ASHRAE Journal - February 2023 - 15

COLUMN ENGINEER'S NOTEBOOK
to be sized correctly results in a grossly oversized system
with higher capital costs. Such a system is also likely to
encounter limitations due to the existing building infrastructure.
From a performance perspective, there would
be challenges with operating a heat pump system sized
for 250% of the actual peak demand.
The analysis also provided an opportunity to understand
other potential issues with the current system
operation. As an example, Building A showed a peak
heating demand of 27 Btu/h·ft2 (85 kW/m2) prior to
the plant upgrades. This value was not in line with our
expectations and data from similar campus buildings.
Evaluation of this data allowed the team to be proactive
in its approach and to make improvements to correct
issues at the load. For example, the design included the
replacement of three-way valves with two-way valves,
eliminating other bypass means and temperature resets.
The post-construction data shows the peak demand to
be 10 Btu/h·ft2 (32 kW/m2).
Beyond looking solely at the peak loads, we needed to
dig deeper into the HHW load profi le. To develop options
for a heat recovery application, the chilled water (CHW)
load profi le should be overlaid on the HHW load profi le
to understand the building's simultaneous heating and
cooling needs. This campus has a central CHW system
available to be used for heat rejection. As such, the project
focused only on the HHW profi le. Figure 1 shows a
HHW system load profi le at Building B in both operating
hours and heating output, compared to the original
design value of 4,500 MBH (1.3 GW) . A majority of the
hours of operation can be seen at 20% - 30% of the measured
peak of 2,041 MBH (598 MW) . Similar load profi les
were observed in all three buildings in this case study.
Heat pumps used for hydronic systems can be applied
as an air-cooled option or water-cooled option and
often come as heat-pump-only or heat recovery units
(i.e., simultaneous heating and cooling). This project
TABLE 2 Existing building heating equipment vs. actual heating demand.
BUILDING
USE
A
B
C
Engineering Classrooms & Labs
Science Classrooms & Labs
Geology and Mathematics
Classrooms & Labs
4,600
4,500
4,760
HEATING CAPACITY
(MBH)
TABLE 1 Existing building characteristics.
BUILDING
USE
A
B
C
Engineering
Classrooms & Labs
Science Classrooms
& Labs
Geology and
Mathematics
Classrooms & Labs
evaluated three options (detailed later in this column).
The evaluation considered fi rst costs and carbon emissions
as the main key performance indicators (KPIs).
This is an oversimplifi ed approach and many other considerations
should be taken into account when determining
the appropriate system type.
The university is transitioning from a central steam system;
the new baseline system will be a natural gas-fi red
plant. It will be sized for N + 1 capacity and will cover all
the buildings' heating needs. The baseline system will use
condensing boilers with a peak effi ciency of 96%, with an
assumed average effi ciency of 89%. The existing building
distribution and terminal equipment were designed for
180°F (82°C) HHW supply temperature.
Analysis of the building's reheat coils was performed to
implement the lower supply water temperature of 135°F
(57°C) used in a condensing boiler system. It was determined
100% of loads could be provided at the one-row
heating coils at a reduced HHW supply temperature of
135°F (57°C).2 The team performed further analysis of
the coils at a 110°F (43°C) supply temperature, the design
operating temperature selected for heat pumps in the
hybrid plant with a gas-fi red boiler for peak loads and a
heat pump chiller for primary heating needs (see Option 3
below). The HHW supply temperature for the heat pump
in this hybrid scenario was selected to maximize effi -
ciency of the heat pump during low-load operation.
INSTALLED HEAT CAPACITY
HEATING CAPACITY
(Btu/h per ft2)
49.45
44.9
37.8
REQUIRED HEATING CAPACITY
PEAK DEMAND
(MBH)
2,543
2,041
1,618
* Upon completion of the system upgrades, including replacement of three-way valves and temperature resets, the measured peak demand was 10 Btu/h·ft2.
F E B R U A RY 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
15
PEAK DEMAND
(Btu/h per ft2)
27*
18.5
12.8
YEAR CONSTRUCTED BUILDING AREA (ft2)
1962
2018
1960
93,000
110,000
126,000

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ASHRAE Journal - February 2023

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