Applications Manual Bosch Geothermal Heat Pumps
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Bosch Thermotechnology Corp.
Data subject to change
Fig. 130 Reheat coil
17.3.2 Sequence of Operation with HGR:
Three modes of operation are available with Bosch
geothermal heat pumps equipped with hot gas reheat:
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Cooling mode only
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Heating mode only
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Cooling mode in conjunction with hot gas reheat mode
During cooling, on a call from the front end controller,
the blower relay is energized through the "G" circuit run
through the normally closed contacts on the reheat relay.
The reversing valve is energized through the "O" circuit.
The compressor contactor is energized through the "Y"
circuit which energizes the cooling relay coil closing the
cooling relay normally open contacts and energizing the
compressor contactor from the "R" side of the transformer.
At the same time the reheat solenoid valve is disabled by
opening the normally closed contacts of the cooling relay.
(Note: On most front end controllers the "O" terminal is
constantly energized when the system’s switch is in the
cooling position or the auto position.)
During heating, on a call from the front end controller,
the blower relay is energized through the "G" circuit run
through the normally closed contacts of the reheat relay.
The reversing valve is de-energized by the absence of the
"O" circuit signal. The rest of the sequence is identical to
that of cooling. (Note: The compressor contactor is still
energized through the cooling relay contacts.)
During reheat, on a call from the front end controller
(humidistat signal) the reheat relay coil is energized
through the "H" circuit. The blower relay is energized
through the "R" side of the transformer run through the
normally open contacts of the reheat relay coil. The "O"
circuit is energized thus energizing the reversing valve. The
compressor contactor is energized though the "R" side of
the transformer run through normally open contacts on the
reheat relay. The cooling relay remains de-energized thus
the reheat solenoid is enabled. (the reheat mode always
operates simultaneously with the cooling mode.)
The best way to control geothermal heat pumps with hot
gas reheat is the one which ts the application.
Most other geothermal heat pump compatible thermostats
in conjunction with a humidistat are acceptable for use.
"O" output for reversing valve energized in cooling
mode required.
When low temperature well water is utilized as the water
source (below 55 degrees F), a means of establishing
two (2) ow rates, one (1) for the cooling/reheat mode
and one (1) for the heating mode is recommended. In
the cooling mode at low entering water temperatures
and standard ow rates discharge pressures and
corresponding discharge gas temperatures are relatively
low. At these conditions when the reheat mode is initiated
the low temperature discharge gas can condense in the
reheat coil thus yielding minimal reheat capacity. A means
to reduce the water ow rate and elevate the discharge
pressure and temperature in the cooling/reheat mode
should be provided. Conversely, at low entering water
temperatures in the heating mode system suction pressure
is reduced causing a loss in heating capacity. A means
of providing higher ow in the heating mode should be
provided. The simplest way to accomplish this is to install
water regulating valves.
Operating pressures and temperatures in the reheat
mode vary slightly from standard cooling mode operating
characteristics. The variations are as follows:
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Discharge Pressure: (-) 5 to 20 PSIG
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Discharge Gas Temperature: (-) 5 to 15 Degrees F.
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Suction Pressure: (+) 5 to 10 PSIG
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Suction Gas Temperature: (+) 5 to 10 Degrees F.
