Bryant 50YEW Design Manual page 10

Water-to-Water System Design Guide
Part II: Load Side Design / Equipment Sizing
Backup Heat
Just like water-to-air systems, which typically have some type
of backup heating capability, water-to-water systems can also
benefi t from the use of supplemental heating to help lower initial
installation costs. Design temperatures are usually chosen for
1%. In other words, 99% of the time, the outdoor temperature
is above the design temperature. If the heat pump is designed to
handle 100% of the load, it is larger than required 99% of the time.
GeoDesigner can determine an economical balance point that will
allow the water-to-water unit to be downsized when a backup
boiler or water heater is used for supplemental heat.
For example, suppose a home in Chicago has a heat loss the same as
the example above [44,423 Btuh, 13,019 Watts]. One 50YEW010
unit has a heating capacity of approximately 10kW [33,000 Btuh] at
32°F [0°C] entering source (ground loop) temperature. According
to GeoDesigner, the water-to-water unit could handle the heating
load 98% of the time. A backup electric boiler would consume
about 326 kWh annually for back up heat [$33 per year at $0.10/
kWh]. Two 50YEW010 units could handle the heating load no
matter what the outdoor temperature is (100% heating – no backup
required). However, this combination would only save about 239
kWh per year [$24 per year at $0.10/kWh], yet the additional
installation cost for a second unit and signifi cantly more ground loop
would never pay back in operating cost savings. In most cases, sizing
for 100% of the heating load is not cost effective.
Cooling
Cooling is not always desired with radiant heating systems. A
water-to-water heat pump system can provide chilled water to
ducted or non-ducted fan coil units. A reversible water-to-water
heat pump can provide chilled water to cool the building, as well
as hot water for the heating system. Buildings with fan coil units
can generally be retrofi tted for cooling quite easily. The diffi culty, as
mentioned in part I, is using existing fan coils for heating, especially
if they were originally sized for high water temperatures.
For optimal cooling and dehumidifi cation, Bryant recommends a
separate water-to-air heat pump for cooling. Controls are much
simpler when a water-to-water unit is used for space heating
and/or domestic water heating, and a water-to-air unit is used for
cooling. Since the water-to-water and water-to-air units can share
one ground loop, the installation cost of using a water-to-air unit
for cooling is simply the incremental cost of the unit. Generally, no
additional ground loop is required (in Northern climates), and the
cost of the water-to-air unit is usually less than the cost of chilled
water/fan coil units, especially if the cost of additional piping/
valving/controls and labor is considered. The cost of a water-to-
air unit is approximately the same as a ductless mini split, and is
much more effi cient. The advantages of geothermal heat pumps
for cooling (no outdoor unit, no refrigerant line sets, longevity, etc.)
should be considered when cooling is required.
Buffer Tank Sizing / Application
All water-to-water units used in heating applications require a
buffer tank to prevent equipment short cycling and to allow
different fl ow rates through the water-to-water unit than through
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the hydronic heating delivery system. A buffer tank is also required
for chilled water cooling applications if the water-to-water unit(s)
is more than 20% larger than the cooling load and/or multiple fan
coil units will be used. Water-to-water units sized for the cooling
load in applications with only ONE fan coil unit may be able
to operate without a buffer tank, but this would be an unusual
situation, since the cooling load is normally much smaller than
the heating load. The best approach is to plan for a buffer tank in
every application.
The size of the buffer tank should be determined based upon
the predominant use of the water-to-water equipment (heating
or cooling). For heating, buffer tanks should be sized at one U.S.
gallon per 1,000 Btuh [13 Liters per kW] of heating capacity at
the maximum entering source water temperature (EST) and the
minimum entering load water temperature (ELT), the point at
which the water-to-water unit has the highest heating capacity,
usually 50-70°F [10-21°C] EST and 80-90°F [26-32°C] ELT. For
cooling, buffer tanks should be sized at one U.S. gallon per 1,000
Btuh [13 Liters per kW] of cooling capacity at the minimum EST
and the maximum ELT, the point at which the water-to-water unit
has the highest cooling capacity, usually 50-70°F [10-21°C] EST and
50-60°F [10-16°C] ELT. Select the size of the tank based upon the
larger of the calculations (heating or cooling). The minimum buffer
tank size is 40 U.S. gallons [150 Liters] for any system.
Electric water heaters typically make good buffer tanks because of
the availability and relatively low cost. However, the water heater
must be A.S.M.E. rated (rated for heating) in order to qualify as a
buffer tank. Attention should be paid to insulation values of the
tank, especially when a buffer tank is used to store chilled water
due to the potential for condensation. A minimum insulation value
of R-12 [2.11 K-m2/W] is recommended for storage tanks.
CAUTION:
Maximum leaving water temperature of the 50YEW series
equipment is 145°F [63°C]. For domestic hot water tank
temperatures or heating buffer tank temperatures above
130°F [54°C], pump and pipe sizing is critical to insure that
the fl ow rate through the heat pump is suffi cient to maintain
leaving water temperatures below the maximum temperature,
and to provide water fl ow rates within the ranges shown in
the performance section of this manual.
Bryant: Whatever It Takes.