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 postconstruction data shows the peak demand to be 10 Btu / h · ft 2 ( 32 W / m 2 ).
Beyond looking solely at the peak loads , we needed to dig deeper into the HHW load profile . To develop options for a heat recovery application , the chilled water ( CHW ) load profile should be overlaid on the HHW load profile 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 profile . Figure 1 shows a HHW system load profile 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 profiles 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 evaluated three options ( detailed later in this column ).
The evaluation considered first costs and carbon emissions as the main key performance indicators ( KPIs ). This is an oversimplified approach and many other considerations should be taken into account when deter-mining the appropriate system type .
The university is transitioning from a central steam system ; the new baseline system will be a natural gas-fired 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 efficiency of 96 %, with an assumed average efficiency 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-fired 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 efficiency of the heat pump during low-load operation .
Table 2 : Existing Building Heating Equipment vs . Actual Heating Demand
Building
A
B
C
Use
Engineering Classrooms & Labs
Science Classrooms & Labs
Geology & Mathematics Classrooms & Labs
Installed Heat Capacity
Heating Capacity ( MBH )
Heating Capacity ( Btu / h per ft 2 )
Required Heating Capacity
Peak Demand ( MBH )
Peak Demand ( Btu / h per ft 2 )
4,600 49.45 2,543 27 *
4,500 44.9 2,041 18.5
4,760 37.8 1,618 12.8
* Upon completion of the system upgrades , including replacement of three-way valves and temperature resets , the measured peak demand was 10 Btu / h · ft 2 .
10 P2S EXTERNAL MAGAZINE SUMMER 2023