ASHRAE Journal - September 2024 - 43

TECHNICAL FEATURE
selections and overall system control strategies cannot
be determined.
For instance, consider the heating and cooling load
requirements for a hospital versus a multifamily
complex. While a hospital requires precise temperature
and humidity level control to ensure optimum occupant
comfort and functionality, a multifamily unit requires
much lower load demands. Similarly, larger facilities
like schools or offi ce towers require vastly different
temperature regulation and energy use.
Modern Hydronic System Design in Commercial Retrofi ts
Modernizing HVAC systems in existing buildings
to ensure occupant comfort and sustain structural
integrity presents an increasingly cost-effective option
for commercial building owners looking to advance
sustainability and carbon neutrality. Roughly 80% of
buildings targeted for decarbonization are existing
structures; some are a result of adaptive reuse plans,
which involves repurposing an existing building for
a new use. According to Commercial Building Energy
Consumption Survey (CBECS) data, nearly 65% of
commercial buildings constructed before 1990 use
hydronic heating and cooling systems. When choosing
the best solution for electrifi cation of an existing fossil
fuel-based hydronic heating system, it's helpful to
understand how the original system was designed and
the assumptions that went into equipment selection.
For instance, when designing for adaptive reuse,
designers need to consider potential limitations of the
building's existing infrastructure. In some cases, limited
physical space or impassable ceiling obstructions may
make it impossible to route piping or place equipment.
Despite challenges and design constraints, retrofi ts
pose a clear advantage for realizing decarbonization and
net zero energy targets. Among other things, achieving
these goals involves the introduction of renewable
energy sources. In most hydronic decarbonization
projects, replacing the energy source, either partially
or entirely, will be a given. The decision to replace or
reuse existing piping and ancillary devices such as coils,
pumps, control and balance valves and air management
components will depend on how the designer alters
the original building operating conditions, with
potential building envelope upgrades and ventilation
improvement strategies having signifi cant infl uence.
Once new building conditions are fi nalized and
associated system loads have been identifi ed-
whether they are unchanged, modifi ed or entirely
new-a detailed assessment of existing hydronic
HVAC equipment and distribution piping should be
completed. To determine what should be reused or
replaced, several factors should be considered:
Age and Condition.
* Will service be required? (parts and labor cost).
* Parts availability (has the product been obsoleted by
the manufacturer?).
* Effi ciency compared to a newer model (simple
return on investment, impact on carbon footprint).
Capacity and Performance Within New Operating
Parameters.
* Modifi cations required to achieve new desired
capacity (increase or decrease).
* Concessions needed for acceptable performance
(gpm/cfm, pressure drop, etc.).
* Will supplemental components be required to
manage defi ciencies?
Pipe Size or Design Layout Adjustments.
* Corrections to keep fl uid velocity and friction loss
rates within industry standards.
* Changes to manage total pressure drops so existing
pumps can be reused.
* Isolation of high pressure drop zones to reduce size
and quantity of new pumps needed.
* Separation of zones requiring different supply
water temperatures.
Adhering to Key Engineering Principles
With the objective of reusing as much existing
equipment and piping as possible, designers should
consider the following established engineering
principles of performance applied in hydronic system
design and operation during their analysis.
Starting with the overall building load profi le, Figure 1
shows a comparison of the percentage of peak design
load to the percentage of expected operating hours at
each load for a typical commercial building in North
America. Realizing that 70% to 100% of peak load occurs
during less than 10% of regular operating hours, the
application of diversity in the required capacities of heat
transfer terminal units and pumps can be advantageous
in maximizing the reuse of existing equipment.
The required fl ow rate for each load can be calculated
using Equation 1 or obtained from the equipment
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