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Bosch Geothermal Heat Pumps
Bosch Thermotechnology Corp.
Data subject to change
Applications Manual
16.5 Bosch Geothermal Water-to-Water Heat
Pump Buffer Tank Application
Typically, all Bosch geothermal water-to-water applications
will require a buffer tank (Figure 121) be placed between
the water-to-water heat pump and the hydronic load to
prevent short cycling and to allow different ow rates
through the water-to-water unit than through a connected
hydronic heating delivery system.
Fig. 121
Bosch geothermal water-to-water heat pumps used in
heating applications typically require a much higher ow
rate (gallons per minute) than the hydronic load (radiant
oor heating system, etc.), and operating the system
without a buffer tank can result in refrigeration circuit
problems. Typically, the safest approach is to plan for a
buffer tank with a water-to-water unit.
A buffer tank is also necessary for chilled water cooling
applications if the Bosch geothermal water-to-water heat
pump is typically more than 20% larger than the cooling
load and/or multiple fan coil units are to be used. Bosch
TW Series geothermal water-to-water heat pumps sized
for the cooling load in applications with only a single fan
coil unit may be able to operate without a buffer tank, but
this is not recommended by Bosch as the cooling load is
normally much smaller than the heating load.
The size of a buffer tank should be determined based upon
the predominant use of the Bosch geothermal water-to-
water heat pump (heating or cooling). For heating, buffer
tanks should be sized at one U.S. gallon per 1,000 Btu/h
of heating capacity at the maximum entering source uid
temperature (EFT) and the minimum entering load uid
temperature, the point at which the Bosch geothermal
water-to-water heat pump has the highest heating capacity,
usually 50-70°F EFT and 80-90°F EFT. For cooling, buffer
tanks should be sized at one U.S. gallon per 1,000 Btu/h
of cooling capacity at the minimum EFT and the maximum
EFT, the point at which the Bosch geothermal water-to-
water heat pump has the highest cooling capacity, usually
50-70°EFT and 50-60°F EFT. Always select the size of the
buffer tank based on the larger of the two calculations
(heating or cooling). The minimum buffer tank size is
typically 40 U.S. gallons for any application.
Electric water heaters typically are good buffer tanks
because of availability and low cost. However, they
must be A.S.M.E. rated for heating in order to qualify as
a buffer tank. Always examine insulation values of the
tank, especially if buffer tank is to be used for storing
chilled water, as the potential for condensation to form
exists. A minimum insulation value of R-12 typically is
recommended for storage tanks. When using an electric
water heat as a buffer tank, there are typically fewer water
connections, and alternative piping arrangements may be
necessary.
Bosch geothermal water-to-water heat pumps will
attain their highest ef ciency when matched with low
temperature distributions systems. Installers should
always avoid geothermal water-to-water heat pumps in
systems requiring design water/ uid temperatures above
120°F.
Depending upon the temperature difference between
the entering and leaving load temperatures, the buffer
tank and/or domestic hot water tank may require lower
settings. For example, if the load pump provides a
temperature difference of 5°F when the total pressure
drop of the system is considered (piping, valves, heat
exchanger pressure drop, etc.), the tank could be set
as high as 140°F. However, if the design temperature
difference is 10°F, the tank temperature must be
lowered to a maximum of 135°F to avoid a leaving water
temperature above the maximum allowed, potentially
causing nuisan ce lockouts. It is always a good idea to
provide a few degrees “buffer” for operating conditions
where the temperature difference could be lower.
