ASHRAE Journal - September 2014 - 20

TECHNICAL FEATURE

TABLE 1 Sequence of events on June 14, 2013.

DOAS
Supply Air

TIME

EVENT

11:00 a.m.

Library FCU blower is running in ventilation mode. Coils are not in use.
Discharge air temperature is 65°F, zone temperature is 73.8°F. Zone
setpoint is 74°F.

11:08 a.m.

FCU for a corridor zone turns on in cooling mode.

11:15 a.m.

Library coils are still not in use. Blower is still running in ventilation
mode. Discharge air temperature is now 56°F, zone temperature is
73.6°F. Total VAV airflow to the 3 zones has not changed. It's likely
that the balance of fresh air to each zone has changed with less DOAS
airflow going to the corridor zone and more going to the library.

11:16 a.m.

Library FCU turns on in heating mode.

11:30
a.m. to
12:00 p.m.

Library discharge air temperature is 92°F - 94°F.
Zone temperature is 73.2°F - 73.6°F.

12:04 p.m.

Library FCU turns off.

12:15 p.m.

Library discharge air temperature is 65°F, zone temperature is 73.8°F.

VAV

Return Air from
Zone 104
Library
Zone 104
FCU

Discharge Air to
Zone 104

Return Air from
Zone 105

Return Air from
Zone 103

Corridor
Zone 105
FCU

Discharge Air to
Zone 105

Corridor
Zone 103
FCU

Discharge Air to
Zone 103

100
FIGURE 9 Diagram of DOAS supply to Zone 104.

ASHRAE JOURNAL

ashrae.org

SEPTEM BER 2014

Temperature (°F)

short time period. Figure 8 (Page 19) shows the power use
by each system during the day.
The operation of the library FCU in heating mode is
of particular interest. Drawings of the ductwork for the
building show that a single DOAS VAV terminal supplies fresh air to three different zones: the library and
two corridors. For each of these zones, the 55°F (13°C)
air from the DOAS mixes with the return air and flows
through the FCU duct to the zone as shown in Figure 9.
The sequence of events that led to the library FCU operating in heating mode is described in Table 1. Figure 10
shows the discharge air temperature, zone temperature
and system setpoint for the library during the day.
This is just one example of how the interactions
between the DOAS and the VRF systems can create a
need for simultaneous heating and cooling that is not
caused by inherent internal or building envelope loads.
In this case, the VRF system is serving to "reheat" the
cool air provided by the DOAS.
Both the GSHP system and the VRF system can provide
simultaneous heating and cooling to various zones of
the building. Figures 11 and 12 show the contributions
of heating and cooling operations to the total VRF and
GSHP system power use. Power uses for the heating and
cooling operations were approximately allocated based
on the nominal capacity of the FCUs or the heat pumps
20

Discharge Air Temperature
Zone Temperature
Setpoint

90
80
70
60

Overcooling by DOAS
Resulted in Heating
Operation of the VRF FCU

50
40

12p

FIGURE 10 Library zone temperatures on June 14, 2013.

running in heating and cooling modes, respectively.
These figures show that when ambient temperatures are
between 50°F and 70°F (10°C and 21°C) the GSHP system
uses less than 0.3 W/ft2 (3 W/m2), primarily for cooling.
In the same range, the VRF system uses over 0.6 W/ft2 (6
W/m2) with much of the power being used for heating.
Although the heat-recovery type VRF system can make
use of otherwise wasted condensing/evaporating energy
to provide space heating and cooling to different zones
without consuming additional power to run multiple
compressors (as the GSHP system does), the longer runtimes and conflicting heating and cooling operations in
adjacent zones in the open office environment due to the
single setpoint control (as shown in Example 1) resulted
in higher power use than the GSHP system when the
weather was mild.

ASHRAE Journal - September 2014

Table of Contents for the Digital Edition of ASHRAE Journal - September 2014