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CONTROL STRATEGIES
Overview
Controls for hydronics applications can be very simple or very
complicated, depending upon the features desired, and the type of
system chosen. Water-to-water units are the most fl exible of all
heat pumps, since there are so many applications that are possible.
Below is an overview of the steps necessary for deciding the best
control strategy for a particular application.
The fi rst step in deciding which control strategy is appropriate
for the application is to decide the type of equipment that will be
used. Bryant offers heating only water-to-water units (50YEW
series) and reversible, or heating/cooling water-to-water units
(50PSW and GSW series). Bryant's recommended approach
includes a dedicated water-to-water unit for heating / hot water
generation, and a dedicated water-to-air unit for cooling. The
approach provides the simplest controls interface, and has the
advantage of redundancy (i.e. the water-to-air unit may be used
for heating in the shoulder seasons if the water-to-water unit is
not operating). Plus, the wide variety of water-to-air units allows
the designer to address retrofi t installations with greater fl exibility.
For example, duct free (console-type) units may be used when
ductwork for cooling is not possible.
Once the type of equipment is determined, the type of water-to-
water unit can be selected. The 50YEW series includes internal
controls specifi cally designed for hydronic heating systems (see
section on 50YEW series controls, below), whereas the 50PSW
and GSW series require external controls. The 50YEW series
is especially suited to radiant fl oor heating systems and the
production of domestic hot water. However, since the 50YEW
series is heating only, the 50PSW or GSW series should be
selected when chilled water is required.
The next decision regarding controls involves buffer tank
temperatures. A fi xed temperature, controlled by an aqua-stat is
the simplest and least expensive type of control strategy to install.
However, outdoor temperature reset (changing the setpoint
temperature of the water in the buffer tank based upon outside
temperature) is the most cost-effective strategy when controlled
by a microprocessor-based controller. This decision can affect
annual operating costs signifi cantly, since the COP of the water-to-
water unit improves as the source and load water temperatures
are closer together.
The next several pages show the various control drawings, as well
as specifi c information on the internal controls available in the
50YEW series heat pumps. No one strategy is best for all hydronics
applications. Individual customer preferences and budgets will help
determine which system is best for each application.
Bryant Geothermal Heat Pump Systems
50YEW Series Controls
The Bryant 50YEW series water-to-water heat pump is unlike
any other heat pump on the market. The large operating map of
the scroll compressor allows high temperature operation, up to
145°F (63°C) leaving load water temperature (even at 32°F [0°C]
entering source water temperature). The combination of a coaxial
(tube in tube) heat exchanger for the source (ground loop) side
and a brazed plate heat exchanger for the load (heating/hot water)
side provides very high effi ciencies. Integral controls for hydronic
heating and domestic water heating avoid the need for external
microprocessor-based controls for outdoor temperature reset,
warm weather shutdown and staging. Below is a summary of the
key components of the 50YEW series internal controls, followed
by a list of control features.
"Smart" module (MPC): Every 50YEW unit includes the Bryant
MPC controller. The MPC is a programmable controller that
takes inputs such as buffer tank temperature, domestic hot water
(DHW) tank temperature, outdoor air temperature, and other
inputs to "decide" when to operate the compressor, pumps
and hot water valve. The MPC is factory-wired to the CXM
compressor control module and user interface.
Water-to-Water System Design Guide
Part IV: Controls
MPC Programmable Controller
55
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