ASHRAE Journal - August 2021 - 60

COLUMN ENGINEER'S NOTEBOOK
There is more than one way to integrate the production
of domestic hot water with HVAC heat-recovery chillers;
the following evaluation of desired design attributes
will clarify why the author and design team arrived at
the solution proposed. The author encourages designers
and owners to carefully evaluate desired performance
metrics on a case-by-case basis to arrive at the optimum
solution for each application.
First Cost
The proposed approach of integrating domestic hot
water production with the heat-recovery chillers yielded
first cost savings of approximately $225,000 compared
to the baseline approach through reduced equipment
and installation costs. This was primarily achieved
through a reduction in the quantity of equipment
required, as well as the elimination of heat pump modules
from the domestic hot water equipment.
Installation cost savings can scale quickly when the
quantity of installed equipment can be reduced. In
this case, a minimum of four of the largest modular
commercial heat pump water heaters available to the
project are required for the baseline approach versus
two custom dual-source tank-type water heaters for the
proposed approach. Although custom equipment often
comes with a cost premium, in this case it yielded significant
equipment cost savings through the elimination
of heat pump equipment from the water heaters and
reduced installation cost (both labor and materials).
Maintenance
Albert Einstein has been quoted as stating that " everything
should be as simple as possible, but no simpler. "
The proposed approach to produce domestic hot water
through a combination of heating hot water preheat and
electric resistance booster heat approaches this ideal
by eliminating additional heat pump components such
as refrigeration compressors, refrigeration circuits and
associated appurtenances, and evaporator fans. This
represents a substantial simplification and reduction
of future maintenance needs compared to the baseline
heat pump water heater system shown in Figure 1.
Energy Efficiency
The design team anticipated that, under conditions
where there is no simultaneous demand for chilled
water production, the baseline approach of using
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ASHRAE JOURNAL ashrae.org A U G UST 2021
dedicated stand-alone heat pumps to produce domestic
hot water could have an energy-efficiency advantage
over the proposed hybrid system that uses HVAC heat
pumps for part of the load with electric resistance heat
to boost the domestic hot water to storage temperature.
The HVAC heat pumps generally have a coefficient
of performance (COP) similar to that of the latest
commercial-grade domestic hot water heat pumps in a
heating-only condition comparison. However, the HVAC
heat pumps used in the proposed system can produce
chilled water simultaneously with heating hot water,
which yields a COP that is substantially higher than the
heating-only COP of either machine.
This is a strategic advantage of the proposed approach
for buildings with substantial simultaneous demand
for chilled and domestic hot water, which can be the
case for medical office buildings in Southern California.
It is also an inherently less wasteful process from an
exergy perspective, as the coupling of domestic hot
water production with heat-recovery chillers eliminates
the potential for the HVAC plant to be rejecting heat
to the atmosphere while the domestic hot water heat
pumps are pulling heat from the atmosphere, which is
anticipated to occur from time to time with the baseline
design shown in Figure 1.
Table 1 summarizes the COP comparison between the
baseline and proposed approaches under a variety of
operational and ambient climate conditions. The relative
operational efficiency of the proposed approach to
integrate domestic hot water with the HVAC heating hot
water system against the baseline approach of providing
dedicated stand-alone heat pumps to produce domestic
hot water is highly dependent on operational characteristics
of the building heating hot water and domestic
hot water demands as well as on the ambient climate
conditions.
Overall, under the operational characteristics and
climate that apply to this case study project, the energy
efficiency of the two approaches could be described as
similar. The difference in annual energy performance
on a project site energy use intensity (EUI) basis is anticipated
to be within 1%, with the dedicated air-source
heat pump water heaters in the baseline design having
the slight advantage according to our energy model during
the design process.
This conclusion makes sense; although the baseline
design has superior COP under heating-only conditions
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