ASHRAE Journal - December 2014 - 18

TECHNICAL FEATURE

FIGURE 5 Estimated monthly heating provided.

FIGURE 6 Estimated monthly cooling provided.

0.6
GSHP

0.4
0.3
0.2
0.1
0.0
JUL. SEP. NOV. JAN. MAR. MAY JUL. SEP. NOV. JAN. MAR. MAY
2011
|
2012
|
2013

the accuracy of these measurements in the field. Choat3
describes a case where the flow hoods gave results that
were 14% lower compared to a measurement made by
traversing the duct with a pitot tube. We chose to rate the
uncertainty of the measurement for each heat pump or
terminal unit as ±11.5%. However, it is important to note
that this does not lead to an uncertainty of ±11.5% for
total cooling or total heating provided. Rather, because
the total cooling or total heating depends on the total
flow, and as described earlier, random errors tend to
cancel each other out when aggregated, the resulting
uncertainty in the total flow is lower, but depends on
the number of units operating at any one time and their
relative capacities. The fewer the number of units on,
the higher the uncertainty. We chose a value of uncertainty corresponding to three units of ±7%.
* The estimated humidity level entering all heat
pumps is approximated as being the zone humidity
level. The estimated uncertainty has two components:
the uncertainty of the sensor (±3% RH) and the uncertainty due to using the zone humidity level: (+3%/-0%).
The latter value is based on the effect (for some units) of
mixing zone return air with DOAS exiting air.
* Humidities leaving the heat pumps are based on
our finding that, for the living lab heat pump, the uncertainty of the measured relative humidity is (to a 95%
confidence level) ±5.5%. This value is taken as the uncertainty for the humidity levels leaving each heat pump.
* Humidity levels leaving the VRF system FCUs are
not measured by the building energy management system. Therefore, we have taken the manufacturer's data
to create a map of SHF that depends on entering wetbulb temperature and the outdoor air temperature. We
18

ASHRAE JOURNAL

ashrae.org

D ECEM EBER 2014

GSHP

1.2
Cooling Provided (kWh/ft2)

Heating Provided (kWh/ft2)

0.5

1.4

VRF

1.0
0.8
0.6
0.4
0.2
0.0
JUL. SEP. NOV. JAN. MAR. MAY JUL. SEP. NOV. JAN. MAR. MAY
2011
|
2012
|
2013

made spot measurements and found the actual unit SHF
to be within ±0.07 of the catalog data, so we have taken
the uncertainty in SHF to be ±0.08. With this uncertainty
in SHF, we can estimate the uncertainty in total cooling
provided at each measurement and for seasonal values.
The resulting uncertainties for the individual heat
pumps vary but are around +23/-18% for cooling and
±12% for heating (when there is no dehumidification).
When aggregated together, the uncertainty in the total
cooling provided is +14/-11% and that for the total heating provided is ±7%. For the VRF system, the uncertainty
in cooling provided by a single FCU is +16/-15% and for
heating it is ±12%. Typically, there are more FCUs running than there are heat pumps, so when aggregated
together the uncertainty in the total cooling provided by
the VRF system is ±5% and that for the total heating provided is ±4%. Compared to the uncertainties in estimating the cooling and heating provided, the uncertainties
in measuring the electrical energy consumed are negligible, and therefore the uncertainties in the calculated
COP and EER are approximately the same as the uncertainties in the total heating and total cooling provided.

Heating and Cooling Provided
The estimated heating and cooling provided by each
system are shown in Figures 5 and 6, respectively. For the
time period from July 1, 2011 until March 31, 2012, which
is the time period during which the conditioning provided by the VRF system could be estimated, the GSHP
system only provided 38% of the heating that the VRF
system provided. During the same time span, the GSHP
system provided 6% more cooling than the VRF system
provided.

ASHRAE Journal - December 2014

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