband between heating and cooling
setpoints we leave for others to judge. The current
control strategy does seem to prevent any single unit
from switching back and forth between modes, but
it does not prevent adjacent FCUs in the open plan
office space from "fighting" each other. Example 1
illustrates this situation.
Example 1
On Wednesday, April 3, 2013 ambient temperatures
were cool with a morning low of 43°F (6°C) and an afternoon high of 63°F (17°C). Figure 4 shows that the power
use by the VRF system was much higher than the power
use by the GSHP system during this day. Only four of the
heat pumps ran during the workday: two heat pumps
operated in heating mode for five minutes each, and
two operated in cooling mode for several hours. The
ASHRAE Journal - September 2014
Table of Contents for the Digital Edition of ASHRAE Journal - September 2014
Contents
ASHRAE Journal - September 2014 - Cover1
ASHRAE Journal - September 2014 - Cover2
ASHRAE Journal - September 2014 - 1
ASHRAE Journal - September 2014 - 2
ASHRAE Journal - September 2014 - Contents
ASHRAE Journal - September 2014 - 4
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ASHRAE Journal - September 2014 - SCover1
ASHRAE Journal - September 2014 - SCover2
ASHRAE Journal - September 2014 - S1
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ASHRAE Journal - September 2014 - S21
ASHRAE Journal - September 2014 - S22
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