Plant in heating mode
In normal heating mode of an air-to-water heat pump, condensation can occur at low
temperatures, causing the evaporator to ice up. This reduces the heat pump’s output
and can lead to low-pressure malfunctions or damage to the evaporator.
B21
B71
Q9
B91
T
T
T
K19
B84
T
K1
Y22
E17
Gas low-pressure
Gas high-pressure
Liquid
Plant in defrost mode (process reversal)
The evaporator is defrosted either with the fan or – as shown in the example below –
with process reversing valve Y22. For process reversal, a HP partial diagram with
process reversing valve (Y22) must be used.
Demand-dependent defrost control ensures that the defrost energy drawn from the
heating circuit in the case of process reversal will be kept at a minimum. During the
defrost process with process reversal, the fan remains deactivated.
B21
B71
Q9
B91
T
T
T
K19
B84
T
K1
Y22
E17
2355Z12
Gas low-pressure
Gas high-pressure
Liquid
Automatic defrost function
When the compressor is in operation, "Duration defrost lock" (2962) and "Time up to
forced defrost" (2963) elapse. If the source temperature (B91) drops below the defrost
release temperature (2951), the defrost release function will be released. The heat
pump can change to defrost operation mode after "Duration defrost lock" at the earliest,
and on completion of "Time up to forced defrost" (2963) at the latest.
When, due to icing up during this period of time, the temperature difference (8477)
between incoming outside air (B91) and evaporator (B84) exceeds the setpoint (8478),
the defrost function is triggered.
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Siemens Switzerland Ltd User manual RVS61.843, RVS41.813 CE1U2355en_02
HVAC Products 6 The settings in detail 28. September 2009
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