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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-to-water
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 redundant. The air-to-water 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 confrim heating supply capacity.
ARTC-SVX009A-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 2 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 air-to-water heat pump system, 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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