Heat Pump Controller
Heat Pump Controller8733819577 (2019/02) Subject to change without prior notice
10 | System Operation
When the thermostat calls for first-stage cooling
(Y1) the loop pump or solenoid valve, if present, is
energized and the first stage of the compressor
capacity starts. The fan ramps up to the first stage
of cooling air flow in 30 seconds.
When the thermostat calls for second-stage
cooling (Y2) the second stage (or full compressor
capacity) is initiated. The fan ramps up to full
cooling air flow.
Once the thermostat is satisfied, the compressor
shuts down and the fan ramps down to either
fan-only mode or off over a span of 30 seconds.
Heating Mode
The first two stages of heating (Y1 & Y2) operate in
the same manner as cooling but with the reversing
valve de-energized. On a call for auxiliary heat
(W1), the fan ramps up to auxiliary heat air flow
immediately and the electric heater package is
energized along with the compressor.
As the thermostat is satisfied, the heaters will shut
off as soon as W1 is de-energized, and the
compressors will remain on until the thermostat
stages are satisfied.
Once the thermostat is satisfied, the compressor
shuts down and the fan ramps down either
fan-only mode or off over a span of 30 seconds. If
the thermostat has two different output points,
one for Auxiliary Heat and a different one for
Emergency Heat the two outputs must be
terminated on W1 units equipped with one stage
of electric heat.
Fan Operation
The fan starts anytime the fan command signal (G)
or a demand for cooling/heating is received on the
thermostat interface block. The fan will run at its
minimum factory speed of 80% in fan-only mode.
The fan remains on during lockouts if there is a
demand from the thermostat. The fan motor will
take 10 seconds to ramp up to operating speed.
The fan can be configured via the Bosch EasyStart
app to run in a range of CFM from 300 to 450 in 25
CFM increments. The controller has airflow
profiles (CFM) for both heating and cooling
operations.
Loop Pump Operation
The Loop Pump (LP) energizes 30 seconds
(configurable from 30 seconds to 240 seconds via
the Bosch EasyStart app) prior to compressor
operation during a mechanical heating or cooling
demand. The LP remains on during low loop water
temperature protection and a high Leaving Water
Temperature (LWT) warning. The loop pump stays
off for the following faults and delay timers:
• High Pressure Switch fault
• Low Pressure Switch fault
• Freeze Coaxial Coil fault
• Freeze Evaporator Coil fault
• Brownout fault
• Condensate Overflow fault
• Anti-Short Cycle delay
• Flow delay
HPC will command the Loop Pump (LP) output to
energize the pump motor whenever a heating or
cooling command (Y1) is received.
Some options will have a built in delay, so
compressor operation is not immediate.
Note that a fault condition initiating a lockout will
de-energize the compressor irrespective of
which stage is engaged.
If the unit compressor locks out for any reason
at this time, the electric heaters will continue to
function normally.
When using a 2-cool, 3-heat thermostat both the
W1 & W2 on the Heat Pump and W2 & EM on
the thermostat must be connected together via
a jumper.
The fan will be commanded to run at fan-only air
flow as long as there is no call for mechanical
heating/cooling or electric heating operations.
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