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Operating Principles
A typical HVAC system has a cooling requirement in the
summer and shoulder seasons and a heating requirement
in the winter and shoulder seasons. A modular air-source
heat pump system provides an efficient means to address
varying cooling and heating demands. The heat pump
system can be sized for the greater of the cooling or
heating demand. When the smaller of the demands is
operating, not all modules operate thereby saving energy
costs and improving efficiency. This also allows for the non-
operating modules to be on standby should any of the
operating modules require servicing. When the HVAC
system demand switches between heating and cooling, the
refrigeration cycle in each operating module is reversed to
produce the required heating or cooling requirement. The
air-source heat pump system does not simultaneously heat
and cool; it produces either heating or cooling depending
on the commanded mode of operation. The ambient air
provides the heat source in heating mode where the air coil
operates as an evaporator and a heat sink in cooling mode
when the air coil operates as a condenser. When operating
in heating mode in colder climates, frost will likely form on
the air coils. The heat pump modules will periodically
defrost the coils by reversing the refrigerant cycle. Modules
are sequentially defrosted to minimize the impact to
heating capacity. The defrost cycle must be considered
when sizing the hydronic system volume to ensure heating
supply capacity.
ARTC-SVX006A-EN
Minimum Fluid Volume and
Avoidance of Short Loops
Adequate system fluid volume is an important system
design parameter because it provides for stable fluid
temperature control and helps limit unacceptable short
cycling of compressors. The heat pump system
temperature control sensors are located in the supply
(outlet) and return (inlet) fluid connections to the building
piping. This location allows the building piping system
volume to act as a buffer to slow the rate of change of the
system fluid temperature. If there is not a sufficient volume
of fluid in the system to provide an adequate buffer,
temperature control can suffer, resulting in erratic system
operation and excessive compressor cycling. The situation
can be more severe during the heating operation when
individual modules switch to defrost mode. The modules on
defrost mode not only stop providing heating capacity, but
they also cool down the circulating hot fluid, increasing the
heating demand and potentially causing a drop in hot fluid
temperature. The defrost cycle can take 3 to 5 minutes to
complete. A minimum of fifty times a module capacity or
four-minute full load water circulation, whichever greater, is
the recommended fluid system volume. So, as an example,
for five 30 nominal ton ASHP bank, 30T x 50 = 1,500
Gallons. Assuming 2.4 gpm per ton for full load flow
rate,150 x 2.4 x 4 = 1,440 Gallons. So, the greater value of
1,500 Gallons is recommended.
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