Bryant 50YEW Design Manual page 6

Water-to-Water System Design Guide
Part I: System Overview
distribution system. When using the various types of hydronic
heat distribution systems, the temperature limits of the geothermal
system must be a major consideration. In new construction, the
distribution system can easily be designed with the temperature
limits in mind. In retrofi ts, care must be taken to address the
operating temperature limits of the existing distribution system.
System Components
The effi ciency, life expectancy and reliability of any hydronic heating
system depends upon how well the various components (heat pump,
distribution system, contols, etc.) work together. The heat pump must
be sized for the building loads; the earth loop must be sized to match
the building loads, ground conditions and climate; the circulating
pumps must be sized for the equipment, piping and ground loop. The
distribution system must be designed to heat and/or cool the building
comfortably. The components must then all be controlled effectively.
Building Heat Loss & Heat Gain
The design must begin with an accurate heating and/or cooling
load of the building. This is the most important step in the design
process. The sizing of the circulation pumps, the distribution
system and the earth loop are all derived directly from the sizing
of the equipment. Overestimating the heat loss or heat gain
means over sizing the system. The extra cost of the oversized
system is unnecessary. In fact, it may result in the selection of a
different type of system. If an oversized system is installed, it may
be ineffi cient and uncomfortable. If the system is undersized it will
not do an adequate job of heating and/or cooling the building.
Loop Design & Installation
Several factors determine the loop design for a specifi c installation.
The energy balance of the building determines how much heat is
taken from and rejected to the earth over the course of a year.
The climate determines the ambient earth temperatures and is a
major factor in the energy needs of the building. The earth itself
(the conductivity of the soil or rock and the moisture content) are
major factors in calculating the size of the loop. The earth can only
take (heat rejected) or give up (heat extracted/absorbed) a fi xed
amount of Btu/hr [Watts] in a given area. The heat exchanger
must have suffi cient surface area.
The design of the loop itself (the size and type of pipe, the
velocity of the liquid circulating in the pipe and the spacing and
layout of the pipe) has a major effect on the heat absorption and
rejection capabilities of the loop. The depth (vertical) or trench
length (horizontal) of the loop must be calculated using IGSHPA
(International Ground Source Heat Pump Association) methods
or approved software. In addition, the type and percentage of
antifreeze can have a signifi cant effect on loop performance.
The workmanship of the installation also plays a large role in the
effectiveness of the loop. All fusion joints must be done properly.
Vertical loops must be grouted properly for good contact with the
earth. Horizontal loops must be backfi lled with material that will
not cut the pipe, and the soil should be compacted around the
pipe for good contact. All closed loop piping systems should be
hydrostatically pressure tested before burial.
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Many factors affect loop performance. Bryant offers training in
loop design and installation, and also provides residential and
commercial loop sizing software.
Controls
The control of a mechanical system determines how it functions. For
the building to work effi ciently and comfortably, the building owner
or manager must understand system functionality and controls.
As Figure 1-5 shows, the effi ciency of a heat pump is a factor
of the difference in temperature between the source and the
load. The heat loss or heat gain of a building varies with the
weather and the use of the building. As the outdoor temperature
decreases, the heat loss of the building increases. When the
ventilation system is operating, the heating or cooling loads
increase. As the occupancy increases, or more lighting is used, or
the solar gain increases, the cooling load increases. At times the
building may require virtually no heating or cooling.
The output of the hydronic heating distribution equipment,
whether it is baseboard radiation, fan coil units or radiant fl oor
heating equipment, is directly related to the temperature and
velocity of the water fl owing through it. Baseboard radiation puts
out approximately 50% less heat with 110°F [43°C] water than
with 130°F [54°C] water. The same is true with fan coil units and
radiant fl oor heating. For example, if a system is designed to meet
the maximum heat loss of a building with 130°F [54°C] water,
it follows that if the heat loss is 50% lower (when the outdoor
temperature is higher), the load can be met with 110°F [43°C]
water. The lower water temperature greatly increases the COP of
the heat pump. Outdoor temperature reset, discussed in part IV of
this manual, is a very cost-effective method of matching the heating
(load side) water temperature with the heat loss of the building.
Other considerations for controls include heating/cooling
switchover, pump control, backup heat (if equipped), distribution
system or zone controls, and priority assignments (e.g. determining
if radiant fl oor heating or domestic hot water will take priority).
The 50YEW series includes internal controls, which makes system
installation much easier. Other Bryant water-to-water heat pumps
must be controlled via external controls.
Bryant: Whatever It Takes.